Let’s start off by playing a guessing game for this week’s column. Question: can you guess what the number one problem is that we have been seeing over the last few weeks at the Johnson County Extension Gardening Hotline?
Here’s a clue: it is not the result of insects or disease. This problem is caused by man.
Got it yet? The answer is the misuse of common lawn herbicides. Drift and over-spray from products used to control broadleaf weeds have a detrimental effect on many landscape plants. For some reason we are seeing more than I can remember this summer.
Leaves affected by herbicide drift are normally cupped and thickened or leathery, and develop an uncharacteristic fan shape. Severely distorted leaves have a mosaic pattern of light and dark green areas. The petioles, or leaf stems are twisted in a curlicue fashion and may even develop small light colored bumps. Common landscape plants most often affected include tomatoes, grapes and redbud. Damage has also been seen on just about every species of trees and shrubs planted in the landscape.
Many gardeners often mistake these symptoms for a disease or insect problem, but the damage is almost always caused by the exposure to broadleaf herbicides. Plant growth regulating herbicides are commonly applied in home lawns early in the season to control dandelions, henbit and other broadleaf weeds. If misapplied and accidentally sprayed on sensitive plants, they can cause severe injury or even death.
Unfortunately these plants do not have to be sprayed directly with herbicides to cause damage. Some broadleaf herbicides such as 2, 4-D are volatile, especially during hot weather, and may drift across the yard or even from adjacent yards. Therefore, you do not necessarily have to be using the product in your yard to suffer damage. 2,4-D is one of the most common consumer herbicides on the market and can be found in most products.
Herbicides may be introduced into the garden by methods other than direct application or drift. Grass clippings collected from a recent herbicide treated lawn and used as mulch around tomatoes may result in damage.
To reduce the chances of herbicide injury, avoid applying them near the vegetable garden and trees and shrubs. Apply the products during calm mornings and cool temperatures. If using grass clippings as a mulch or soil amendment make sure they are herbicide free. Allow the clippings to fall to the turf for two or three mowings after application before catching. Composting herbicide residual grass clippings for a couple of months are sufficient for reducing problems.
The results of the herbicide drift vary depending on the amount of damage. Severely stunted or distorted plants may not die, but often to do not produce well. Many plants will simply grow through the problem as the season progresses.
One of the best ways to avoid spring herbicide damage is to treat the lawn in the fall for prevention of early season weeds. When the chemicals are applied in the fall, mid-October through early November, the drift is reduced as the air temperatures are normally lower. Also, plants at that time of the year have pretty much shut down for the season and are not experiencing periods of rapid growth. Basically they are not affected by the drift vapors. Fall is the ideal time to control spring weeds.
- Dennis Patton