Crop and Soil Environmental News, April 2008
Authors: Mike Goatley, Extension Turfgrass Specialist, Virginia Tech and Shawn Askew, Extension Turfgrass Weed Specialist, Virginia Tech
Accompanying the spring resurgence in growth of lawn grasses is a host of weeds that also are responding to warming temperatures and spring rains. There are both opportunities and concerns in postemergent (POST, i.e. after weed emergence) weed control in the spring. Let’s detail the best management practices in spring weed control that promote a healthy, thick lawn while protecting our environment.
What is a weed? A weed is simply defined as a ‘plant out of place’. A weed might be a broadleaf such as a dandelion or clover, or perhaps it is a grass, possibly even another turfgrass species. The primary reason a plant is considered to be a lawn weed is because it somehow distinguishes itself from the lawn grasses. While bright multi-colored plants in the landscape beds are very often desirable, most people value uniformity as a key characteristic of what defines a great-looking lawn. Weeds disrupt this uniformity because of differences in shape, size, and color. And of course, some weeds are serious pests and warrant control efforts because they are invasive (wiregrass, bermudagrass), potentially injurious (lawn burrweed) or poisonous (poison oak or ivy).
Identify the weed and determine why it is present. One must know what they are working with to understand control alternatives. For instance, many winter annual weeds (henbit, chickweed, bittercress, annual bluegrass and so forth) are in the last stages of their life cycle and will soon die as temperatures warm. A postemergent herbicide application now will likely provide some control, but not 100% kill because the plants are now quite mature and in the process of flowering and producing seed. After flowering, why treat with a postemergent herbicide? The plant will soon die anyway. In the future, a more reasonable plan is to treat winter annual broadleaf weeds in the fall (either with PRE or early POST materials) to optimize their control, and know that about the only way to chemically control annual bluegrass in lawns is by PRE applications in early fall.
Cool-season perennial weeds such as dandelion, clover, and plantains can be controlled in the spring with appropriately selected and timed herbicide applications. The key to success is knowing the weed and its life cycle to make an appropriate herbicide choice and application. Your local extension office can aid in weed identification and control alternatives and the internet provides ready access to a host of resources, most complete with color pictures of the weeds. Virginia Tech’s TurfWeeds.net features photos and descriptions of the region’s most prevalent turf weeds in all stages of growth and development.
Next, do some investigative work to better understand why the weed is present. Weeds exploit poor quality turf. The best weed control is a thick, healthy turf, so your detective work needs to uncover what factors are limiting turf performance. Are there soil-related problems such as pH, fertility, poor drainage, or compaction? Could the limiting factor be shade? Was the turf previously damaged by an environmental extreme or pest attack? Weeds possess incredible capacities to occupy and dominate environments where other plants can not survive. Therefore, we need to focus on providing growing conditions that optimize turf growth and development, and that is best done by making smart decisions with fertility, mowing, irrigation, etc.
What are some general guidelines in spring postemergent weed control. It has been said that “weeds begat weeds” and it is true! Once a weed dies (either naturally or by herbicide control), the opening in the turf canopy is an open invitation for the next season’s weed seed to germinate. A preemergent (PRE) herbicide can be used to control germinating weed seed, but consider that most of these products will also control any grass seed that has (or is) to be applied. If a PRE is not used, be prepared to apply POST herbicides later this spring after you get the turf actively growing and before the weed gets very large. With patience and proper management, lawns can eventually be restored to their desired quality levels.
Choose wisely when selecting postemergent herbicides. The label specifically indicates what chemicals are in the product, how to properly and safely apply them, and what weeds will be controlled. Particularly in the spring, there are many ‘weed and feed’ products available on store shelves containing a variety of herbicides. Be sure to distinguish the products that are PRE (traditional ‘crabgrass herbicides’) versus those that are POST (traditional ‘dandelion herbicides’). A PRE does nothing to control an existing weed and a POST is useless to control germinating weeds. And the importance of carefully following the label can not be overstated in terms of product performance and safety to you, the turf, and the environment. Are there any restrictions on grasses to which the product can be applied? Is irrigation required or is it detrimental? Any restrictions on air temperature during application? These are but a few of the important considerations. For instance, many standard POST broadleaf herbicides will seriously damage warm-season grasses if applied during spring transition. Most broadleaf herbicides require dew on the leaf surfaces when applied as granules to insure absorption into the leaves. The label points out these precautions. Similarly, these same POST broadleaf herbicide products can be very particularly injurious to desirable ornamental plants following spring treatments if they are not applied carefully. Follow the label and you will achieve the desirable response with virtually no risk of undesirable consequences.
Where can I get more information on specific weed control products for lawns? In addition to your local extension office, log on to the Virginia Tech website (www.vt.edu) and search for ‘PMG’ to bring you to the 2008 Pest Management Guide. Choose the ‘Home Lawns’ tab for access to the latest pest control recommendations from Virginia Tech scientists. Another option is to search for ‘Turf and Garden Tips’ to access a podcast blog site specifically developed to serve homeowner needs in responsible lawn and landscape management programs.
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