Ask an Expert! Answers to Landowner Management Questions

Question:

I have knapweed and rush skeletonweed on my property (which isn’t huge, 80 acres, and the infestation isn’t enormous, just enough that it’s an issue for me). I spray, re-spray and re-re-spray, using 2,4-D and Milestone at 2 oz and 3/8 oz, respectively, per gallon, the dose the county weed guy recommends. The weeds never seem to die. I don’t think I’m wrong when it seems like I treat the same weeds year after year.

Do I increase the amount of chemical? Use a different chemical? Do something radical like dig up the weed and, with a tiny artist’s brush, paint undiluted 2,4-D directly on the root? Am I doing something wrong? Thanks.

Answer: 

Weeds never seem to die.

Thanks for your question and your determination to manage a couple of troubling noxious weeds.

Do I increase the chemical? A recommendation, that is part of an integrated weed management approach, would be that you monitor your upcoming season spray application closely to observe the impacts of your treatment. The rates and materials you describe should work well on the knapweed and skeletonweed. Treatment timing is also very important for these two perennial weeds. Treatment timings should include both the spring and fall to reduce the amount of new seed production. Timing must also consider periods when soil moisture is adequate and plants are actively growing to get good movement of herbicide into the plants. Both of these weeds are known for their abundance of seed produced and the seed viability in the soil (3 years for rush skeletonweed, 8 years for spotted knapweed).

Spotted knapweed and rush skeletonweed
Spotted knapweed (left) and rush skeletonweed (right) are aggressive weeds that can easily take over disturbed areas and rangelands. Photos:  Washington State Noxious Weed Control Board.

Taking the time to identify the boundaries of the weed populations and then working through the area in a grid pattern to minimize misses is time well spent. Additional time should also be given to scout outside the area to pick up any outlying plants. After the initial treatment, return to the site in about two weeks, to spot treat any new seedlings or any plants previously missed and beginning to bolt. Treatments in the fall season to target any plant rosettes that survived through the summer is very effective in reducing these perennial weeds.

In addition, always evaluate and consider the planting of competitive native vegetation in the area of the infestation to compete with these invasive weeds. If the weed site is frequently disturbed and can’t be managed differently, the location may require some annual maintenance to keep the weeds from spreading to other areas.

Distribution maps of spotted knapweed and rush skeletonweed.
Distribution maps of spotted knapweed (left) and rush skeletonweed (right) across Washington State.  Photo: Washington State Department of Agriculture.

Given the scale of your acreage and infestation size, individual treatments of digging, cutting and daubing stems with herbicide wouldn’t be very efficient for these fast moving weeds. However digging may be appropriate for those single, outlier weeds that are found elsewhere on your acreage.

By Bill Wamsley, Noxious Weed Control Board Coordinator, Lewis County, bill.wamsley@lewiscountywa.gov