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Issue No. 5, Article 6/April 22, 2005

Weed Species of Interest

With the warm temperatures across much of the state over the past couple of weeks, weed vegetation in fields that have not been tilled or treated with a herbicide has grown significantly. As field preparation and planting continues, a few weed species that may catch your attention are described in the following paragraphs.

Annual bluegrass (Poa annua) can exist as a winter annual species and appears to be becoming more common in many Illinois fields (especially fields not tilled). At the present time, this species is beginning to "flower" or "head" and will soon begin to die back. Some observations have indicated that burndown herbicides provided less-than-satisfactory control of annual bluegrass. It seems likely that since the plant is entering the later stages of its life cycle, it may be somewhat less susceptible to burndown her-bicides than had the same treatment been applied earlier in its development. Annual bluegrass typically completes its life cycle by the end of May or first week of June, so re-treating to improve control may not always be necessary.

Poison hemlock (Conium maculatum) is a biennial species commonly found in pastures and along railroad rights-of-way but becoming more common in no-till fields. During its first year of growth, poison hemlock forms a rosette (a dense cluster of leaves growing close to the ground), then bolts to produce seed during its second year. The leaves are alternate, 4 to 5 times compound, and toothed, giving the leaves a "lacy" appearance. The stems are smooth and hollow, with purplish spots or blotches. At maturity, the plant may reach 6 feet or more. The entire plant is poisonous. Poison hemlock is sometimes confused with another common biennial weed species, wild carrot (Daucus carota). The species can be differentiated by the purple spots on the smooth, hairless stem of poison hemlock and the presence of hair on the stem and leaf margins of wild carrot.

Butterweed (Packera glabella) generally emerges in the fall, overwinters as a rosette, then flowers in the spring. At the time of this writing, butterweed (also known as cressleaf groundsel) had begun to flower, although the flowers have yet to fully expand in the northern areas where this species can be found. The bright yellow flowers and basal leaves of butterweed often mislead people to assume the plant is a mustard species, when in actuality it belongs to the Aster (Asteraceae) plant family. In fields where the population of butterweed is sufficiently heavy, the field appears to be a "sea of yellow." The stem is hollow and can be either green or purple.--Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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