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Herbicides are a valuable garden tool, but their use and misuse constitutes a serious threat to desirable plants. Each week at our diagnostic clinic, we see numerous samples from well-meaning gardeners, who through misuse of chemicals cause severe damage to their own plants.

The word herbicide itself means "to kill the plant." Herbicides don't distinguish between good plants and bad plants, and weedkillers, as such, do not exist. If someone truly invented a weedkiller that you could spray over the entire yard and only weeds would die, he would be a millionaire many times over. Herbicides rely on the user and selective application techniques to control undesirable plants. Dozens of different weedkillers are on the market and are divided into different categories based on chemical composition and use.Pre-emergent herbicides are applied prior to seed germination and kill sprouting plants. These products are only effective if applied before plants start growing and usually don't cause serious problems if used according to label directions.

Weedkillers generating the most complaints are lawn weedkillers and long-term vegetation controls, or soil sterilants. Lawn weedkillers contain several different ingredients with similar modes of action, such as 2,4-D, MCPP and dicamba. These products selectively kill broad-leaved plants and when used at correct rates do not kill turf grass. Product guidelines generally prohibit use when temperatures exceed 85 degrees Fahrenheit, in windy conditions or underneath the drip of trees or around sensitive trees. Lawn weedkillers cause twisting or cupping or make parallel vein patterns on the leaves. Tomatoes, grapes, beans and roses are extremely sensitive and severely damaged by exposure to lawn weedkillers, but all broad-leaved plants can be damaged by these chemicals.

Another major problem is with long-term vegetation controls, or soil sterilants. These products include the triazine herbicides, such as pramitol, atrazine and others. Soil sterilants, as they are commonly called, represent a serious hazard to desirable vegetation. I am familiar with one landscape where this product was misused 14 years ago and the trees still show damage. The herbicide damage shows as a yellow leaf with green veins. The damage appears similar to iron chlorosis except the veins look as though they have been painted in with a broad marker or paintbrush rather than the narrow veins that show with iron chlorosis.

Damage can be severe or fatal and often affects trees and shrubs years after it was originally applied. Plants are severely damaged when the roots grow into treated soil and pick up the herbicide. Tree roots extend 21/2 times the height of the tree, so a 50-foot tree could pick up the herbicide from 125 feet away.

Triazine herbicides are often used underneath asphalt to prevent weed growth. Roots grow under the pavement and pick up the chemical. Once the product is in the soil and plants are damaged by the herbicide, there is no effective antidote for the poisoning. Never use these products, because the potential risk is far too great to outweigh any benefits.

Roundup can also severely damage desirable vegetation. It is less hazardous because it does not travel through the soil, nor does it persist after application. As long as it is kept off desirable foliage, the risks to plants are minimal.

Herbicides are an effective garden tool to reduce unwanted vegetation and save time in the garden. Use them improperly and desirable plants are the victims. My garden has enough troubles with the pests and problems that Mother Nature provides without me adding others by misusing herbicides. Carefully read and follow all label directions as you use herbicides and avoid using soil sterilants. Your reward: healthier plants, better production and fewer frustrations from preventable damage.

- Lawn water use this week is 1.75 inches.

If you are following integrated pest management techniques outlined for controlling codling moth on apples and pear trees, apply a second spray of Imidan this week. Pick up and destroy any fruits that fall to the ground.

Spray for peach tree borer now. Use Dursban and spray only the trunk. Do not get sprays on the fruit or foliage.

- Thursday garden get-together: Bugs, slugs and other things, Red Butte Gardens and Arboretum, Thursday, July 8, 12 noon at the amphitheater. Jerry Goodspeed, Larry Sagers and specialists from Utah State University will diagnose any problems you bring to the seminar.