In my last column, I referenced a couple weeds to look for during post-harvest hay field evaluations. Hemp dogbane was one of them, and deserves attention for numerous reasons.

Hemp dogbane is a perennial potentially growing to heights of three to five feet and may be confused with milkweed species due to their similar exterior appearance and the presences of milky sap when leaves are removed or stems cut (note: only fresh plants will show sap). Stems have a reddish tint at maturity and become woody at the base. The term dogbane is said to refer to the plant being poisonous to dogs. The same plant resins can also harm cattle under the right conditions.

Because of the plant’s long, horizontal rootstocks, you have not only plants, but in many cases plant colonies. Plants start as a single taproot that can grow to a depth of almost six feet plus spread laterally as much as ten feet in a single season. Its vigorous growth and a long growing season (it flowers from May-September) makes it a formidable foe in forage stands.

As long as stands remain competitive, plant populations may remain low and of little concern. If post-harvest recovery is slowed, Hemp dogbane is excellent at filling “open space” and can become a big problem. When possible, manage stands to prevent dogbane from gaining a foothold by encouraging a competitive grass stand.

Already established – and colonies expanding? You have a couple of options. According to work out of the University of Missouri, a mid-August mowing could help to reduce the size of weed patches the next season. More frequent mowing can help reduce plant vigor and seed production, although mowing in general isn’t likely to eliminate it.

Chemical control options include many of our common active ingredients: 2,4-D, dicamba, fluroxypyr, and triclopyr. Missouri research suggests 2,4-D or fluroxypyr. Limited research in Kansas shows an advantage to products containing fluroxypyr.

Herbicide efficacy can be enhanced (and forage injury reduced) when good growing conditions are present prior to and for a time after herbicide applications. Always use an appropriate surfactant and apply when plants are in the 12- to 15-inch range or shorter.

For identification aids, see the Hemp dogbane page at the Kansas Wildflowers and Grasses website – https://www.kswildflower.org/flower_details.php?flowerID=112.