Henbit (Lamium amplexicaule) is a cool season, ANNUAL broadleaf weed with square stems and pink to purple flowers. It is mostly noticed in the early spring after it has had a chance to grow all winter in the lawn and then begins to bloom. These blooms produce seed which will lay dormant over the hot summer, then begin to germinate in the fall as the temperatures cool. The plant itself dies completely in hot temperatures and does not come back from the roots. Products labeled for pre-emergent control of Henbit include Hi Yield with Dimension and Pendulum 2G.
Wild Carrot (Daucus carota) is a biennial broadleaf weed which behaves as an ANNUAL in our climate. Known as Wild Carrot or Queen Anne’s Lace, it was introduced from Europe, and is an ancestor of the cultivated carrot. It has now spread throughout the United States. The seeds germinate in the fall when temperatures are cool. The plant forms a “winter rosette” close to the ground, eventually sending up erect single stems 1-3 feet tall. The plant blooms in spring though early summer, producing a white, flat-topped bloom which bears many thousands of seeds. The seeds have hooked spines like Velcro that cling to animal fur and clothing, spreading the plant far and wide. Seeds can remain dormant in the soil for 2-5 years. When pulled, the taproot smells like carrot. The entire plant dies after blooming and setting seed. It does not come back from the roots. Unfortunately, I have not found a pre-emergent labeled for control of this weed. Pull young plants before they bloom and set seed or use a selective post-emergent labeled for wild carrot.
Carolina Geranium (Geranium carolinianum) is a broadleaf ANNUAL weed that germinates from seed as the temperatures cool in the fall. The plant grows in the winter months from a single taproot, sometimes reaching 24” across. Small white blooms appear in early spring producing seeds that will lay dormant until temperatures cool in the fall. The entire plant dies in high temperatures and does not grow back from the roots. Products labeled for pre-emergent control of Carolina Geranium include Hi Yield with Dimension and Pendulum 2G.
Annual Bluegrass This clumping ANNUAL grass germinates from seed as the temperatures cool in the fall. We notice it more in the spring because it has produced light colored seed which is more visible. You will likely notice that the seed is produced close to the ground, so mowing will not lessen the number of seed produced for the next season. It will die off completely when it gets hot. Products labeled for pre-emergent control of Annual Bluegrass include Hi Yield with Dimension and Pendulum 2G. Fortunately, it is easy to pull up if you have the time to do so.
Rescuegrass is a cool season ANNUAL grass that has flattened seed spikelets and leaves are slightly hairy on the upper sides of the blades. It has become a common weed in lawns. It will die off completely in the hot summer months, but can be tenacious during the winter months. While not labeled specifically for Rescuegrass control, Pendulum 2G will control a wide range of grass species. Two applications in the fall, the first in mid-September and the second in late November may provide the best control. Digging the entire plant, making sure to remove all rhizomes and all roots may aid in control.
Common Chickweed is yet another cool season ANNUAL weed that we may not notice until spring when it blooms. It will die off completely when it gets hot. Products labeled for pre-emergent control of Common Chickweed include Hi Yield with Dimension and Pendulum 2G.
More to Know
All of these weeds have something in common. You may have noticed that I highlighted ANNUAL in red. Annual cool season weeds germinate in the fall, live until they produce seed, then die when the temperatures get hot. The weeds that you saw in your lawn last winter produced seeds that have been dormant all summer, just waiting for cool weather to arrive to germinate.
Unfortunately, we often do not notice cool season weeds until the spring when they are blooming, but that is NOT when they should be controlled! They are easily controlled by the application of an appropriate pre-emergent in the FALL before the seeds have begun to germinate. Cool season weeds begin to geminate when the soil temperatures consistently drop below 70°F. Timing of pre-emergent application is critical and should occur when soil temperature begins to drop in the fall.
What if I use pre-emergent and still have weeds?
Young seedlings that do germinate may be controlled by an appropriate selective post-emergent herbicide after the lawn is dormant in the fall. It is much easier to prevent these weeds than to control them after they have germinated. Since not all seeds germinate at the same time, undoubtably some will come up, so keep your expectations reasonable. Encouraging healthy growth of you lawn with proper watering and fertilization practices will encourage the lawn to crowd out the weeds and should always be the first step in weed control.