Controlling Weeds in Nursery and Landscape Plantings

Bulletin 867

Causes Of Herbicide Injury

If selected and applied properly, the herbicides listed in this publication can be safely used in and around a wide variety of ornamental plants. However, herbicide injury may occur if one of the following mistakes is made:

1. An herbicide is applied to a plant that is sensitive to it. Gallery is safe for use over the top of many ornamentals but for some unknown reason severely injures winged euonymus (Euonymus alatus), which is very sensitive.

2. An herbicide is applied at too high a rate. This can be due to the selection of an excessive rate, a mistake in application caused by poor calibration of application equipment, or uneven distribution of the herbicide during application. Princep (simazine) provides safe, excellent, low-cost weed control when applied at the proper rate, but it will injure or kill many ornamentals if applied at high rates.

3. An improper application is made. Some oil-based herbicides are safe to use over the top of plants prior to budbreak, but may injure the foliage if applied during active growth. Pennant can be applied over white pine in early spring, but if applied as the new growth is expanding, it will cause considerable injury to the new growth.

4. A nonselective postemergence herbicide is applied over the trunk of young trees with thin bark. Finale, Reward, and RoundupPro can be freely used around the base of trees with mature bark but must be used with caution around the base of trees with young, thin bark. Finale and Reward can cause sunken cankers where applied, while Roundup Pro may cause slight to severe bark splitting.