Herbicide Treatment of Container-grown ‘Gloria’ Azaleas and ‘Merritt Supreme’ Hydrangeas

Abstract

Abstract Dormant budded plants of ‘Gloria’ azaleas (Rhododendron obtusum, Planch) and ‘Merritt Supreme’ hydrangeas [Hydrangea macrophylla (Thunb.) Ser.], and vegetative summer stock of ‘Gloria’ azaleas, were treated with various rates of chlorimuron, imazaquin, oxyfluorfen, and chloramben. Chlorimuron minimally injured both greenhouse crop species, but 0.14 kg a.i./ha was necessary to provide >70% preemergence control of the broadleaf weed species in peat-based or bark-based soilless media. Large crabgrass [Digitaria sanquinalis (L.) Scop.], redroot pigweed (Amaranthus retroflexus L.), Pennsylvania bittercress (Cardamine pensylvanica Muhl. ex. Willd.), common chickweed [Stellaria media (L.) Vill.], yellow woodsorrel (Oxalis stricta L.), and creeping woodsorrel (Oxalis corniculata L.) were the weed species tested. Imazaquin provided excellent control of all weed species at 0.56 kg a.i./ha for 14 weeks, but was phytotoxic to vegetative azaleas in both growing media. No difference in damage was detected when applications were made immediately after repotting and pinching or after 1 month of growth. Oxyfluorfen provided excellent control of all weed species for 4 weeks at 2.2 kg a.i./ha and, on all species except chickweed, for 14 weeks at 4.5 kg a.i./ha with no phytotoxicity to azaleas or hydrangeas for media drench applications. Chloramben drench and foliar spray applications at 6.7 kg a.i./ha injured vegetative azaleas regardless of application time. Foliar and drench applications of chloramben at 3.4 kg a.i./ha produced slight to moderate injury on vegetative azaleas. More injury resulted when applications were delayed until 4 weeks after pinching. Chloramben was not toxic to either dormant-budded species. Chemical names used: 2-((4[4-chloro-6-methoxypyrimidine-2yl)amino carbonvl]amino sulfonyl))benzoic acid, ethyl ester (chlorimuron); 2-[4,5-dihydro-4-methyl-4-(1-methyl-ethyl)-5-oxo-1H-imidazol-2yl]-3-quinolinecarboxylic acid (imazaquin); (2-chIoro-l-(3-ethoxy-4-nitrophenoxy)-4-(trifIuoromethyl) benzene (oxyfluorfen); 3-amino-2,5-dichloro-benzoic acid (chloramben).