Annual Bluegrass and Butterweed

May 5, 2000
Several samples and calls were received this past week regarding a couple of weed species that are currently common in Illinois no-till fields. Annual bluegrass (Poa annua) can exist as a winter annual species and appears to be becoming more common in many Illinois fields. At present, this species is beginning to or has already reached the "heading" stage and will shortly begin to die back. Some callers have indicated that a burndown herbicide application had failed to control annual bluegrass and were curious as to why. It appears that, since the weed is entering the later stages of its life cycle, it may be somewhat less susceptible to a burndown herbicide application than it would be if the same treatment had been applied earlier during its development. Annual bluegrass typically completes its life cycle by the end of May or first week of June, so re-treating for this species may not always be necessary.

Butterweed (Senecio glabellus) has begun to flower, and, in fields where the population is sufficiently heavy, the field appears to be a "sea of yellow." The bright yellow flowers and basal leaves of butterweed often mislead people to assume it's a mustard species, when in actuality it belongs to the aster (Asteraceae) family. The stem is hollow and often purple in color. Butterweed was discussed in issue no. 2 of the Bulletin, "A Sea of Purple," and the Web version of this article has several pictures of this species.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague