Revegetation Guidelines for Western Montana - Global Restoration ...

Mowing and weeds
Besides affecting competitive vigor of desired vegetation – resulting in high-maintenance, unhealthy
roadsides that are prone noxious and nuisance weed invasions, erosion, and repeated herbicide
applications – improper timing of mechanical mowing can also facilitate spread of noxious weeds.
This can occur when roadside maintenance crews mow roadside weeds, usually with flail mowers,
after the weed seeds have matured. Similarly, many maintenance crews, more out of habit than
proven need, mow healthy roadside communities before seed maturation. This inhibits desired
plant seed dispersal for next year’s stand and the flail mowers expose the soil for the weed seed,
providing a competitive advantage for the weeds, cultivating even more weeds to spray in the
future. Activities that give weeds an opportunity to spread must be avoided and prevented (Callicot
and Lore 2000).
However, properly timed mechanical mowing can be an effective weed management tool by
favoring desired plant growth and decreasing competitive vigor of weeds. Proper timing of mowing
is based primarily on the growth stage of the weeds and secondarily on the growth stage of the
desired plants (Sheley et al. 2001). The most effective time to mow noxious weeds is when the
desired plants are dormant and the weeds have reached the flowering stage, well before seed
production. Mowing during this time can encourage unrestricted growth and seed production of
desired plants and weaken the weeds while preventing seed production. Long-term, repeated
mowing of weeds after they have invested a large amount of energy for bolting (when the stem
extends from the center of the rosette upwards two to four feet) and producing reproductive
structures can eventually deplete root reserves and weaken the infestation. If regrowth bolts again
and produces flowers, an additional mowing will be necessary for mowing to be effective (Sheley et
al. 2001).
When the dominant vegetation is a noxious weed, mow two inches high when the weed is between
the early bud to early flowering stage. However, in some cases, noxious weeds will reach the
appropriate stage for mowing but the grasses have not reached dormancy. If so, mow the weeds at
a height above the desired plants. Mowing above the height of actively growing grasses allows
continued vigor and defoliating the weeds reduces seed production and vigor, increasing resources
available for neighboring grasses (Sheley et al. 2001).
Carefully timed roadside mowing may reduce the seed bank of noxious weeds (Tyser et al. 1998)
and in a Montana State University study, mowing as a single management tool decreased spotted
knapweed density by 85 percent (Rinella et al. 2001). A further reduction in density would be
anticipated when integrated with a herbicide treatment applied to the rapidly developing regrowth,
one month after mowing. Consider mowing and applying a herbicide in a single entry with a wet
blade mower. This mower has a blade that cuts the plants while also applying a herbicide.
Cavitation pulls the herbicide into the stem where it then moves into the vascular system of the
plant. The advantages of wet blade mowing include reduced herbicide rates, run-off, and drift
because the blade precisely places the herbicide only on the stems of the cut plants. Excellent results
have been documented with many noxious weeds including Canada thistle (Cirsium arvense),
Dalmatian toadflax (Linaria genistifolia ssp. dalmatica), leafy spurge (Euphorbia esula), Russian
knapweed (Centaurea repens), and salt cedar (Tamarix spp.).
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