Attack winter weeds Perennials can be kept in check

Black medic is a clover that reseeds readily to form large patches within two years if the original plants are not removed.

Cold weather does not stop many perennial weeds that grow in the Lowcountry. Some, like blue-eyed grass, grow year round, while others, like dandelion, go dormant in summer. Here are several perennial weeds that are most active or noticeable in lawn grasses during the winter.

It looks like a miniature iris. Although most people say it is an annual, I am sure that plants have survived more than one year in my yard, so I agree with those who say it’s a short-lived perennial.

Blue-eyed grass (Sisyrinchium rosulatum) produces blue flowers in the spring that are followed by small, spherical seed capsules. The flowers of the weedy blue-eyed grass are about the same size, and as showy as the ornamental, native, perennial blue-eyed grass (Sisyrinchium angustifolium). If desired, the time to pull up blue-eyed grass is when it starts to flower.

This is a biennial, which means plants grow for two years and flower in the second year. In the first year, fleabane produces narrow, toothed leaves with tiny hairs that grow in a ground-hugging rosette. In late spring of the second year, it sends up thin flower stalks topped by daisy-like flowers with narrow petals.

Fleabane grows in patches that get larger over time. Fleabane spreads by seeds and by short, horizontal stems that produce new plants. Because it is difficult to pull up, fleabane is best managed with herbicides.

French for “lion’s tooth” because of the jagged edges of its leaves, dandelions (Taraxacum officinale) are a common winter perennial weed in the Lowcountry. Although most of them go dormant in summer, they survive for several years because of their thick, fleshy taproots. In the past few years, it seems that more and more plants are still green in summer. Perhaps they are adapting to warmer temperatures.

Removing dandelions without using herbicides is tedious but possible. The taproot is fragile, which may be an adaptation for survival. A weeding implement with a long, narrow blade can be used to loosen the taproot, and then the plant is pulled up. I rarely am able to remove more than an inch of the taproot when pulling dandelions, and I haven’t checked to see if these plants regrow, but they don’t seem to.

Both are in the bean family. These weeds capture nitrogen and may release a bit of it into the soil. Their flowers feed honey bees. However, those are not reasons to let them be. Both white clover and black medic reseed readily and grow so thickly that they choke out turf grasses.

White clover (Trifolium repens) has typical three-part clover leaves (very rarely four-part) and white flowers. It spreads by stolons, stems that grow horizontally along the ground and root along their length. It is a perennial that persists and grows year-round. White clover should be pulled up as individual plants appear.

Black medic (Medicago lupulina), a biennial, looks like a small, flat clover with yellow flowers and red stems. It is named for its black seed pods.

When spraying winter weeds such as black medic and fleabane, homeowners should be cautious about applying certain herbicides to centipede and St. Augustine grass during the winter. Partially dormant grass is sensitive to some herbicides, like Image (active ingredient imazaquin), that are safe to use on actively growing grass.

To avoid injury to semi-dormant grass, winter weeds need to be spot-treated with herbicide in a hand-held sprayer. Spray only until the weed is wet; don’t saturate it. If larger areas need to be treated, use a hose-end sprayer, but again, only the area infested with weeds should be sprayed.

Most lawn herbicides can be applied only twice per year, so if an herbicide is sprayed on winter weeds, it can only be used once on summer weeds. Many lawn herbicide products are mixtures of three or four active ingredients, and each active ingredient may be applied only twice per year. Read labels carefully.

After spraying an herbicide, do not allow children or pets to walk (or roll) on the lawn until the herbicide is dry. This may take several hours during the cool days of winter.

With a combination of hand weeding and judicious herbicide use, winter weeds can be managed.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He also is an avid gardener. Contact him at tknth@clemson.edu.