Issue No. 2, Article 4/April 9, 2010
Identifying Early-Season Weeds
Ample soil moisture and warming temperatures are promoting rapid growth and development of many early-season weed species. Most weeds currently growing in fields emerged last fall and successfully overwintered (winter annuals, biennials, and perennials), but several early-season summer annual species recently have emerged. Existing weed vegetation should be controlled before planting by using tillage, herbicides, or a combination of tactics so the crop can become established under weed-free conditions.
Field scouting to identify the weeds present and their relative densities will provide the information needed to tailor a burndown herbicide program for any particular field. Many species can be present in any particular field, and accurate identification is sometimes challenging. Following are brief descriptions and photographs of weed species common to Illinois fields at this time of year.
Annual bluegrass is a shallow-rooted grass species that often roots at the lower nodes. Plants grow to about 12 inches tall and often have begun to flower by the time of crop planting. Leaves lack auricles and possess a membranous ligule. Leaf blades are glabrous on both surfaces and keeled with sharp-pointed tips.
Over a dozen species of buttercup exist in Illinois, which can make accurate identification a bit challenging. Smallflower buttercup is one species that can be found across much of the state. Its lower leaves are rounded and broad with toothed margins, generally bright green in color and borne on long petioles. Upper leaves are deeply lobed, with much shorter petioles. Flowers have yellow petioles, and seeds are contained within a cone-shaped structure known as an achene.
Butterweed (aka cressleaf groundsel) is a winter annual species that produces bright yellow flowers in later spring. The plant overwinters in a rosette stage and "bolts" in the spring to produce a hollow stem that is green or red and smooth to the touch. From a distance the yellow flowers of butterweed resemble those of yellow-flowered mustard species, but butterweed is not a mustard species.
Common chickweed often forms very dense mats of vegetation that can make planting difficult. It has a shallow, fibrous root system and a stem that branches extensively and often roots at the nodes. Leaves are small, opposite, and pointed at the tip. A perennial chickweed species, mouseear chickweed, can be differentiated from common chickweed by the dense hairs covering its stem and leaf surfaces.
Dandelion is a simple perennial species that forms a large, often deeply rooted taproot. The leaves have irregular margins, are often deeply lobed, and form a basal rosette. The flower is large and yellow.
Field pennycress begins as a basal rosette of leaves, then produces an erect flowering stem that is smooth and tends to smell unpleasant when bruised. Leaves on the upper stem are alternate and sessile about the stem, with earlike projections and either entire or slightly toothed margins. Field pennycress is one of several mustard species in Illinois that produces white flowers consisting of four petals. Other mustard species that produce white flowers include shepherd's-purse and Virginia pepperweed. The most common yellow-flowered mustard species in Illinois fields is yellow rocket.
Henbit and purple deadnettle are close botanical relatives; both exist as winter annuals, and both have square stems. Henbit is more common throughout Illinois, while purple deadnettle appears more often in about the southern half of the state. The lower leaves of henbit are 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.
Fall-emerging horseweed plants (aka marestail) form a basal rosette that represents the plant's overwintering stage. In the spring, plants bolt by rapidly elongating the main stem. Leaves, very numerous and hairy with toothed margins, alternate around the stem and become progressively smaller toward the top of the plant. The leaves on mature plants lack petioles and have entire or slightly toothed margins. As the plant matures, leaves toward the base of the plant deteriorate and fall off the stem.
Kochia is one of the summer annual weed species that emerges earliest. Its leaves are opposite, simple, and very hairy. Stems are erect and often become grooved as the plant ages. Mature plants can vary in color from green to red.
Poison hemlock is a biennial species becoming increasingly common in reduced-tillage environments. Leaves are arranged alternately on the stem, triangular in outline, four to five times pinnately compound (fernlike), without hairs, and borne on petioles. Basal leaves have a long petiole, while upper leaves have a much shorter one. Stems (produced during the second year) are erect, much branched, smooth (without hairs), and hollow except at the nodes. Stem color is light green; lower portions of the stem are spotted or streaked with red or purple blotches. The inflorescence of poison hemlock is a compound umbel consisting of multiple white flowers, each flower bearing 5 petals.
Prickly lettuce, a winter or early summer annual with a large taproot and stem, grows up to 6 feet tall. Leaves are large and coarse, with prickles along the margins. Older leaves also have a row of prickles along the underside midvein. The stem, roots, and leaves exude a milky sap when injured.
Speedwell species are generally considered winter annuals, although some perennial species exist. Some species grow close to the ground and form dense mats of vegetation, whereas others grow more upright. Leaf shape varies by species, but most speedwell species have lower leaves that are opposite and upper leaves that are alternate.
Wild carrot is a biennial species that most often occurs along the edges of reduced-tillage fields. Typical of biennial species, its first year of growth occurs as a rosette of leaves followed by rapid stem elongation and maturation during the second year. Some confusion may arise in trying to differentiate poison hemlock from wild carrot. Both are biennial species that produce an umbel inflorescence, but poison hemlock is more toxic. Differences include plant height (poison hemlock may reach 6 to 10 feet at maturity; wild carrot rarely exceeds 4 to 5 feet at maturity); stem coloration (poison hemlock stems have red to purple blotches; wild carrot stems lack such coloration); stem hairs (poison hemlock has stems with no hairs; wild carrot has stems with bristly hairs); flowering time (poison hemlock flowers in mid- to late spring; wild carrot flowers later in summer); the pungent odor of poison hemlock foliage; and the hairy leaves of wild carrot.
Two members of the smartweed family that emerge during early spring are Pennsylvania smartweed and prostrate knotweed. Both species (as well as most members of the Polygonaceae family) have swollen nodes covered with a membranous sheath called an ocrea.