Winter weeds and cool-season grasses quickly colonize lawns when desirable warm-season turf enters its dormant, brown phase. Plants like Annual Bluegrass or Henbit create an unsightly patchwork of green against the dormant lawn. Controlling these persistent winter invaders requires a proactive, multi-season strategy combining good lawn maintenance with properly timed chemical applications.
Identifying Common Winter Weeds
Winter weeds are classified by their life cycle, which dictates the best time for control. The most problematic are winter annuals, which germinate in the cool soil of fall, live through the winter, and produce seed before dying in the spring.
A prominent offender is Annual Bluegrass (Poa annua), a bright, light-green grass that grows in clumpy tufts and produces white seed heads. Broadleaf weeds include Henbit (Lamium amplexicaule), which has a distinct square stem and rounded leaves, often displaying small pink or purple flowers. Common Chickweed (Stellaria media) is a low-growing, mat-forming weed with small, oval leaves and tiny white, star-shaped flowers.
Cultural and Manual Removal Techniques
Maintaining a thick, healthy lawn is the most effective defense against weed establishment. A dense turf canopy naturally shades the soil, blocking the sunlight winter annual seeds need to germinate. Promoting a deep root system through proper watering helps grass outcompete weeds.
For warm-season grasses like Bermuda or St. Augustine, raising the winter mowing height to two or three inches maximizes shade and suppresses weed growth. Watering should be deep but infrequent, often only every 10 to 14 days in the winter, as excessive moisture encourages winter annuals. For small, isolated patches, manual removal is a viable, non-chemical solution. When hand-pulling, ensure the entire root system is extracted, which is easiest when the soil is moist.
Timing Prevention with Pre-Emergent Herbicides
The most effective long-term strategy for controlling winter weeds is preventing their seeds from sprouting using pre-emergent herbicides. These products do not kill existing weeds; instead, they create an invisible chemical barrier in the top quarter to three-quarters inch of the soil. As weed seeds germinate and encounter this barrier, the herbicide disrupts the cell division process, halting growth before the seedling can emerge.
Timing is crucial, as application must occur before weed seeds germinate in the fall. The ideal window is when the soil temperature consistently drops to approximately 70°F (21°C) for several consecutive days, which typically occurs in the late summer or early fall. After applying a granular pre-emergent, it must be activated by moisture, requiring at least one-half inch of water to establish the barrier. If the application is delayed until after germination, the herbicide will be ineffective, allowing the weeds to establish themselves.
Eliminating Established Winter Grass
Once a winter weed or grass, such as Poa annua, is visible, a post-emergent herbicide is necessary for elimination. This approach presents a challenge in warm-season turfgrasses because Poa annua is biologically similar to the desirable lawn grass when it is active. Selective control requires using specific active ingredients based on the type of lawn grass present to kill the weed without harming the turf.
For broadleaf weeds like Henbit and Chickweed, a selective broadleaf herbicide containing 2,4-D, dicamba, or mecoprop is effective and safe for most turf types. Controlling established Poa annua in warm-season grasses is more complicated and requires specialized active ingredients.
Selective Control Options
Products often required for selective control in Bermuda or Zoysia grass include:
- Ethofumesate (Prograss)
- Bispyribac-sodium (Velocity)
- Sulfentrazone plus metsulfuron-methyl (Blindside)
If the underlying warm-season lawn is completely dormant, a non-selective herbicide like glyphosate can be used for spot treatment. However, this method carries the risk of killing the desirable grass if it is not fully dormant.