Green winter weeds mar the smooth evenness of brown, dormant Southern lawns. If that is not sufficient motivation to take them seriously, weeds are taking advantage of this time of year. Dormant lawn grasses do not compete with weeds, so weeds have access to all of the water and sunlight they need to grow and reproduce.
Most winter weeds grow in cool soil that remains moist. Since cool temperatures lead to moist soil, because there is less evaporation, it is not surprising that winter weeds flourish in these environments. A season like this winter with above-average rainfall (so far) promotes growth of winter weeds in low spots in lawns.
Patches of winter weeds will enlarge from year to year or spread to new spots if weeds are allowed to set seed. Annual weeds reappear close to the same spot where the weed grew last winter, where the seeds fell.
Many of these weeds are sensitive to warm temperatures. Temperatures in the low 70s speed up reproduction instead of leaf growth, meaning that plants remain smaller when it is warmer, but they produce seeds faster.
Here is a list of some common winter or cool-season weeds found in the Lowcountry.
Bitter cress (Cardamine sp.) is a small weed in the mustard family. It has dark green leaves in a “basal rosette,” a flat cluster of leaves near the ground. The small, white flowers rise above the leaves on short stalks. Bitter cress dies out when temperatures reach 75 degrees. Before they disappear, however, even tiny plants barely an inch in diameter will flower and produce a few seeds.
Bitter cress’s “claim to fame” is its ability to shoot seeds several feet away from the plant. One spring as I walked through the grass at a rural tourist attraction, I was pelted by what sounded like hundreds of bitter cress seeds that my feet released.
Vetches (Vicia species) look like miniature pea vines except they have narrow, oval leaves instead of round leaves. Different vetches are large (up to 12 inches) or small (4 inches) and have lavender or white flowers. Like peas, they make seeds in pods that turn dark when ripe.
Carolina geranium (Geranium carolinianum) has broad, deeply indented leaves with reddish stalks and small, pale pink flowers. Based on my experience, this is one of the few weeds that can be eliminated by pulling. By removing all Carolina geranium plants before they set seed this year, noticeably fewer plants will emerge next year. If dedicated weeding is repeated, hardly any plants will appear the third year.
Two other annual winter weeds growing now are common chickweed and mouse-ear chickweed. Although both are called chickweeds, they look different.
Common chickweed (Stellaria media) has thin, spreading stems and tiny white flowers. It is very tolerant of cold.
Mouse-ear chickweed (Cerastium sp.) grows more upright and denser than common chickweed. Most importantly, it has tiny hairs that make the leaves look like, yes, furry mouse ears.
Henbit (Lamium amplexicaule) is a winter weed in the sage or mint family. It is easily recognized by its square stems, heart-shaped leaves that wrap around the stem, and clusters of narrow, lavender flowers.
Annual bluegrass (Poa annua) emerges in early spring and lasts until May. It is a bright green, short grass that forms small clumps and a typical “grassy” seed head. Like most cool-season weeds, it is common in soil that remains moist. Because annual bluegrass does not last that long, and its grassy appearance blends with lawn grasses, many experts suggest not bothering to control it.
Many other winter annual weeds are found in turf and beds at this time of year. The Weed Science Society of America has a public access photo gallery to assist with identification (wssa.net/weed/weed-identification).
To remove seeds of winter weeds from the lawn, mow weedy patches with a lawn mower set to cut at the lowest recommended height for the type of grass and collect and dispose of the cuttings.
This technique works relatively well for upright weeds, like henbit, vetch and annual bluegrass, but, unfortunately, does not remove low-growing weeds like common chickweed and bitter cress.
My next column will cover perennial winter weeds and herbicides as another option to keep winter weeds in check.
Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. Contact him at firstname.lastname@example.org.