Evaluation of selected preemergence herbicides in field-grown landscape crops in Kentucky

Research was conducted during 1990 and 1991 at Lexington, to evaluate the efficacy of herbicides available for use on woody landscape plants (simazine and oryzalin), newly labelled herbicides (isoxaben, isoxaben + oryzalin, metolachlor and metolachlor + simazine) and those not widely labelled for the nursery industry (oxyfluorfen and dithiopyr). Herbicide phytotoxicity was also evaluated in 10 species of woody nursery crops. Results 6 weeks after treatment (WAT) indicated that the standard of simazine at 1.1 kg ai/ha (1.0 lb/A) + oryzalin at 3.3 kg (3.0 lb) provided adequate control of most common broadleaved weeds (98%) and annual grasses (98%) but not yellow nutsedge (31%). Isoxaben + oryzalin at 4.2 kg (4.0 lb) (DF formulation) and isoxaben + trifluralin at 4.2 kg (4.0 lb) (granular formulation) gave excellent full-season control of nearly all broadleaved and grass weeds with the exception of galinsoga (Galinsoga ciliata) and field bindweed (Convolvulus arvensis). In addition, yellow nutsedge (Cyperus esculentus) control was acceptable, ranging from 63 to 96% in these treatments. Isoxaben alone also provided excellent broadleaf control (93%) 6 WAT in 1990 but poorer (Setaria faberi), galinsoga and velvetleaf (Abutilon theophrasti) by 12 WAT. Dithiopyr (granular formulation) provided excellent full-season residual control of broadleaved weeds, nutsedge and grasses at the highest rate of 2.2 kg (2.0 lb). Oxyfluorfen at the higher rate (2.2 kg; 2.0 lb) also controlled all species except yellow nutsedge. Poor broadleaf control was obtained 12 WAT with 3.3 kg (3.0 lb) metolachlor or metolachlor + simazine at rates up to 4.4 kg (4.0 lb) in both years. Pronamide [propyzamide] at 2.2 kg (2.0 lb) did not provide adequate control of either broadleaf or grasses at 6 or 12 WAT. No visible phytotoxicity to woody ornamentals was observed in either season up to 8 weeks after application. However, growth reductions were noted in both common lilac (Syringa vulgaris) and red oak (Quercus rubra) stem or trunk diameter following treatment with isoxaben at 1.1 kg (1.0 lb)