With noxious weeds, it’s OK to be a control freak

There is a weed war raging around our gardens at the moment, but it isn’t the war against the usual dandelions, crabgrass or chickweed. This war involves plant thugs that make our ordinary garden weeds look like wimps. Thugs with names like knapweed, rush skeletonweed, thistle and a few dozen others that are termed noxious weeds.

By legal definition in Washington, a noxious weed is a non-native plant that has been introduced to the state by human action. The state of Idaho defines a noxious weed as a plant that a government agency determines as injurious to public health, agriculture, recreation, wildlife or property value.

In Washington, noxious weed control is under the purview of the state-level Noxious Weed Control Board and county weed boards. In Idaho, the Idaho State Department of Agriculture oversees the Noxious Weed Program and the county level Noxious Weed Management Supervisors.

In both states, these entities establish the lists of plants considered noxious weeds and have the legal authority to establish and enforce their control or management. The enforcement of weed controls is graduated depending on the class the weed falls in and the severity of the infestation. According to Dave Mundt, coordinator of the Spokane County Weed Board, the ultimate responsibility for the control of noxious weeds does lie with the landowner of the property containing the weeds.

There are 125 plants broken into three classes on the Washington list and 36 on the Idaho list. “Of the 125 on the Washington list, about 25 are of serious concern here in Spokane County,” says Mundt. He considers five weeds in particular to be at the top of the problem list for Spokane County. They include knapweed, Canada thistle, rush skeletonweed, common bugloss and Dalmatian toadflax.

Noxious weeds take over because they out compete native plants or make an agricultural crop unusable in some way. Canada thistle has spines that keep animals from eating it and hairy leaves that can keep herbicide laced water from coming into contact with the leaf’s surface. When the herbicide can’t reach the leaf surface, it can’t act on the plant.

Knapweed and Dalmatian toadflax disrupt nutrient use and availability in native plants. Recent research on knapweed is finding that the plant has the ability to steal carbon from other plants thus disrupting their ability to function and produce food. This weakens the native plants and allows the knapweed to move in.

In the case of Dalmatian toadflax, research is indicating that where there are a significant number of plants, there is a reduction in the amount of grass cover found in the same area. The toadflax is altering the environment in some way and the grass cannot compete.

Knapweed and some other weeds have the ability to regenerate flowers and seed heads if the first crop is eliminated. In the case of knapweed, if it is mowed off, it simply grows another set of flowers and seed heads conveniently slightly lower than the mower blade.

Controlling noxious weeds can be a challenge. Nonchemical methods such as cultivation can actually increase the problem by bringing fresh seed to the surface where it germinates readily.

Hand pulling can work for some small patches but may not be practical for large infestations. With some weeds, like rush skeletonweed, pulling the main plant breaks the roots and stimulates the growth of a new plant at the end of each broken root piece. Where you had only one plant before, now you have dozens.

Research into the biological control of weeds is receiving increasing attention. Many of our worst noxious weeds are a problem here because in their native habitat in Asia or Europe, there were insects and diseases that kept them in check. When they were placed in our environment, they had no enemies and just went wild.

While the idea of introducing biological controls seems like a simple answer, it can have just as many negative consequences as the initial introduction of the weed created. A bug that may control the weed may also attack other desirable plants or upset the balance of other elements of the ecosystem. Painstaking research has to be done to make sure that they know what will happen when the bug is released into the environment.

Chemical control of noxious weeds may be the only viable option to get an infestation under control. Once it is under control, other non-chemical management can be used quite effectively. The use of chemical controls should only be undertaken after the weed in question has been properly identified and the appropriate chemical is selected and applied according to the directions. Improper application of chemicals can cause considerable environmental damage in its own right.

For more information on the management, identification and control options for noxious weeds, contact your county weed authority.