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The Garden Plot for June 20, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Herbicide Injury”
Distorted, puckered, and curling leaves on garden and landscape plants can be caused by herbicides used to control weeds. We see quite a bit of herbicide injury to plants in the Missoula County Extension Plant Clinic. It takes a little detective work to diagnose herbicide injury. First, it’s important to know how the injured plant should normally appear. For example, certain herbicides produce leaf symptoms that look like the variegation common to some cultivars. Next you need to find out what herbicides have been sprayed.
There are a few hints for detecting herbicide damage. Yellowish or dark spots on leaves can be caused by disease or, if found on leaves of the same age, can indicate spray drift from a contact herbicide. On the other hand, discolored areas between leaf veins and brown leaf margins may mean that an herbicide which can move inside the plant has been applied. Symptoms often begin on the same side of the tree or shrub where the herbicide was sprayed.
The following is a list of the symptoms associated with commonly applied herbicides.:
Roundup (glyphosate) can cause an overall yellowing of leaves, especially on new growth. Roundup may also cause delayed leafing in the spring and small, distorted leaves. Roundup injury usually occurs as a result of spray drift during application.
2,4-D causes twisted, cupped leaves, distorted and stunted fan-shaped leaves, prominent leaf veins, and splitting or cracking of bark. 2,4-D can be absorbed by the roots of shrubs and trees.
Dicamba and triclopyr are also absorbed by the roots of shrubs and trees. These herbicides cause cupped, distorted leaves, delayed leafing in the spring, and splitting or cracking of bark.
If you suspect herbicide injury, you can add organic matter, such as compost, to the base of the injured plant. Incorporate the compost into the soil carefully to avoid disturbing roots. Soluble herbicides may be removed from the soil by leaching through with irrigation water. Otherwise, be patient and keep herbicide-injured plants well-watered. Most plants outgrow minor herbicide injury.
Sometimes we see herbicide injury to garden plants and no herbicide has been sprayed. Manure from animals that have grazed on, or been fed, hay from grassland treated with certain herbicides may contain enough residual to inhibit plant growth. While herbicides that break down rapidly in the soil should not be a problem, others such as Picloram (Tordon) can be problematic. Picloram is a persistent herbicide that breaks down very slowly in the soil. Picloram in manure applied to gardens and landscapes can cause severe stunting, abnormal growth, and death to broadleaf plants. Particularly susceptible plants include potatoes, tomatoes and bell peppers. Picloram may persist in manure and in soils for several years.
For more information on herbicide injury, contact your local county extension agent.
Helen Atthowe's new short program of gardening tips
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