Preemergence Weed Control In Container-Grown Herbaceous Perennials

During the 2008 season, three preemergence herbicides were applied to two container grown herbaceous perennials and one woody plant and evaluated for weed control, phytotoxicity, and effect on plant growth.  The herbicides and rates were:  BAS 659H G (dimethenamid + pendimethalin) 2.97, 5.94 and 11.88 kg ai/Ha; BAS 656H EC (dimethenamid)  1.09, 2.18 and 4.36 ai kg/Ha; Mesotrione (Mesotrione EC) 2.35, 4.70 and 7.05 kg ai/Ha; Broadstar 0.25G (flumioxazin) 0.42, 0.84 and 1.68 kg ai/Ha; V-10142G 0.84, 1.68 and 3.36 kg ai/Ha.  Herbicides were applied to LITTLE TRUDY^® Catmint (Nepeta ‘Psfike’^PP18,904), MONGOLIAN BELLS^® Clematis (Clematis integrifolia) and FROSTY ^® Potentilla (Potentilla fruticosa ‘Monsidh’).  Treatments were applied twice with six weeks between applications.  The entire experiment was repeated twice with two weeks between the experiments.  Plants were evaluated for phytotoxicity after one week, two weeks and four weeks after applying herbicide treatments.  In this study, BAS 656H EC was safe to use with Clematis and Potentilla at the recommended rate.  BAS 656H EC should only be used on Nepeta after it is established. BAS 659H G and Broadstar 0.25G were safe to use on all species tested in this study at the recommended label rate.  Mesotrione was not acceptable for Clematis or Potentilla and marginal for Nepeta.  V-10142 would not be recommended at any rate for any of the species tested.  Generally, effective weed control was provided by BAS 659H G, BAS 656H EC, Broadstar and Mesotrione. For treatments with V-10142, weed efficacy (grasses in particular) was significantly less at the 1x and 2x rates.