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What Weed Is That?

April 26, 2002
The warm weather during the week of April 15 encouraged (yes, they grew pretty fast in a week's time!) 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.

Several weed species in the mustard (Brassicaceae) plant family are commonly found in Illinois. Members of this family have either white or yellow flowers, and the flowers consist of four petals that form a cross.

Although this family contains many species, some of the more common members found in no-till fields are wild mustard (Brassica kaber) and yellow rocket (Barbarea vulgaris),

field pennycress (Thlaspi arvense),

Shepherd's-purse (Capsella bursa-pastoris), and the pepperweeds (Virginia and field).

Two members of the smartweed (Polygonaceae) family that emerge during the early spring are Pennsylvania smartweed (Polygonum pensylvanicum)

and prostrate knotweed (Polygonum aviculare).

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 (Polygonum persicaria L.) 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 in Illinois. Common chickweed (Stellaria media) exists primarily as a winter annual but may sometimes emerge in the early spring. Mouseear chickweed (Cerastium vulgatum) is a perennial species and is generally not as abundant in no-till fields as common chickweed. Both species are similar in appearance; however, mouseear 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 (Lamium amplexicaule) and purple deadnettle (L. purpureum) 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.

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.

Kochia (Kochia scoparia) 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 (Conyza canadensis), 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 (Ornitho-galum 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 (Conium maculatum) is a biennial species that is commonly found in pastures and along railroad rights-of-way but is 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, 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.

Dandelion (Taraxacum officinale) is a simple perennial species that forms a large, often deeply rooted taproot.

Leaves have irregular margins, are often deeply lobed, and form a basal rosette. The flower is large and yellow. 2,4-D is often used for burndown control of dandelion, but control is generally more complete and consistent when 2,4-D is applied in the fall.

Prickly lettuce (Lactuca serriola) 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.

Several species of buttercup (Ranunculus species) exist in Illinois, but one that appears to be very common is smallflower buttercup (Ranunculus abortivus). 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.

Other weed species that you might find in no-till fields include speedwells (Veronica species),

annual bluegrass (Poa annua),

catchweed bedstraw (Galium aparine),

and plantains.

--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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