Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

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EVALUATION OF GRANULAR HERBICIDES IN CONTAINER-GROWN WOODY
ORNAMENTALS. S. Barolli, Imperial Nurseries, Granby, CT and J. Ahrens,
Connecticut Agricultural Exp. Station, Windsor.
ABSTRACT
Seven granular herbicide products were evaluated in 2006 for efficacy and
tolerance by seven woody ornamentals. The ornamentals included hydrangea
(Hydrangea macrophylla ‘Endless Summer’), dwarf burning bush (Euonymus alatus
‘Compacta’), rose (Rosa ‘Carefree Marvel), Snowstorm Spirea (Spiraea hybr.
‘Snowstorm’), Tinkerbelle lilac (Syringa hybr. ‘Bailbelle’), weigela (Weigela florida ‘My
Monet’ and butterfly bush (Buddleia davidii ‘Black Knight’). All plants were transplanted
into 1-gallon pots on 6/19/06 with media consisting of 70% pine bark, 15% peat and
15% sand by volume, and were actively growing at treatment on 6/21/06. Three pots of
each plant and six plantless pots were included in each plot and the herbicide
treatments and untreated control were arranged in randomized complete blocks with
four replications. Large crabgrass (Digitaria sanguinalis L. Scop) and common
groundsel (Senecio vulgaris L.) were seeded only in the plantless pots on 6/21/06, and
were pulled and counted on 7/19 and 8/18.
The herbicides included OHP-31906 at 100, 200 and 400 lb product/A,
oxyfluorfen 2% + pendimethalin 1% (OH-2) at 100 lb/A, isoxaben 0.5% + trifluralin 2%
(Snapshot 2.5TG) at 150 lb/A oxyfluorfen 2% + trifluralin 3.0% (Weedfree 75) at 100
and 200 lb/A, flumioxazin 0.25% (BroadStar) at 150 lb/A, trifluralin 2% + isoxaben
0.25% + oxyfluorfen 0.25% (Showcase) at 100 and 200 lb/A and oxadiazon 2%
(Ronstar 2G) + napropamide 10% (Devrinol 10G) at 200 + 40 lb/A. The herbicides were
applied with a calibrated auger-feed granular applicator on 6/21/06 over dry plant foliage
and reapplied on 9/7/06. On 9/8/06 annual bluegrass (Poa annua L.) and common
groundsel were seeded into the plantless pots. Overhead irrigation for 25 minutes was
applied starting within 10 minutes of the last herbicide (oxadiazon).
Reduction in weed counts and % control based on numbers varied with herbicide
and dosage (Table 1). Weed population were so high in controls on 7/19 (4WAT-1) that
pulling and counting removed soil, as well as weed seeds, resulting in low populations
in the controls thereafter.
Hydrangea was the only species injured, and the only species that trapped
granules in whorls. All herbicide treatments injured the hydrangea (Table 1.) and this
injury was reflected in plant vigor reductions. The lowest rate of Weedfree 75,
Showcase and OH-2 caused the least injury. The Ronstar + Devrinol combination which
has been the standard for hydrangea at this nursery, caused more injury than in
practice, probably because in practice granules are brushed off the leaves before
irrigation and this was not done in this experiment. Following the August ratings the
ornamentals were pruned and retreated. None of the treatments injured the mature
hydrangea foliage following the second application.
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