(Peat free) Growing Media - Hardy Plant Society

Garden
The
Growing media 2010–2012

gardenscience
arguments
for going
Peat-free
l Many peat-free or peatreduced
growing media prove
suitable for a range of
purposes, such as rooting
cuttings (top) or potting-on.
l Reducing the amount of
peat gardeners consume will
help ensure the protection
of lowland raised peat bogs
(above), which are rare habitats
in the UK, home to plants such
as sundews and many mosses
and sedges.
l As a result, peat bogs can
continue to act as valuable
carbon sinks, storing carbon
in a form that does not
negatively contribute to
climate change.
l Peatlands also play a role in
the global water cycle – their
destruction may lead to
increased risk of flooding.
Time for an
alternative
When choosing
multipurpose
compost (left)
it is possible to
find peat-free
formulas, such
as those based
on coir, or peatreduced
mixes
peat?
What
future
for
Andrea Jones
Gardeners have been using peat-based
composts since the 1970s, but environmental
concerns makes future use uncertain. Susie
Holmes provides an update on the issue.
Photography by Tim Sandall
Peat-based composts are, for a
generation of gardeners, synonymous
with gardening. Almost all the plants
we buy are grown in them and they
constitute the vast majority of composts
sold – in fact home gardeners account
for about 70 percent of all UK peat use.
Our love affair with peat-based
compost began in the 1970s when peat
took over from loam-based John Innes
mixes, transforming the horticultural
industry in the process. Until recently
most ‘multipurpose compost’ and
growing bags were 100 percent peat,
with lime and fertiliser added, although
many products now contain at least
10–20 percent other materials as
manufacturers (encouraged by the UK
Government) try to reduce peat use.
Peat and the environment
So why can we not carry on using peat
to grow plants? There are two major
concerns over peat extraction.
The loss of biodiversity from rare lowland
peat habitats has historically been
the main problem, and Government
targets for peat extraction are part of
the UK Biodiversity Action Plan,
developed as a response to the 1992
Convention on Biological Diversity,
encouraging sustainable development.
The second and more recent concern
is the loss of peat as an important store
of carbon, once bogs are drained and
peat extracted. This peat, over time,
gradually breaks down, giving off CO 2
,
a greenhouse gas. As a result, new
Government targets for peat replacement
will be linked to the ‘Act on CO 2
’
campaign (see News, p147). As such a
large proportion of horticultural peat is
used by home gardeners, there is
pressure on the gardening public to do
their bit, in addition to efforts by the
professional sector.
Although a UK Government target
set some years ago to make 90 percent
of composts peat-free by 2010 will not
be met, limited progress has been made
– a reduction of around 25 percent has
so far been achieved.
Arguments that far more peat is
burned in power stations globally than
used in horticulture are unlikely to make
any difference, either – in the battle to
reduce greenhouse gas emissions,
every bit saved helps. P
178 March 2010 March 2010 179

Gardenscience
Growing media options
British gardeners often use the word
‘compost’ when they actually mean
‘growing medium’; technically ‘compost’
is the product of a composting process,
which in most cases growing media are
not, so this often causes confusion.
We should not assume replacing all
or some peat in a mix is bad. Blending
in materials such as bark and wood fibre
can improve drainage, while green
compost or loam improves water and
nutrient holding, creating mixes which
are actually better for growing in than
pure peat. However, there is no mix ideal
for every situation, and gardeners will
find some blends are better for certain
uses; a coir-based mix is good for seed
sowing and propagation, for example,
but a blend with bark or wood fibre
will probably suit outdoor planters.
There is nothing new about this, in
essence – gardeners have long blended
their own media for specific uses, but
the materials manufacturers are adding
to shop-bought media may be in some
cases unfamiliar. Moreover, it is not
always easy to ascertain the precise
ingredients in a bag of ‘compost’.
Manufacturers, however, are now improving
labelling so it should soon be
easier to see what the peat content is
Peat-free media can be
used for many purposes,
including topdressing for
plants in containers
‘different blends
suit different
people, depending
on the types of
plant you grow
and how you water
and feed them’
– and which other materials are used in
the mix. Some are even adopting a barchart
system, in much the same way
packaging of certain foods shows the fat
and salt content. The Government is also
encouraging more in-store information
to make it easier for gardeners to make
‘greener’ growing media choices.
The main source of independent
information on different brands of
growing media is the annual Gardening
Which? test report but, as with any peatbased
mix, different blends suit
different people, depending on the
types of plants you grow and how you
water and feed them – do not be put off
if the first alternative mix you try does
not suit you.
Adapting to new media
Often, the biggest problem gardeners
have when changing to a new growing
medium is with watering. Some reducedpeat
or peat-free media dry quickly on
the surface yet are still moist below,
making it all too easy to overwater plants.
However, an advantage of this drier
surface is reduced moss and liverwort
growth compared to peat mixes. It may
also be necessary to feed plants differently
in reduced-peat or peat-free media.
Coir-based mixes may need feeding
earlier and mixes containing green
compost may last longer without feeding
because of the slow-release nutrients
that the compost supplies.
The materials outlined opposite (p181)
are alternatives used by manu fac turers
in peat-reduced, and even peat-free,
growing media likely to be found at
garden centres. These four alternatives
are considered to have a significantly
reduced impact on the earth’s climate
system in comparison with peat.
Other materials
Mineral-derived materials such as perlite
and rockwool can be used in growing
media, but either need to be added at a
low percentage (because of their low
water-holding capacity) or used in
hydroponic systems. High energy is
needed for their manufacture; they do
not degrade after use.
Loam is used at a low percentage in
some mixes to improve water and
nutrient retention, for example in John
Innes products. However, good quality
loam is scarce and not renewable.
No one material can replace peat, but
the quality of alternatives has improved
hugely and as a result the RHS is committed
to helping gardeners reduce
peat use. If you are not confident, try a
reduced-peat mix first, especially for
specialist plants. For tubs of shrubs or
summer bedding, quality peat-free
media should perform well.
The key to success with new growing
media is managing watering and nutrition,
and not expecting them to behave
like peat. The Government will continue
to encourage gardeners to use less peat,
which should mean a wider choice of
reduced-peat and peat-free products,
with better information on packs and in
store. Our reliance on peat is fairly recent
– gardeners have adapted to changes
before, and so can again. n
Susie Holmes is an independent
growing-media consultant who
specialises in peat alternatives
@
i
For more information, visit:
www.rhs.org.uk/peat
This year, a project at RHS Garden
Wisley will review how to water, and
manage nutrients in, peat-free products.
l Further reading The March issue
of The Plantsman, quarterly sister
publication to The Garden, includes a
more detailed review of peat alternatives.
For subscription details, tel: 0845 062 1111
or visit: www.rhs.org.uk/plantsman
susie holmes
Does what it says? The labelling on
growing-media bags is beginning to
improve. Some products found in garden
centres and nurseries clearly state if a
product is peat-free (above); other
reduced-peat mixes may need closer
examination (below right and bottom).
Peat-substitute materials include bark
(above right), which is usually added to
peat mixes and broken up into small
particles before use. Wood fibre (right) is
a relatively recent material, added to
compost mixes by some manufacturers.
Coir (below) and composted green waste
(bottom right) are better known, and have
been used in home-made mixes
Coir
Wood fibre
Bark
Green compost
paul alexander / rhs
growing Media
components
l Bark
A by-product of the timber industry,
bark can be considered a sustainable
resource because forestry plantations
are replaced in a cycle of harvesting
and replanting. Pine or spruce barks
generally are the main types used;
bark of deciduous trees is often toxic
to plants. Bark is useful in mixes for
plants that are outdoors over winter
and for species that need good
drainage, but usually must be blended
with more water-retentive materials.
l Wood fibre
Manufactured wood fibre is a recent
addition to the peat-alternatives range
in the UK. It is produced by subjecting
wood chips to pressure, which expands
them to a fluffy fibre. Wood fibre has
the advantage of being a consistent
material from a renewable resource
and various products now contain it.
l Coir
Coir pith dust is a by-product of coirfibre
production in countries such as
India and Sri Lanka. Fibre from coconut
husks is used in ropes, mattresses and
matting, but the dust is a waste product
that was once burned or accumulated
in heaps. Coir has been used in growing
media since Victorian times and is
closer to peat in terms of its air-towater-capacity
ratio than other
alternatives. Coir is imported in a
compressed state and reconstituted in
the UK, making transportation cheaper:
coir’s ‘carbon footprint’ is not too bad
as it is transported mostly by ship.
Coir can be used in reduced-peat
blends and even undiluted: it has good
air capacity as it holds air within the
‘honeycomb’ structure of its particles.
Its texture promotes root growth so is
suitable for seed mixes and cuttings,
and for potting indoor plants.
l ‘Green compost’
This is the term for compost from green
waste materials, such as brushwood,
grass clippings and tree prunings,
made at large composting facilities.
Green compost is already used at a low
percentage (typically 10–20 percent)
in many multipurpose products and
growing bags. It behaves like loam,
holding nutrients and water well,
making it useful for summer planters
and hanging baskets.
180 March 2010
March 2010 181

GARDenPRACTICe
Getting
to grips with
peat-free
A range of peat-free media has
been available for years, yet
gardeners are still confused
which to choose and how
best to manage them. rhs
experts Paul Alexander and
Nick Morgan offer practical
examples to help guide us.
Photography by tim sandall
January 2011 51

gardenpractice
As the need to reduce the use of peat
becomes more pressing, home gardeners
can face a daunting choice of alternatives
(see The Garden, March 2010, pp178–181).
These numerous products make a variety of
claims regarding their use, and the selection
of growing media on offer is further
complicated by the presence of peat-based,
peat-reduced and peat-free products.
The RHS promotes sustainable use of
peat alternatives and will support progress
towards the Government’s target of a total
withdrawal of peat use by home gardeners
by 2020. Whatever your own thoughts on
peat use in horticulture, there is a need for
gardeners and growers to understand how
to manage peat-free growing media.
Consider these key points
when choosing a compost:
l What is it to be used for? Multipurpose
compost (peat-based or peat-free) is suited
for potting and growing on (see below) as
well as raising seed; more specialist seed
and cutting composts (see pp54–56) may
provide better results.
l How easy is it to wet the growing
For many gardeners, changing to
peat-free media can prove testing.
RHS scientist Paul Alexander (left)
outlines the issues of finding an
acceptable peat replacement, and
makes suggestions for a smooth
transition, based on his research
medium properly and how well does it
then hold onto water?
l Are nutrient levels available in the
medium suitable for the plants you wish
it to support?
Media components
Most growing media (including peat-based)
are blends of one, or possibly two, main
ingredients with a small volume of a variety
of other materials for specific beneficial
properties. The principal materials tend to
dominate how growing media perform
although this can be unpredictable due
to a number of factors:
l Different materials behave differently to
each other, but the packaging of many
growing-media products does not state what
the main components (feedstocks) are.
l Variation can occur between batches of
any one component supply.
l Quality control for peat-free media is
often more difficult due to this potential
variability in components.
In addition, peat-free media may cost more
than peat-based products due to production
and quality-control issues. Competitive
Making the right choice of growing
medium and then knowing how best to
treat it is important if you want to get
the most from the plants you grow in it
pricing in-store often means that manu facturers
(both peat and peat-free) use cheaper
materials, which can lead to variable quality.
Plant responses
Recent research by the RHS has focused on
the principal material from which composts
are manufactured, as we seek to understand
the basis for differences in performance
rather than simply compare what is
available. However, poor labelling of some
peat-free compost adds to confusion.
The RHS believes that this information
would be extremely helpful to gardeners
and is actively working with the Growing
Media Initiative to support this.
Research is further complicated by
variable responses shown by different plants
to different materials. Using plants that are
familiar and frequently grown is helpful, but
results cannot be seen as typical for all
plants. This can be confusing for gardeners
who want a compost that performs well
across their needs, for all plants.
When using growing media, water management
is critical (as the research detailed
below begins to reveal); this is especially
the case with peat-free materials.
There are other points to consider, such
as how best to feed plants grown in these
composts, as the primary components
may supply more nutrition than ordinarily
found in peat-based composts. In all, we
still need to learn how to get the best from
these media, be they used for general
potting or plant propagation. As we build
experience and peat-free products become
more main stream, these issues will be less
of a problem. n
Paul Alexander is RHS Principle Scientist
– Horticulture, and has a particular interest
in soils and growing media
l See Nick Morgan’s tests with seeds and
cuttings in different media (pp54–56) P
Which media
to choose?
The range of peat-free media
on the market can be daunting.
Here are some points to consider:
l Various materials are used as
substitutes for peat in growing media.
Most commonly offered are coir (a byproduct
of coir-fibre production, made
from coconut husks), composted green
waste, and wood fibre (made from
pressure-treated wood chips). Others
include composted wool fibre and bark.
Loam-based media are also offered.
l Packaging of many peat-free and
peat-reduced media is poorly labelled.
If possible, choose one that clearly
states its principal ingredients (below)
or ask suppliers before you buy.
l Be prepared to pay more for
peat-free growing media to secure
the best-quality product.
l Try different brands to familiarise
yourself with the range of available
products, and experiment before committing
to expensive or difficult plants.
l Look for specially formulated media
suited for specific purposes, such as
seed and cutting compost for
propagation (see p56).
RHS research:
Watering peat-free
media
rhs / rachel tanner
Paul Alexander’s experiment (left) assessed different
growing media for growing plants when given
differing amounts of water. Petunia and fuchsia
plants were grown through last summer in multipurpose
media based on peat, wood fibre, coir and
composted green waste. Water was applied at five
different rates for each medium, the control treatment
being optimum watering for that medium,
while the other four treatments were this value +25
percent, +50 percent, ‐25 percent and ‐50 percent.
For research reasons, optimum watering was
taken to be when the container is wet through
but not waterlogged.
The experiment is currently being analysed, but some effects have already been observed:
Plant quality
All the media tested were
capable of producing goodquality
plants. When comparing
plants watered at the control
rate, plants appeared slightly
bigger in coir and peat, but leaf
colour, size and flower number
were good in all four.
Effects on plant quality
With plants that were under- or
overwatered, quality reduced
most quickly in composted
green-waste and wood-fibre
composts. Peat- and coir-based
media proved more forgiving.
If using composted green waste
or wood fibre composts it is
important to water more carefully.
Water requirements
Peat and coir were able to retain
far higher volumes of water than
wood fibre and composted green
waste. Good water retention
offers greater flexibility, as
frequency of watering can be
reduced. However, as water
resources become more valuable,
media needing less water to
produce good plants could be
preferable to those that need
more. Under optimum watering,
green compost and wood fibre
plants were still good quality but
only received around one third of
the water that the peat and the
coir plants received.
l The findings of this research will
be published on RHS Online at a
future date: www.rhs.org.uk
January 2011
January 2011 53

gardenpractice
se
The plant requirements of any type
of media may seem basic; it must provide
moisture, oxygen, plant nutrients and
some thing into which the roots can anchor.
However, these essentials have to be
married together carefully in order to
create useful compost for plant raising.
Blending different grades and types of
bulky organic materials, and possibly
adding mineral aggregates such as grit,
sands or perlite, should create media
that hold moisture without becoming
waterlogged, and provide sufficient pore
spaces to trap air for the roots to absorb.
Assessing the textural qualities of a
medium by running it through your fingers
is a good indicator of its ability to meet
those requirements. It should flow freely,
with variable particle sizes, and be neither
coarse and lumpy nor too fine; it should
feel moist without being wet and sticky.
With the right textural qualities it should
be easy to fill any size of container and
produce a level, smooth seedbed. The
filled containers should wet up quickly
either by watering overhead or from
below by standing pots in a water bath.
Stable nutrient levels
At the propagation stage plants are
susceptible to fluctuations in nutrient
levels: they can be deprived of, or oversupplied
with, essential plant foods if the
organic content in compost is not stable
and is still breaking down. The organic
content in peat-free media is not as
decomposed as peat and therefore more
likely to create plant-nutrient deficiencies
or toxicity. Gardeners depend on the
manufacturers to make media that are
stable with nutrient levels suited to
seedlings and rooting cuttings.
Nick Morgan (left) from the Glass
Department at RHS Garden Wisley
tested various different media,
including traditional peat, peat-free
and loam-based composts for use
in raising seeds and cuttings; here
he observes their varied properties
Media compared
At RHS Garden Wisley we ran a test to
make a simple comparison between
some proprietary peat-based and peatfree
propagation and multipurpose
composts; it also included traditional
loam-based (John Innes) propagation
media. The main organic ingredients in
the peat-free media were bark, coir
and green waste. Some also included
sterilised soil in their mixes
To see how well media performed we
sowed containers with different easy-togrow
flower and vegetables seeds.
Small seeds (lobelia) were mixed with
fine sand to aid sowing; this mix was
broadcast over the surface and left
uncovered. Medium-sized seeds (lettuce)
were surface-sown and covered with
5–10mm ( 3 ⁄16– 3 ⁄8in) of medium. The large
bean seeds were simply pushed straight
into the compost.
All were put in a warm propagator.
Seedlings and cuttings
The speed and overall germination rate
was observed along with the general
quality of the seedlings produced to a
stage ready for pricking out. We also
prepared pelargonium cuttings (see p56)
to compare how well they rooted. The
cuttings were inserted around the edge
of a small pot, and watered in to settle
the compost around the cuttings; these
were then placed in a warm propagator to
allow them to develop root systems. Our
evaluations were based on the time taken
to root and the quality of the root system. n
Nick Morgan is Horticultural
Specialist for the Glass Department
at RHS Garden Wisley
seeds sown in
lobelia
lettuce
beans
Peat-based
The peat-based propagation
medium had a fine texture,
producing a smooth seedbed,
and wetted up easily.
A high proportion of all seed
types germinated. Seedling
development was rapid and
the growth a little soft, making
it urgent to prick them out
before they deteriorated.
A peat-based multi purpose
compost was also tested and
provided similar results,
although some particularly
coarse fibres and hard lumps
had to be picked out in order
to make a smooth seedbed for
the finest seed.
widely available growing media
loam-based
Loam-based propagation
media are made from a mixture
of finely screened loam and
sand. The media tested
produced a fine, smooth
seedbed and wetted-up easily
by standing in a water bath,
which preserved the fine,
smooth surface from capping
(formation of a surface crust).
Germination was slowest in
this medium; the fine surfacesown
seed struggled to anchor
into the dense surface. Larger
seeds, however, performed
much better, producing goodquality
seedlings at the
pricking-out stage.
green waste
This medium, formulated for
propagation, was mostly green
waste with some composted
bark and mineral aggregates.
It had a moist, crumbly texture;
producing a level seedbed was
not easy. On wetting, the
medium soon retained water;
the pots needed a long period to
drain well before the surface was
suitable for sowing. Germination
levels were high, producing
good-quality seedlings soon
ready for pricking-out.
A multipurpose equivalent of
this medium proved coarse and
lumpy, making it difficult to
prepare a suitable seedbed.
wood fibre
Predominantly derived from
shredded wood, with a coarse
texture arising from fibrous
wood fragments, it was difficult
to create a level seedbed with
this medium – the surface was
open and uneven, so smaller
seeds might not have good
contact with the medium.
Water drained through quickly;
seedlings required frequent light
watering to keep the immediate
rooting zone moist. Larger
seeds, however, fared well.
The manufacturers do state
that this medium is unsuitable
for small seeds; the test served
to confirm this.
coir
Coconut fibre-based media
with added vermiculite look
and handle similarly to peatbased
composts. The coir
propagation medium tried here
was easily handled, producing
a smooth, level seedbed. It
wetted-up and drained quickly.
Germination was erratic with
low counts of small and
medium seeds; larger ones
germinated slowly and
produced compact plants.
These findings were surprising,
as good results have been
previously observed with coir at
Wisley. A multi purpose compost
gave similarly poor results. P
54 January 2011 January 2011 55

gardenpractice
pelargonium cuttings:
how the Media fared
Peat-based
propagation media
Rooting took about 14 days and cuttings were ready
for potting into individual pots a week later. During the
propagation phase, the foliage remained green. The top
growth was quite ‘soft’, with expanding new leaves and
some stem stretching. The root system was extensive
and had moved through the medium, penetrating almost
to the bottom of the container.
Loam-based
propagation media
This medium was dense and heavy; after 14 days,
rooting had started but had not penetrated far into the
compost, and the top growth had become hard and
pale. At 21 days the cutting growth remained poor; the
amount of root had not increased significantly and had
not penetrated more than 3cm (1 3 ⁄8in) into the medium.
Green-waste
propagation media
Cuttings produced roots in 14 days, and these continued
to develop in number and length over a 21-day period
prior to potting up. The foliage remained green; there
were not any obvious signs of new vegetative growth,
the stem hardening a little. The root system was quite
extensive and had penetrated well into the medium.
Wood-Fibre
multipurpose media
This fibrous medium posed some problems in keeping
the cutting upright, and therefore they had to be inserted
quite deeply. Rooting after 10 days was quite extensive
and at 21 days had increased in volume and penetrated
to the bottom of the container. The leaves remained
green, new leaves expanded and stem length increased.
Coir propagation media
After 10 days, the cuttings had produced a large callus
and some rooting had started to penetrate the medium.
However, the rooting was slower and more limited than
experienced at Wisley in the past with coir. The leaves
held a good green colour; new leaves were expanding
and the stems remained quite soft.
www.rhs.org.uk For more information, search
@ ‘Peat-free media’ on the homepage of RHS Online
growing and
propagating
using peatfree
media
Paul Alexander’s research and even
the simple tests done by Nick Morgan
indicate that changing to peat
alternatives will require gardeners
to make changes in cultivation,
especially in propagation, when
plants are at their most sensitive.
Key points to consider:
l Different requirements may not be
served by the same bag of compost.
Media specifically manufactured for
propagation performed better for
this purpose than multipurpose
composts, particularly for smaller
seeds. Multipurpose composts,
despite their descriptive name, are
not formulated specifically for seed
sowing or raising cuttings.
l Coir-based composts are the most
peat-like of the products tested here.
l Watering regimes of peat-free
media differ from those of peat. For
example, propagation media made
from composted green waste were
found to be easily overwatered.
A light touch is needed when
preparing containers of these
products to avoid smearing and
compacting the surface.
l Multi purpose composts based on
wood fibres need to be finely milled
and well composted to be suitable
for seed sowing.
l As supplied, the composts were
compressed in their bags. Ensure that
the medium is ‘fluffed up’ and aerated
before transferring to the containers.
Media that feel warm are still decomposing
and should not be used.
56 January 2011

Peat
Left to Right: Rhs / rachael tanner; science photo library; RHS / Paul Alexander
Commercial harvesting
of peat in Cumbria.
Aerial view of commercialscale
peat extraction at Bolton
Fell Moss, Cumbria (operations
set to cease here in Nov 2013).
RHS science update
replacing peat
The new generation
of growing media
When buying growing media, the gardeners’
predicament is not knowing how they may perform.
Feedback from the RHS Members’ Advisory Service
suggests that the performance of many peat-based,
peat-reduced and peat-free growing media is variable.
Some of this may be related to manufacturers trying to
reduce costs by sourcing cheaper ingredients, reducing
the quality of the final product.
One idea from the industry is a ‘quality standard’. This
could be expensive to adhere to and administer, but
should allow gardeners to make a more confident choice
(quality standards are already in use in the Netherlands
and in Australia). Currently, labelling of products in the
UK is at best variable – some packaging includes content
details, others do not even say whether the material is
peat-based or peat-free. Some manufacturers argue that
consumers interpret the label ‘peat-free’ as meaning the
product is lacking in something, so do not use this
description for fear of negative connotations.
With the Government aiming for the elimination of peat from growing media, how
will gardeners and the horticultural industry cope with the alternatives?
Author: Paul Alexander, Principal Scientist, Horticulture, RHS Garden Wisley
The vegetation of areas
from which peat has been
harvested can regenerate, but
this may be a slow process.
Difficulties in the marketplace
Good quality peat-reduced and peat-free alternatives
for the bulk of general gardening tasks are now
available, but competition is forcing prices
down, making it difficult for them to
compete with established peat-based
formulations. A recent RHS survey
suggested that consumers remain price
driven when buying growing media.
RHS Garden Wisley uses a commercial
peat-free product that is more expensive
than comparable ‘off the shelf’ products,
because it is of an assured quality.
rhs / tim sandall
Peat has become a hot
topic in UK horticulture.
Since the 1940s it has
been the main ingredient
of choice for growing
media (or ‘potting composts’).
Before then, loam-based John
Innes formulations were the norm.
Ingredients are chosen mainly for
their performance (how well plants
grow in them), their availability and
the costs of transporting them.
Peat’s popularity in the UK has
been based on its cheapness, and its
chemical and physical consistency.
It requires relatively little processing
once harvested to form a growing
medium, and is locally available to
the UK from within Europe. The
chemical and physical properties of
peat are well suited to its use in
growing media. It is physically stable
and low in fertility, so manufacturers
can add different blends of nutrients
to tailor their growing media to
particular plant types. However, even
peat benefits from the addition of
other materials to improve its
properties – composted bark, for
example, is often added to improve
a mix’s drainage properties.
So if peat is cheap, available and
ideal, why is it being phased out?
Peat and environment
The impacts of peat extraction are
often debated. Cases can be made to
support, and contradict, most issues.
What is not in doubt is that pristine
peatlands are increasingly rare
habitats supporting uncommon and
specialist plants and animals in
diversity. They aid freshwater
management by helping mitigate
flooding, and many preserve
valuable archaeological archives.
There is also debate over peatlands
and climate change, and whether
they store or release greenhouse
gases. The precise balance is complex
and often specific to particular sites,
but most researchers agree that
peatlands as a whole store carbon.
Living peatland vegetation absorbs
carbon dioxide from the atmosphere.
When it dies under waterlogged
conditions the process of peat
formation begins, which locks up
more carbon than it releases. When
we harvest peat and take it from the
wet, low-oxygen conditions in which
it formed and persists, it begins to
break down and releases this stored
carbon. Primarily, this is as carbon
dioxide, an important greenhouse
gas linked to climate change.
»
rhs / leigh hunt
For most purposes, RHS Garden
Wisley uses a commercial
peat-free product.
International
research
In parts of the USA and Australia peat is
an expensive ingredient (due primarily
to transport costs), so has not
commonly been used in growing
media. Wood-based materials such as
composted bark (right) have dominated
growing media in these areas.
Increasingly, international research is
looking into materials that are sustainable, and
rhs advisory Services
ideally considered to be waste. North American research
focuses on various wood wastes, making use of different
tree species, different portions of the tree (such as bark
or needles) or designing mixes by particle size (coarse
materials mixed with finer particles).
In Spain, recent research has trialled composted gorse
as a growing-media ingredient – not only sourcing
materials locally, but finding a use for a ‘problem’
(invasive) plant. Other materials being studied as
potential constituents for growing media include cotton
gin ‘trash’ (the waste from cotton fibre production), olive
waste, rice hulls, carpet waste and brewery waste.
68 The Garden | January 2012
January 2012 | The Garden 69

Peat
Sourcing peat
We are using huge quantities of peat.
In 2010, UK horticulture used nearly
3 million cu m (106 million cu ft) of
peat, almost 70 percent of this by
home gardeners. Currently, 60 percent
of the annual total is imported
from the Republic of Ireland,
32 percent is from within the UK
and 8 percent from northern Europe.
As an industry, commercial
horticulture in the UK recognises
that it must reduce its environmental
impact and improve its sustainability.
Much UK peat comes not from
uplands but lowland raised bogs:
these now extremely rare, specialist
ecosystems support a diversity of
rare plants and animals.
Figures based on global peat
reserves suggest that considerable
volumes of peat are available
worldwide. This may well be true, but
the cost of extracting and shipping
make most reserves too expensive:
they are simply too far from the
markets and consumers.
Alternative materials
Any material used in growing media
has to satisfy the chemical and
physical qualities of peat, but must
also be: commercially available at
suitable volumes; available year
round; consistent; free of
contaminants; and ideally need little
Government
view on peat
The recent Government white
paper, ‘The Natural Choice’,
announced its aim of – voluntarily
– eliminating peat use in
horticulture by 2020 for home
gardeners and by 2030 for
professionals. To achieve this, a
cross-industry task force of
growers, manufacturers,
non-governmental organisations
and retailers was set up.
The Sustainable Growing Media
Task Force aims to identify the
barriers that have held back peat
replace ment to date, and to
suggest how these can be
overcome. In June this year it will
publish its recommendations
towards a voluntary phase-out.
rhs / Paul Alexander
Trials of peat-reduced and
peat-free growing media
at RHS Garden Wisley.
processing. Many alternatives to peat
fail to meet some of these criteria
(and often cost more than peat).
Peat has been so successful in
multi-purpose compost that
replacing it is challenging. Because
no single material satisfies all the
requirements, most new growing
media are blends of one or more
main ingredients and a number of
others at smaller volumes. For peatfree
products, we may need to return
to different media formulated for
specific groups of plants. Developing
and trialling new media is costly and
time consuming. Balancing the
chemical and physical properties of
naturally variable organic materials,
ensuring they are ideal for the end
use, and that the product is stable to
allow some storage, is so complex it
can take three or more years before a
product is ready to market.
In the UK, growing media
ingredients now include green-waste
compost, composted bark, wood fibre
and coir. Research continues into
how they are best blended to produce
reliable, consistent growing media.
Other studies (see panel, p69) are
looking at ‘designing’ growing
media from specific materials, such
‘Commercial
extraction
of peat at
current rates is
unsustainable’
as food-waste compost or coarser
woody materials.
Many growers, both professional
and home gardeners, regularly report
they need to alter their management
practices with the newer ingredients.
Watering, feeding, and pest and
disease management can all vary
with growing media made from
different materials. The Society’s own
studies and trials will continue and
the results publicised in The Garden
and elsewhere as a priority.
www.rhs.org.uk/peat For more
information on peat use, the RHS policy
on peat, and alternative media, visit
the pages at RHS Online.
✤ Growing media buying habits For
results of the June 2011 RHS survey
see The Plantsman, March 2012.
Europe and trade
The desire to replace peat not only affects the import of material for bags
of growing media, but also impacts on professional plant growers.
An unfortunate side effect of replacing peat in the UK is that our own
professional growers must compete with European growers who face less
pressure to reduce their peat use.
It is widely believed that growing peat-free is more expensive, so UK
growers may not be able to compete on price with imported plants grown
in Europe. This issue is something that the industry and the Department
for Environment, Food and Rural Affairs (defra) wish to prevent happening.
Impact on growers
A number of professional
growers are peat-free, some
have peat-free lines, some have
peat-reduced lines and nearly
all will have tried peat-free
growing at some stage. The
issue of increased costs, due
to changes in management
practices for example, are cited
as currently preventing most
growers increasing their use of
peat-free media. It is hoped that
over time, as growers get used
to new blends, this will change.
rhs / rachael tanner
Scoring petunia plants
at the end of the trial.
rhs / Paul Alexander
rhs / neil hepworth
View from
Roger Williams,
RHS Head of Science
The RHS is acutely conscious of the
adverse environmental impacts of the
use of peat in horticulture, and how
important it is to support the nation’s
gardeners and horticultural industry in
reducing peat use. That is why we are
pleased to be contributing to defra’s
Sustainable Growing Media Task Force.
Thanks to the efforts of RHS curatorial staff and
scientists, our own gardens are now 97 percent peat
free. We only use peat-based media to propagate certain
plants and to maintain a small number of our specialist
plant collections. Peat is never used as a soil improver or
mulch in RHS Gardens, and at our flower shows we have
banned the use of peat for staging exhibits.
We have taken these steps because we think
commercial extraction of peat at current rates is
environmentally unsustainable, as it removes peat at a
much faster rate than it forms, leading to the irreversible
destruction of peatlands. This matters because,
worldwide, peatlands are important for biodiversity,
carbon storage and flood risk management.
Quality control
We are committed to informing, encouraging and
helping gardeners to use peat-free alternatives but, in
our experience, the variable quality of peat-reduced and
peat-free growing media are of concern to gardeners.
Consumer confidence is critical and needs to be
improved by better quality control of such media.
Information about which peat-free products are
appropriate for specific applications and plants is often
lacking at the point of sale. The Society believes that
labelling of growing media needs to be improved.
Packaging should be explicit about its peat
content and all other ingredients in the mix.
More work needed
Addressing such issues would help
gardeners make better-informed
choices, and enable the RHS further to
improve its advice to gardeners on how
best to manage the increasing number
of types and mixes of growing media.
For example, we have examined how
plants grown in different peat-free media
respond to different watering regimes (see The
Garden, Jan 2011, pp51–56). The results suggest that
good plants can be grown in a wide range of media but,
with some mixes, watering must be managed carefully
for best results. This RHS research is continuing,
examining other factors such as different methods of
irrigation, the response of different plant types, and
trialling more peat-free substrate mixes.
Finally, we see a need for further research where peat
replacement is considered most difficult – for ericaceous
plants, the propagation of vegetables, and the supply of
fresh herbs to supermarkets by commercial growers.
rhs / tim sandall
70 The Garden | January 2012
January 2012 | The Garden 71

Sowıng
Garden practice
without peat
By adding vermiculite, perlite or grit to peat-free
growing media, RHS Garden Wisley improved the
success of seed germination and rooting of cuttings
Author: Nick Morgan, Horticultural Specialist, RHS Garden Wisley. Photography: Tim Sandall

In the commercial horticultural world, a growing
medium (usually known as ‘potting compost’ in garden
centres and nurseries) is formulated to meet specific plant
requirements. Manufacturers blend ingredients to produce
media intended for propagation, for growing on and for
myriad niche applications.
By contrast, most peat-free composts sold to gardeners
are described as ‘multipurpose’. Manufacturers recommend
these for a wide range of use from seed
sowing to repotting of mature plants.
At the moment, the most readily
available multipurpose peat-free
composts are formulated to contain
varying amounts of composted green
waste, conifer bark, wood waste or
coir fibre. Manufacturers blend one
or more of these products to create a
workable, open yet moisture-retentive
growing medium, adding a fertiliser
mix suitable for the type of bulk
ingredients used.
Fine seedlings
of Begonia
semperflorens
need an
even-textured
peat-free compost
to germinate well.
44 The Garden | February 2012
Peat-free in use
Unlike peat-based products, peatfree
multipurpose growing media are
often variable in appearance and need
different handling. Where the main
constituent is a blend of composted
bark and wood waste the
growing medium may
appear to be loose and
open; high levels of
green waste, on the
other hand, make a
compost dense and heavy;
coir is most peat-like but
can appear dusty and
without substance.
This variability is a real problem
in propagation, which requires a seed
bed for sowing fine seeds or an open
compost for rooting cuttings. Home
gardeners, however, can easily
manipulate a ‘multipurpose’
compost by using different additives
to improve the texture and so
give more consistent results.
To demonstrate this, our team at
RHS Garden Wisley grew begonia
and lettuce seedlings and rooted
fuchsia cuttings in a range of peatfree
multipurpose composts. We
sieved some composts first and then
added perlite, vermicu lite or grit (see
box, p46) in varying ratios, depending
on the initial texture, to make them
all more versatile (and more suitable
for containers, including smaller
modules) at modest extra cost.
Nick Morgan:
‘When seed sowing, it seems that coir is a good
peat-free medium to use, but adding vermiculite
gives a more even germination. For rooting
cuttings, it is – on balance – better to add perlite to
coir or green waste-based media, rather than grit.’
Adjusting
texture
Multipurpose growing
media vary in texture
due to the nature of
the main ingredients
and the mix in particle
sizes. Some initial
preparation can make
the product more
uniform, suitable for
fine seed germination
and propagation
by cuttings.
1
In the bag: Growing media can become
quite compressed during transportation
and storage, so tip it out to break it down.
If dry, it should be thoroughly wetted and left
to rehydrate for some time before use.
Coir + vermiculite
Overview: Coir is a user-friendly material
derived from coconut husks and now widely
available as a growing medium or an
ingredient in other mixes. We broke down
blocks of compressed coir-based compost by
immersing them in a bucket of water.
Treatment: Coir does not require sieving as
the particles are small enough. We added one
part medium-grade vermiculite to eight parts
compost by volume, to improve aeration and
help regulate the nutrient availability in the
mix that would benefit seedlings.
Germination results: Both fine seed of
begonia and larger lettuce seed germinated
evenly. At pricking-out, the root systems were
well developed and manageable. Particles of
compost remained attached to the roots,
which would aid a quick establishment into
their new growing medium.
2
Break down lumps: After it is fully
hydrated, rub the growing media
between hands to break up lumps
and ensure a uniform and aerated product
ready for tray and pot filling.
Green waste
+ vermiculite
Overview: Composted green waste is used in
varying amounts in many peat-free growing
media. Particle sizes vary from dust-like to
angular pieces several millimetres across.
Treatment: We used a 6mm (¼in) sieve to
remove larger particles, and added mediumgrade
vermiculite to the compost at a ratio of
1:4 by volume. The mix was lighter, flowed
easily between the fingers and water drained
through quickly when containers were
wetted-up prior to sowing. Green wastebased
composts can be nutrient rich, which
may be detrimental to germinating seeds;
vermiculite helps to buffer the compost by
holding on to some plant nutrients.
Germination results: Medium and fine seed
germinated excellently and transferred
to new compost with ease.
3
Sieve: Pass through a medium (about
6mm / ¼in) mesh to remove large particles
that can create voids. These can
reduce surface contact and moisture uptake in
finer seeds and inhibit root growth in cuttings.
Peat-free propagation
Sowing seed with peat-free growing media
Vermiculite is the most useful additive for mixing in at different proportions. It improves aeration and drainage in any
multipurpose peat-free compost and helps achieve the desired texture for sowing. In the Wisley test, seed germination
(Begonia semperflorens, below) was successful in all three types of peat-free composts but the rate of growth varied.
Wood fibre
+ vermiculite
Overview: Wood-based growing media may
include stripped bark, pulverised wood,
sawdust or even paper waste. There are
identifiable pieces of wood and bark in the
mix, making it lightweight and free draining.
Treatment: We removed a high proportion of
the larger particles with a 6mm (¼in) sieve and
mixed in one part medium-grade vermiculite
to four parts compost to help retain moisture.
The final seed bed was acceptable even
though fine slivers of wood were visible.
Germination results: Fine and medium seed
germinated freely but development was
slower. This may be attributed to a lack of
nitrogen, possibly depleted as the wood
products continue to decay. Applying a
dilute liquid fertiliser after germination
may have helped.
»
4
Mix in additives: Add vermiculite, perlite
or grit to replace the volume of coarse
material removed by sieving or aim to
add about 15 percent. This maintains or
improves drainage and moisture retention.
February 2012 | The Garden 45

Peat-free propagation
Adapting peat-free for cuttings
An ideal rooting medium for cuttings (here Fuchsia) needs to be open and
free draining, to balance air and moisture supplies for developing roots –
different ratios of perlite (or grit) can be used to achieve this. From this
demonstration, it appears removing large particles and adding perlite helps.
Coir + grit
Coir can appear dry on the surface when still
wet lower down; we added 3mm (⅛in) grit at
a ratio of 1:8 by volume to improve drainage.
Result: All the cuttings had produced
roots within 18 days.
Green waste + grit
We passed the growing medium through a
6mm (¼in) sieve, then added one part 3mm
(⅛in) grit to eight parts compost by volume.
Result: In 18 days, the cuttings had developed
a robust, expanding root system.
Wood fibre + grit
We adjusted the open texture by sieving to
remove coarser particles, then replaced with
3mm (⅛in) grit at 1:8 parts by volume.
Result: After 18 days rooting was adequate
but roots appeared quite fine and fragile.
Coir + perlite
We added one part medium-grade perlite
to 10 parts compost by volume.
Result: Cuttings had produced roots within
18 days, with more extensive root systems
than those raised in the coir and grit mix.
Green waste + perlite
After sieving the green waste-based compost,
we added medium-grade perlite at 1:8
parts by volume.
Result: Root growth after 18 days was similar
but top growth was more developed.
Wood fibre + perlite
Once the compost had been through a 6mm
(¼in) sieve, we added perlite at a ratio of 1:10
to improve the moisture and air balance.
Results: Rooting was similar to that in the
wood fibre and grit mix after 18 days.
Vermiculite, perlite
or grit: which to
choose?
Vermiculite is a naturally occurring
mica mineral that is mined, heated and
crushed to create multi-laminated
particles, a high-energy process. It is
often added to growing media for seed
sowing because it maintains air spaces
to assist drainage. The laminated
particles create a large surface area
on which moisture can be trapped,
and it helps retain nutrients.
Available in coarse, medium and
fine grades, vermiculite is valuable as
a covering after sowing: it insulates
the seeds, keeps them in contact
with the moist compost and does
not impede emergence.
Perlite is derived from igneous rock
which is mined, then heated to
extreme temperatures, requiring a
high-energy input. As trapped
moisture is driven out it expands to
many times its original volume,
creating a lightweight product which
is then processed into granular
particles in various sizes. Perlite does
not have the moisture- and nutrientholding
capacity of vermiculite; it is
added to growing media to improve
drainage and aeration.
Horticultural grit derives from
aggregate (such as granite), which is
washed and lime free (so does not
raise the compost pH). It improves
drainage and adds weight to the
compost. Useful sizes for adding to
growing media range from 3–6mm
(⅛–¼in); they often have a proportion
of smaller particles in the mix.
More from the RHS For more
comparisons of seeds sown in different
composts, see The Garden, Jan 2011,
pp51–56; read the feature online via
‘Issues’ at www.rhs.org.uk/thegarden
46 The Garden | February 2012