By Ruth Ann Hales, Colorado State University Cooperative Extension
Summer is the time for weeds. And with weeds comes various strategies for controlling these invasive plants. In my previous column I extolled the virtues of mulch in order to control weeds in your garden. However, instead of mulching the garden many homeowners choose to use various herbicides. Herbicides, when used in moderation and according to label directions can successfully help control weeds. Unfortunately, many home owners rely exclusively on herbicides and this, itself, creates problems during hot summer days.
Over the last ten days, several different plant samples showing signs of herbicide injury have been brought to the extension office. The growing tips of these plants are most often twisting and curling in an irregular pattern. This twisting is most characteristic of injury caused by a herbicide known as 2,4-D. 2,4-D is sold in most dandelion control products. Plants that have been showing this injury are tomatoes, strawberries, geraniums, Blue Mist spirea, daisies, phlox, and poppies. However, any broad leaf plant can be affected by this herbicide.
There are many different formulations of dandelion control products. And some formulations are more volatile than others. Volatility simply means that when temperatures get to a certain degree the liquid will evaporate and form a vapor. This vapor, even though you and I can't see it is still there and can harm plants.
As the air currents and wind move around, so does this invisible vapor. When the herbicide comes into contact with a broad leafed plant, the plant absorbs it through its leaves and stems and is injured.
Many forms of 2,4-D volatilize above 85° F. This means spraying weeds on a hot summer day will create vapor clouds that can harm nontargeted, innocent plants. Spraying weeds in the cool of the morning may also not be effective. Herbicide that is not absorbed by the weed as outside temperatures reach 85° F will form a vapor and can injure nearby plants.
To reduce 2,4-D drift injury, reduce the use of this herbicide during the hottest summer months. If you must use this herbicide, apply it during cool evening hours and follow the label directions carefully. The best time to apply 2,4-D is earlier in the spring when dandelions are up and growing and temperatures are still cool.
Some plants injured by 2,4-D drift will struggle with injuries but survive. Others eventually die back completely. Vegetable gardens affected by 2,4-D drift may struggle, survive and produce fruit. Produce from these gardens should not be eaten.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010