Unwanted cool-season grasses and broadleaf weeds disrupt lawn uniformity, especially when warm-season turf is dormant. These invaders, often called “winter grass,” thrive in cooler temperatures and moist soil conditions that suppress desirable lawn species. Effective management requires preventative cultural practices combined with targeted chemical treatments. This guide outlines how to eliminate existing winter weeds and prevent their return.
Identifying Common Winter Grasses and Weeds
Accurate identification is necessary because treatment methods vary significantly between grass and broadleaf weeds. The most common winter grass is Poa annua, or annual bluegrass, a prolific winter annual that germinates in the fall and produces seeds throughout the spring. It is recognizable by its lime-green color, which stands out against established turf. Poa annua produces numerous small, white seed heads even when mowed closely, giving the lawn a patchy appearance.
Common invaders include broadleaf weeds like henbit and chickweed, which germinate in the fall. Henbit belongs to the mint family and features characteristic square stems, rounded leaves with scalloped edges, and small pinkish-purple flowers. Chickweed forms dense, low-spreading mats, identifiable by its bright green, small leaves and tiny white, star-shaped flowers. These weeds flourish when the desirable turf is not actively growing.
Cultural and Mechanical Control Methods
Non-chemical control relies on altering the turf environment to create conditions unfavorable for winter weed germination. Adjusting the mowing height is a preventative measure, as winter weeds, particularly Poa annua, thrive in short turf. Maintaining a height of three to four inches helps the desirable turf shade the soil surface, inhibiting the light required for many weed seeds to germinate.
Proper watering techniques are important since winter weeds have shallow root systems and prefer consistently moist soil. Instead of frequent, light watering, which keeps the topsoil wet, water deeply and infrequently. For dormant lawns, water only every two to three weeks to hydrate turf roots without encouraging weed seed germination near the surface. Hand-pulling is an effective mechanical method for small infestations, especially if done before the plants produce seeds.
For large, localized areas, soil solarization provides a non-chemical means of killing established weeds and seeds. This technique involves covering the area with a clear plastic sheet for four to six weeks during the hottest part of the summer. The plastic traps solar energy, raising the soil temperature above 140°F, which kills weed seeds and pathogens near the surface. The soil must be moist before covering to enhance heat conduction.
Chemical Strategies: Pre-Emergent Application
The most effective strategy for controlling winter annual grasses is prevention, achieved through the application of pre-emergent herbicides. These products create a chemical barrier just beneath the soil surface. This barrier targets germinating seeds by disrupting cell division in the young root system, preventing the seedling from establishing.
Timing is crucial for pre-emergent success, as the product must be applied before the seeds begin to germinate. For Poa annua, germination begins when the soil temperature consistently drops below 70°F. This window typically occurs in the late summer or early fall. The herbicide should be applied two to three weeks before the soil reaches this temperature threshold to allow time for activation.
Common active ingredients include prodiamine, pendimethalin, and dithiopyr. After application, the herbicide must be watered into the soil with a quarter to a half-inch of irrigation or rain within 24 hours for the barrier to properly form. Since the protective barrier eventually breaks down, a second application may be necessary in late winter or early spring to control other annual weeds like crabgrass.
Chemical Strategies: Post-Emergent Application
When winter grass has emerged, a post-emergent herbicide is necessary to kill the actively growing plants. These chemicals are absorbed through the leaves and translocated throughout the weed, making product selection dependent on the weed type and the desirable turf species. Non-selective post-emergents, such as glyphosate, kill nearly all vegetation they contact, making them suitable only for spot treatment or in areas of fully dormant warm-season turf. Confirm the turf is completely brown before using non-selective products, as they can severely damage green grass.
Selective post-emergents are designed to kill specific weeds without harming the surrounding turf, but controlling Poa annua can be challenging since it is a grass itself. For lawns with warm-season turf like Bermuda or Zoysia, professional-grade selective options are used to target Poa annua.
- Certainty (sulfosulfuron)
- Revolver (foramsulfuron)
- Mesotrione (Tenacity)
These selective products require careful application, as some, like Mesotrione, are not safe for all turf types.
Broadleaf weeds like henbit and chickweed are easily controlled with common three-way herbicides containing 2,4-D, dicamba, and MCPP. Post-emergent treatments are most effective when the weeds are young and actively growing, and when air temperatures are between 65°F and 85°F. Applying herbicides when temperatures are too cold makes them ineffective, while high temperatures can increase the risk of damage to the lawn. Multiple applications may be required to eliminate established Poa annua.