Issue No. 3, Article 7/April 9, 2004
Everything's Coming Up Green (and Purple)
Various shades of green and purple have begun to appear in many fields in response to recent rains and warm temperatures. It is our goal to keep ahead of the weeds and tell you what is emerging each week to make scouting fields easier. In the Web version of the Bulletin, color photographs accompany weed descriptions. So, to keep up the annual tradition of reviewing the identification of these early-season weeds. . . .
One of the earliest grasses to appear in no-till fields is downy brome (Bromus tectorum). Downy brome is a winter annual, with a dense, soft mat of hairs covering the upper and lower leaf surfaces and the stem. A unique characteristic of downy brome leaves is that they twist clockwise. The ligule is very short and membranous, while the leaf sheath is closed (typical of brome grasses). The droopy panicle, or seed head, begins to appear in April and can have a green to reddish purple tint.
Another grass that is early to emerge is annual bluegrass (Poa annua). Annual bluegrass has very narrow leaves, with a short (1- to 2-millimeter-long) membranous ligule. Leaves and stem do not have any hairs present. A distinguishing characteristic of annual bluegrass is the end of the leaf blade, which is keeled like the bottom of a boat. This grass is highly tillered and is likely to be found in clumps.
Two species of chickweed are common to Illinois. The most prevalent is common chickweed (Stellaria media), with mouseear chickweed (Cerastium vulgatum) coming in second. Common chickweed is a winter annual that forms very dense mats. The leaves are smooth, opposite, and round to egg-shaped, with a pointed end. Mouseear chickweed is a perennial with leaves very similar to common chickweed, except that dense hairs cover the leaf surfaces.
Curly dock (Rumex crispus) is a perennial weed that begins as a rosette, with smooth, egg-shaped leaves. Some of the younger leaves may have red and purple spots on them, although the speckling will become more pronounced with maturity. Older leaves will develop wavy margins, and an ochrea (papery sheath) will be easily seen near the base of the petiole.
Two biennial weeds that are hard to distinguish when small are wild carrot (Daucus carota) and poison hemlock (Conium maculatum). Both form basal rosettes and have leaves that are pinnately dissected (very divided). Leaves of wild carrot (otherwise known as Queen Anne's lace) are smooth on the upper surface yet have very short hairs on the lower leaf surface. In the second year, vertical hollow stems with very few leaves are produced from the rosette. Poison hemlock leaves and stems are hairless, with purple speckling on the stems. In the second year, leaves are present on the stems, and the plant appears fernlike. When the stem or leaves are crushed, they emit a pungent, parsniplike odor. Poison hemlock leaves tend to be larger, 20 to 40 centimeters long, compared with wild carrot leaves, which may only reach 15 centimeters.
Leaf shape is the easiest way to differentiate buckhorn and broadleaf plantain. Both plantains are perennials that have leaves that form basal rosettes. Buckhorn plantain (Plantago lanceolata) has very narrow, lanceolate leaves with parallel venation. These leaves can be smooth or hairy, and there is no petiole. Broadleaf plantain (Plantago major) has broad, oval-shaped leaves, with three to five prominent veins. The older leaf margins are wavy and may or may not have hairs. There is a petiole present on broadleaf plantain.
A weed less common to Illinois is western salsify (Tragopogon dubius). This biennial looks very grasslike, with long, narrow leaves arranged in a rosette. The leaves have a keeled tip (similar to annual bluegrass) and clasp around the stem. The stem is smooth and very fleshy with occasional hairs. Western salsify leaves, stems, and roots (large taproot) exude a milky sap when injured.
Horseweed (marestail) has also taken over many fields already this spring. Horseweed seedlings develop basal rosettes. The earliest leaves are egg-shaped, with soft, short hairs. With time, the leaves become linear to elliptic and crowded around the stem. The edges of the leaves are toothed. We encourage you to pay special attention to horseweed due to the presence of glyphosate-resistant biotypes in Indiana and Kentucky.
Many other weeds have emerged or begun to emerge, including purple deadnettle, henbit, yellow rocket, shepherd's-purse, Virginia pepperweed, field pennycress, butterweed, buttercup, kochia, prickly lettuce, star-of-Bethlehem, pineappleweed, dandelion, Pennsylvania smartweed, prostrate knotweed, speedwells, catchweed bedstraw--and the final weed in our list is giant ragweed!