Attack weeds this fall to reduce next year’s impact

By Bonnie Orr
WSU Chelan/Douglas County Master Gardener

Bonnie Orr
Bonnie Orr – WSU Extension Chelan/Douglas County Master Gardener – photo by Don Seabrook, Wenatchee World

It is nearly Labor Day, which means it’s time for the county fairs and time to organize our fall garden tasks.

We think about planting trees and shrubs in the fall and harvesting our garden’s bounty. One of the tasks we often do not think about in the fall is weeding because we are just so tired of dealing with weeds and wish they would go away.

Three to four inches of mulch makes an effective germination barrier for annual weeds.

Different strategies are needed to beat back perennial weeds. Fall is an optimum time to kill perennial weeds rather than just pulling them out to keep them under control. The reason that fall is the time to deal with these weeds is that the plants are still growing vigorously. They have large root systems, often with storage roots to sustain plants through winter.

It is nearly impossible to dig out these root systems. I have tried and succeeded with morning glory, but it was hours of work and took three years! Most of us have better things to do with our time. Herbicides and patience to the rescue!

An herbicide is drawn down into the storage roots and weakens the perennial weed and eventually kills it.

So this is the perfect time to use an herbicide of your choice. First, read the instructions on the package and make sure it is labeled to kill your target plant. Then decide on the delivery of the herbicide to be sure you do not damage desirable plants. The WSU Master Gardeners can help you make decisions about herbicide, or you can visit the website PICOL: Pesticide Information Center OnLine, It is a label database that helps you target a weed and select an herbicide that kills it. WSU Hortsense will also recommend weed control.

After you have applied the pesticide, check in a week to 10 days to see if another application is needed to knock back the growth. If the weeds have been persistent for several years, it may take more than one season to eliminate the plant. That means that herbicide is applied in the fall, the next spring and the succeeding fall before you notice a major diminishment of the plant.

The most pernicious garden weeds are horsetail (Equisetum arvense), field bindweed or wild morning glory (Convolvulus arvensis), quack grass and dandelions. All of these weeds respond to herbicide applications. But the product must be very carefully applied to prevent damaging desirable plants. Be conscious of heat over 80 degrees that can cause the product to volatilize — that is, turn into a gas — and affect nearby plants. Also, even a breath of breeze can move the product to nearby plants.

Other Washington state-listed noxious weeds on uncultivated and disturbed lands — such as Canada thistle, Dalmatian toadflax and Kochia — should be cut back to eliminate their seed heads (which need to go in the trash). Then treat the remaining stems and leaves with herbicide.

You can continue to treat perennial weeds until a frost kills the tops or until the ground temperature is 50 F, at which point most plants stop growing.

A word about annuals: Don’t forget to deal with real “baddies” such as Russian thistle and the knapweeds before they distribute their seeds. These are annuals or biennials, and you can begin to control them by cutting the weed to the ground and stuffing the stems and seeds into the trash. Then in the spring, grubbing out the new plants in the roseate stage will eliminate the weed’s new generation.

Look forward to a more weed-free life next year.