Vol. 54—2000 - NorthEastern Weed Science Society

CLOPYRALID EFFECTS ON FIELD-GROWN WOODY ORNAMENTALS
Todd L. Mervosh and John F. AhrensI
ABSTRACT
Five deciduous ornamental shrubs wereexamined fur their tolerances to clopyralid,
sprayed over the top of plants on April 8, 1999. The shrubs (and the condition of their budson
April 8) were the following: redosier dogwood [Comus sericea 'Red Twig') (buds tight); dwarf
burning bush [Euonymus a/atus 'Compacta') (buds Yoto Yoinch long); forsythia [Forsythia x
intermedia 'Lynwood Gold') (bods Yoto Yoincb long); panicle hydrangea [Hydrangea paniculata
'PeeGee') (buds tight); and weigela [Weige/aj/orido 'Java Red') (buds tight). The shrubs were
planted in 1998 in a sandy loam soil, were sheared I to 2 weeks prior to clopyralid application,
and were all less than 6 inches tall at the time of treatment. Clopyralid was applied in a volume
ono gallA at l1IIes of 0.125, 0.25, or O.5Ih1Aa.i, The primary weed was horseweed [Conyza
eaoadensis (L.) Cronq.), the rosettes of which hada diameter of I to 2 inches.
All rates of clopyralid completely controlled horseweed. Plant injury (0 = no injury; 10 =
dead) was evaluated on June 4 and July 14, and plant vigor ratings were taken on August 31 (0 =
dead;10 = excellent vigor). Weigela and dwarfhoming bosh were highly sensitive to clopyralid,
which, at all application rates, caused substantial injury (ratings on.o or higher) to these two
shrubs. Clopyralid at 0.5 IhIA nearly killed most of the weigela plants. Plants less severely
injured displayed leaf curling or cupping. Panicle hydrangea was intermediate in susceptibility
to clopyralid. Injury ratings fur hydrangea ranged from 1.5 to 5.0 depending on clopyralid rate,
hot plants treated with 0.125 or 0.251h1Arates recovered to have vigor ratingsofS.5 or higher
by the end of August. Redosier dogwood and forsythia were !he most tolerant shrubs to
clopyralid. Injury ratings did not exceed 2.0 for these plants treated with clopyralid at 0.125 or
0.25 IhIA, and plant vigor ratings were at least8.25 fur all treatments.
Because clopyralid was highly active on seedling horseweed at even the O.l251h1Arate,
it would he of interest to evaluate tolerances of dormant shrubs at even lower clopyralid dossges.
'Assistant Scientist and Weed Scientist Emeritus, The Connecticut Agricuhurai Experiment
Station, Valley Lahoratory, Windsor, CT 06095.
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