Those brilliant fields of butterweed brightening the landscape this spring are deceptive. Not only is the daisy-like plant a weed, but it can be toxic to livestock.
University of Missouri weed specialist Kevin Bradley says butterweed, also known as cressleaf groundsel, is one weed that’s been on the rise over the last several years. "It seems to thrive in moist, saturated soils. So that might explain why we’ve seen more of it over the past several years," says Bradley.
Butterweed can cover large sections of fields, but the big concern is when it shows up in quantity in pasture settings. Although not considered to be as toxic as its western cousin, tansy ragwort, it still produces toxic alkaloids. Poisoning symptoms are usually chronic signs of decline rather than rapid and lethal.
A winter annual, butterweed germinates in the fall and grows throughout the winter months. Infestations become evident this time of year when it starts to bloom. Although often identified as some type of mustard, this weed is actually a member of the Aster family. You can tell by counting petals. Mustards have four petals per flower and butterweed will have 7 to 12 petal like ray flowers. Seed heads look like small dandelion puff balls and the seeds are wind dispersed.
Fortunately butterweed is flowering and starting to senesce naturally on its own right now. It grows best in cool wet conditions and will die out in periods of hot and dry conditions. Weed scientists recommend noting where butterweed was an issue this spring and addressing it with a fall control program.
"Bottom line, most fall herbicides with residual activity do well on this species," Bradley says. "In a pasture setting, there are also several herbicidal options as well."