What Weed is That?

The following article was prepared by Aaron Hager and Christy Sprague and printed in the April 26, 2002 edition of the Univ. of Illinois Pest Management and Crop Development Bulletin. It gives an excellent description of some of the weeds we have in Wisconsin and will make you thankful that we don’t have some of the weeds they present - at least not yet! And that’s part of the reason I asked their permission to print their article in the Crop Manager. Knowing what’s happening on our flanks can be great protection. The on-line version of the article has many excellent color pictures of most of the weeds described. To go there, click on this URL: and click on "What Weed Is That."

The warm weather during April encouraged the growth of many weed species in no-till fields. Trying to figure out what some of these early-season weed species are can sometimes be difficult, so we thought a short review on identification would be beneficial.

Illinois has several weed species in the mustard family (Cruciferae, referring to the four petals that form a cross) most of which have either white or yellow flowers. Although this family contains many species, the more common ones in no-till fields are wild mustard, yellow rocket, field pennycress, shepherd's purse and the pepperweeds (Virginia and field).

Several species of buttercup exist in Illinois, but one that appears to be very common is smallflower buttercup. The lower leaves are generally broad and rounded, bright green in color with toothed margins, and borne on a long petiole. Upper leaves are borne on short petioles and deeply lobed (generally three to five lobes). Flowers have yellow petals, with seeds contained in a structure known as an achene.

Another species with a bright, yellow flower is butterweed (Packera glabella), also referred to as cressleaf groundsel. Although the yellow flowers may lead you to think butterweed is a mustard, this species actually belongs to the Asteraceae family. Butterweed has a hollow stem that can be either green or bright red in color.

Two members of the smartweed (Polygonaceae) family that emerge during the early spring are Pennsylvania smartweed and prostrate knotweed. Both species (as well as most members of the Polygonaceae family) have swollen nodes (the genus name Polygonum means "many knees"), covered with a membranous sheath called an ocrea. Look closely where the leaf petioles join the main stem. Ladysthumb is very similar in appearance to Pennsylvania smartweed, and the two can be distinguished from one another during early vegetative growth by examining the ocrea. Ladysthumb has a fringe of hairs at the top of the ocrea, whereas Pennsylvania smartweed does not.

Two species of chickweed are commonly found. Common chickweed exists primarily as a winter annual but may also emerge in the early spring. Mouse ear chickweed is a perennial species and is generally not as abundant in no-till fields as common chickweed. Both species are similar in appearance; however, mouse ear chickweed is covered with dense hairs on the leaf and stem surfaces, while common chickweed plants lack hairs. These species (especially common chickweed) can form very dense "mats" of vegetation that can make tillage and planting operations difficult. Chickweed flowers consist of 5 petals that are white and deeply lobed, giving the appearance of 10 petals.

Henbit and purple deadnettle are close relatives; both exist as winter annuals and both have square stems. Henbit is more commonly found throughout Illinois, while purple deadnettle appears more often in the southern half of the state. The lower leaves of henbit are petiolate (attached to the stem with petioles), while the upper leaves grasp the stem (i.e., lack petioles). The upper leaves of purple deadnettle, however, are attached to the stem with petioles, are more triangular than those of henbit, and are less deeply lobed. As the name implies, purple deadnettle has a distinctive reddish to purple coloration of the foliage and stem.

Kochia is an early-emerging summer annual species. It is a herbaceous dicot and member of the Chenopodiaceae family (the same botanical family as common lambsquarters). Kochia leaves are alternate with simple blades that are highly pubescent. Stems are erect, highly branched, and often grooved on older plants, and they vary in color from green to red, often with both colors present on an individual plant. Kochia was introduced into North America from Europe as an ornamental because of its red color in late summer and fall (hence kochia's other common name, "fireweed").

Horseweed, or marestail, exists as a winter or early summer annual species. Seedlings develop a basal rosette of leaves, and the leaves are covered with short hairs and have toothed margins. Control of this species with burndown herbicides can be difficult, especially if applications are made under cool conditions or without 2,4-D. Recent reports have identified biotypes of horseweed that are resistant to glyphosate in Delaware and Tennessee.

A species that is not as common as others described here but can be difficult to manage with burndown herbicides is star-of-Bethlehem (Ornithogalum umbellatum). This species is a bulbous perennial that is frequently sold as an early-flowering ornamental that has escaped into agricultural fields. All parts of the plant are poisonous if ingested. Emerging star-of-Bethlehem shoots resemble wild garlic or wild onion but lack the characteristic odor of these species. The mature leaves are dark green and frequently have a prominent white midrib. Plants typically produce bright white flowers beginning in early to mid-May, then die back for the remainder of the season.

Poison hemlock is a biennial species commonly found in pastures and along railroad rights-of-way but is now found in no-till fields. During its first year of growth, poison hemlock forms a rosette of leaves, then bolts to produce seed during its second year. The leaves are alternate, four to five times compound, and toothed, giving the leaves a "lacy" appearance. The stems are smooth and hollow with purplish spots or blotches. The entire plant is poisonous.

Prickly lettuce can exist as a winter or early-summer annual. Leaves on young plants are long and tapered at the end, with margins that are finely toothed. A row of prickles can be found along the midrib on the underside of the leaf. A milky juice is present in leaves, stems, and roots. Other weed species that you might find in no-till fields include speedwells (Veronica species), annual bluegrass, catchweed bedstraw, and plantains.

May 2002