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Perennial Weed Control in Hot, Dry Weather (06/24/21)

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I have been receiving several questions regarding control of noxious weeds during this year’s drought. Many are concerned about both injuring drought-stressed grass, as well as getting poor weed control due to hardened-off weeds. Since many folks are comparing this year to 1988 and 1989, I thought it would be good to take a look at Dr. Rod Lym’s research data from those years to determine the best course of action. Luckily, Dr Lym wrote an article for Crop & Pest in August 2017 addressing this same issue. No need for me to reinvent the wheel, so here are Dr. Lym’s words about perennial weed control in 1988:

QUESTION: I am wondering about continuing to treat leafy spurge in the dry conditions we are experiencing this year. The leafy spurge plants I have treated with Tordon do not look like they are dying. Should I continue to treat, use a different chemical, or just stop until we get some rain?

ANSWER: While herbicides are typically not as effective when plants are under moisture stress, I think it is better to continue to treat leafy spurge rather than stop. My reasoning comes from the lessons learned during the drought in 1988 (that was the year Yellowstone burned). The entire state was very dry and temperatures were often in the 90’s and even over 100 with very little rain. No one really knew if they should continue to spray weeds or just stop and save the money. In my own research program, we stopped the field work in July and moved to lab and greenhouse research because the plants in the field did not appear to respond to any treatment. Some counties continued their treatment program and others stopped applications. What did we learn?

The following spring of 1989 the counties that quit treating leafy spurge had large increases in acres infested. Since the grass and most other species also stopped growing, it appeared the leafy spurge roots with no competition continued to spread. I remember leafy spurge was just about the only green plant in the Badlands. The counties that continued to treat (using Tordon + 2,4-D) had less than average control, but the leafy spurge did not spread and infestations were similar in size to 1988. Tordon has fairly long soil residual, so even if the chemical is not absorbed or translocated immediately after application, the herbicide will still reduce regrowth following moisture. Based on this experience I recommend land managers continue to treat leafy spurge, but if possible, wait until the temperatures cool and perhaps rain lessens the drought. The lesson from 1988 is: Do not stop the treatment program!

Joe Ikley
Extension Weed Specialist