Winter Appearing Weeds

Most of the weeds that we encounter in turfgrass are either summer annuals (which germinate in spring, grow and set seed in summer, then die in the fall), or, perennials (which persist for multiple seasons). There are, however, a few winter annual weeds in turfgrass, such as annual bluegrass, common chickweed, and henbit. Less common winter annual weeds in turf include shepard's purse, veronica, and purple deadnettle.                                     In some years, these weeds may warrant control and some understanding of the ecology and life cycle of these weeds can be of assistance in determining control strategies. Winter annuals germinate in the fall and bloom and set seed in the spring. Like summer annual weeds, they spread by seed, not vegetatively like ground ivy or other perennial weeds.  Proper identification of a weedy species is the first step to determining the best control strategy. Use the information in the figure captions to identify common chickweed and henbit, which are the two most common winter annual weeds in turf in Ohio (Top two pictures). There are also a few perennial weeds that are more visible in late winter or early spring, such as wild onion, wild garlic (bottom picture), and Star-of-Bethlehem.

Henbit and Common Chickweed

Both are highly competitive in thin or dormant turf or in newly seeded areas. However, a vigorous lawn will tend to out-compete them. So, the best defense against henbit and common chickweed is to properly maintain your turfgrass. This includes selecting the right species for the location and usage and proper cultural practices (proper mowing, fertility, irrigation, and aeration).

If, however, a herbicide is required, the timing of application is very important for optimal control. The chief concern is that a dense mat of winter annual weeds, after they die in the spring, will open a bare spot in the turf that may be filled in by crabgrass and other summer annual weeds. Since winter annuals thrive in cool, moist conditions, germination depends on a combination of lower soil temperature and increased precipitation. In years in which there are very warm temperatures and little precipitation in early fall, germination is delayed. Therefore, timing of pre- and especially postemergence herbicides would need to be delayed.

Both species can be controlled preemergence with a fall application. However, this application will likely degrade and not be effective against annuals next spring so a specific preemergence application targeting these weeds would only be justified in the most severe cases of infestation. Best control of winter annuals with postemergence herbicides is achieved if applications are made in the fall to actively growing plants. Remember to read and follow manufacturer directions and recommendations on the label. Ideally, henbit and chickweed can be controlled with the same fall application that targets dandelions and other perennial broadleaf weeds. However, if the application is made too early in the fall, these species will continue to germinate from seed, which will require reapplication for effective control. If necessary, postemergence herbicides can be used on winter annuals in the spring. You must use extra caution with spring applied herbicides, since newly emerging leaves of ornamentals are extra sensitive to broadleaf herbicides. Avoid applications when temperatures are above 80°F or when it is windy. Also, you must target the application no later than when the weed is flowering. Since they are annuals, control after it has set seed is not warranted.

Annual Bluegrass

Poa annua may be annual or perennial. The annual biotypes are opportunistic, that is they germinate at just about any time of the year. However, annual bluegrass is technically a winter annual with a primary flush of germination in October. Development of 100% effective annual bluegrass controls has been elusive, but there are some recommendations. Velocity® is an example of a postemergence herbicide for annual bluegrass control. Consult the label for specifics. Some superintendents report good success with sequential applications of Prograss® postemergence. In addition, some of our preemergence herbicides are also labeled for annual bluegrass control and most research shows that you can achieve an 80-90% reduction in germination. It is very important to consult the label and begin preemergence applications in early September, before annual bluegrass begins to germinate. The problem with using preemergence controls may be one of economics and also safety to creeping bentgrass (if attempting to control annual bluegrass on a putting green).

Wild Garlic and Wild Onion (Allium sp.), and Star of Bethlehem (Ornithogalum sp.)

These species become quite noticeable in turf in late winter or early spring, because they green up and grow a few week before the turf does. As a result, they tend to be visible as clumps of foliage up to 12 inches in height in otherwise still dormant turf. They are monocots, like our grasses, but they are not grasses. Just as sedges are a different than grasses, so to are the Allium and Ornithogalum species. In fact, these are more closely related to amaryllis and daffodils. And there in lies the problem.there are not scads of research dollars being devoted to develop herbicides that effectively control daffodils or related species. Thus, dedicated selective controls for these weeds do not exist. If you have a serious problem, you can attempt to treat them with the three way post-broadleaf herbicide of your choice, but with the caveat that the herbicides are not specifically designed to control these weeds and therefore your results may be variable. 2,4-D alone is also a good choice. Your best option may be to keep them cut short, either by mowing or with hand shears. As bulbs, they persist vegetatively for a period after flowering and then the foliage dies back for the season. If you mow or otherwise defoliate them early, you weaken the plant and diminish its ability to develop next year.

Winter annual and perennial weeds are usually not our most serious weeds in turf. However, they can, on occasion, become quite noticeable. The key is to recognize the uniqueness of the life cycle, and target your herbicide application for the right time of year.

Posted by David Gardner