For warm-season lawns, late winter is the usual time of year to get started with a weed control program. This generally involves pre-emergence herbicides that will control a lot of lawn weeds such as crabgrass. This protective herbicide will attack these weeds when they begin to germinate. Weed control may also involve the use of post-emergence herbicides that will control weeds already actively growing.
Dormant lawns should continue to be watered during dry periods to prevent winter injury. Prolonged dry conditions can kill warm-season turfgrasses, even when dormant. Although it may appear dormant, St. Augustine survives the winter in a non-dormant state and does not have below ground shoots. This makes it particularly susceptible to injury due to dry conditions.
While watering is important for winter lawn care, so is weed control. This is because soil moisture is reduced by weed infestations that use up what little water is available. A procedure commonly used in bermudagrass lawns during February is to apply a product containing glyphosate to control all weeds that are green at the time.
Glyphosate is the active ingredient of products such as Roundup or Roundup Pro. Because of the weather conditions we've been experiencing, application of these products should be done with extra care. Bermudagrass is normally dormant during February, meaning that no green leaves are found on the plants. This turf will be sensitive to the glyphosate treatment and its use may result in turf injury. If you are considering using this treatment, it would be wise to check the lawn carefully for areas showing green shoots.
If you find the bermudagrass is showing green leaves, consider using other post-emergence products such as 2,4-D type herbicides to control winter weeds. An additional note is that the glyphosate treatment is approved and recommended only for bermudagrass and should not be used on other warm season turf species such as St. Augustine, zoysiagrass or centipede. Some of these species are very sensitive to glyphosate and its use may cause severe lawn injury. If you are not sure what grass species you have, contact your County Extension Office or ask a lawn care professional to give you an estimate to help identify your grass type correctly.
If you decide not to use glyphosate, you have other choices. Of the various weeds growing in the lawn at this time, the broadleaf weeds can be controlled with herbicides in both warm season or cool season grasses. Some of these weeds are just unsightly, others can be a serious nuisance. A case in point is a weed called spurweed or lawn burweed. If you have this weed you will find bothersome burs appearing in the lawn in April or May. The problem with burs is that they don't show up until later on, but now is the time to take action.
Burs are a tiny plant with parsley-like leaves that seem inocent enough. The concern with this weed is the spiny seedheads or burs that the plant produces. Once these burs form there is no way to remove them for the current growing season. Although you can kill this winter annual in April or May with a herbicide, doing so will not eliminate the burs already produced. The most effective strategy is to kill this plant before it flowers, or at least before seeds are set. Now is the time to use a broadleaf post-emergent weed killer to kill lawn burweed, prior to flowering/seed head set. Either Trimec (2,4-D + dicamba + MCPP) or Weed-B-Gone (some formulations identical to Trimec, others have only 2,4-D and MCPP) are effective as broadcast sprays. Use these products as directed on the label. This broadleaf treatment can also be incorporated into a tank mix with a glyphosate containing product as a part of a broad spectrum post-emergent winter weed control program in bermudagrass. As discussed earlier, check carefully to be sure bermudagrass is completely dormant!
Pre-emergence herbicides, those that control weeds at the time of weed seed germination, should generally be applied in February for maximum weed control in the summer lawn. Because of the milder winter temperatures this year, one might wonder if weeds will germinate earlier this year than normal. This is certainly a possibility. It is best to get preemergence products applied as soon as possible. Delaying application into March increases the chance of poor results from pre-emergence herbicides since weed seeds may germinate and establish before that time.
Apply these products when the soil is moist and follow with at least 2 light waterings. If applying a spray mixture of post-emergent and pre-emergence herbicides, wait about 3 - 5 days following spray application before watering the lawn. This will give the post-emergent herbicide time to begin working before irrigating to activate the preemergent herbicide portion of the mix.
Some lawns that haven't been watered may have suffered winter kill due to the dry conditions, even though temperatures haven't been particularly cool. In such cases, the use of certain preemergence herbicides may be detrimental to the recovery of the lawn. A better choice may be to skip pre-emergence application at this time. After the grass greens-up and allowed to become re-established, a post-emergence application of MSMA can be used along with a pre-emergence herbicide to prevent additional weed establishment.
Regardless of the weed control program you choose, best results will be obtained if it is used in combination with a good overall lawn management program.