The most effective weed control program involves maintaining healthy, dense turf. One of the required factors for most weed seeds to germinate is sunlight so the "shade" effect of a dense turf canopy is a very effective means of reducing seed germination. However, achieving the necessary turf density for weed control is often difficult if not impossible to do for both cool and warm-season grasses because they simply are not growing fast enough to fully cover in the months of March and April around Virginia. Thinner turfs are more prone to invasion by all weeds. For this reason, a spring pre-emergent herbicide application is another way to ensure a weed free turf.

Appropriate Timing

Careful observation can provide useful tips regarding pre-emergent herbicide applications. Perhaps the most important phenological sign is the blooming of the forsythia. Studies show that pre-emergent weed control response is optimized sometime between 100% forsythia bloom to 50% bloom drop. Of course, pre-emergent herbicides can be applied earlier if desired but the duration of control through the prime months for weed seed germination will be shorter with an earlier application date. 

Pre-emergent Herbicide Options

The following pre-emergent herbicides are most commonly available to homeowners and/or professional lawn care managers (see table below). Not all of these products will be available at the garden centers of large retail stores so be prepared to shop at your local garden center that specializes in turf and landscape products. The labels will tell you precisely what uses are legal with each product so be sure to read them carefully before you buy.

Professional lawn care operators have some unique options available to them that can be used at spring seeding events with some important restrictions. Tupersan (siduron) can be used for crabgrass control for spring seeding of cool-season grasses ONLY. It cannot be applied to seeded warm-season grasses. Drive (quinclorac) has a much broader label in terms of weeds controlled AND species it can be applied to because it can be used on most warm and cool-season turf grasses at seed establishment. Follow the label carefully with either product to maximize performance and minimize the potential for unwanted effects on your desirable turf grasses.

What about Weed and Feed materials? These products are often the first part of a 4-step program touting a great looking lawn. The ability to apply fertilizer and pre-emergent weed control in a single application is certainly attractive in this first part of the program, but there are still some things to consider in choosing the convenience of these programs. First of all, since most of the fertilizer carriers of the pre-emergent herbicide contain 70-80% water soluble nitrogen and 20-30% water insoluble nitrogen, there is going to be a significant growth and greening response with the product application. At the recommended use rate for these materials they are typically supplying approximately 1 pound of N/1000 sq ft. For cool-season grasses, this is not an unrealistic value IF this is the only application made in the spring in our transition zone climate (i.e. skip steps 2 and 3 in the program). The application timing on warm-season grasses for Virginia is somewhat troubling because the product must be applied before transition from winter dormancy to actively growing grass because of the need to get the pre-emergent herbicide out at the proper time. The N in the "weed and feed" would be much more beneficial to both roots AND shoots if it was applied after this transition. Certain parts of the commercially available 4-step programs simply do not fit optimal transition zone fertility management programs for either cool- or warm-season grasses.

Corn gluten meal is an organic pre-emergent herbicide with activity on crabgrass. However, the best crabgrass control it has provided in Virginia Tech research is 60%. Is that satisfactory? It depends on your expectations of turf quality. By its composition, corn gluten meal is also a 10-0-0 fertilizer and will be delivering approximately 1 lb of N per 1000 sq ft at its labeled application rate. This N will be mostly water insoluble, so that is a plus. However, this material, just like the synthetic weed and feed products, is a significant nitrogen source and must be managed accordingly.

With any pre-emergent herbicide, whether it is a weed and feed or pre-emergent material alone, it is important to water-in the product as soon as possible in order to establish the weed control barrier in the soil. Many synthetic products readily photo-degrade within the first 24 hours after application, so follow label directions for watering closely.

Post-emergence Herbicide Strategies

What about winter annual broadleaf weeds that are blooming now such as deadnettle, henbit, and chickweed? If these plants are in flower they have essentially completed their life cycle and there is no reason to treat them now. However, there are many perennial broadleaf weeds such dandelion that will soon initiate very rapid growth and can be controlled as temperatures warm. There are several two and three-way combinations of the traditional hormone-type herbicides (2,4-D, Mecoprop, etc.) that can be used. However, be careful with timing because many of your most desirable (and tender) landscape plants are just initiating spring growth, too, and they are especially sensitive to injury from these herbicides at this time of year.

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For more information about these and other landscape gardening topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.