If Weeds Could Talk

IPM Indicator Weeds in Home Lawns

By Mr. Ryan Adams, Extension Associate, Center for Integrated Pest Management, NC State and Dr. Danesha Seth Carley, Associate Professor,Department of Horticultural Science, NC State

All weeds have a story to tell. Their mere presence communicates valuable information on recent maintenance practices employed. Cultural practices that promote a healthy and dense turf stand are one of the most important and least recognized weed control tools. In fact, many pest infestations are triggered by problems associated with upkeep including mowing height and regularity, irrigation timing and frequency, and fertilization (usually too much or too little). Improper practices can stress the turf, giving weeds a competitive edge. Although herbicides may seem like a simple solution to weed problems, killing the weeds with an herbicide only treats the symptom without identifying the underlying cause.

In order to treat the cause of a weed infestation, it is important to consider prevention, mechanical and biological options, and cultural practices that encouraged weed establishment in the first place. We will take a look at common indicator weeds in North Carolina turf and unpack what their presence can tell you.

Mowing

Mowing is the most time consuming maintenance practice, and many aspects of mowing are misunderstood and in some cases, can be performed incorrectly. Because lawns often look attractive after mowing it is easy to assume that grass thrives on mowing. In reality, mowing is a very destructive process that injures the grass plant. Each mowing temporarily reduces root growth, decreases carbohydrate availability, increases water loss, and decreases water absorption by the roots. Therefore, it is important to minimize these stresses with sound mowing practices.

Ideally, the decision to mow or not to mow should be based on the growth of the grass, rather than a set schedule. As a general rule, mow as often as needed so that no more than one-third of the total leaf area is removed in a single mowing. For example, a bermudagrass lawn at a height of two inches should be mowed when the grass reaches three inches. If you are seeing dog fennel, chicory, blackberry, and wild carrot, it is likely you are mowing too infrequently, since most of these weeds are removed with consistent mowing. Conversely, if you mow your lawn too frequently, you will increase the likelihood of several weeds such as annual bluegrass, moss, crabgrass, chickweed, and goosegrass.

Soil Moisture

Proper irrigation timing and techniques play a critical role in weed establishment and encroachment. One of the most common issues is late-day irrigation, which increases periods of leaf wetness. A simple change to morning or early day irrigation will reduce the likelihood of several turf diseases as well as weeds that prefer wet conditions.

Dry Soils

Spurge, black medic, oxalis, goosegrass, bahiagrass, knotweed, bindweed and lespedeza are the most common weeds in nonirrigated areas. Drought-like conditions further encourage weed issues. The installation and proper use of an irrigation system can help prevent weeds altogether. Alternative options in drought prone or non-irrigated areas would include use of droughttolerant turf species such as bermudagrass or zoysiagrass.

Wet Soils

In contrast to dry soils, excessively irrigated or poorly drained sites are usually littered with annual or rough bluegrass, chickweed, dollarweed, doveweed, barnyardgrass, moss, sedges or kyllinga. These weeds are frequent inhabitants of areas near water bodies, drainage ditches, or low areas in a yard.

Yellow nutsedge grows rapidly during the hot summer months and is often found in wet or poorly drained soils before spreading to dryer areas. This warm-season perennial weed is common, and easy to identify. The growth habit similar to grasses except that it has a triangular stem, and typically will spring up quickly after a rain event. The upright grass-like leaves are light green to yellowish in color and shiny in appearance.

Soil Conditions

Compaction:

Turf areas that receive foot or tire traffic commonly become compacted over time. Soil particles become so densely packed and compressed, forcing air, water, and roots from pore spaces. With poor water infiltration and anaerobic conditions, the remaining roots become shallow and thin, making it difficult for them to obtain necessary water and nutrients.

The best way to combat compaction is through aerification, which opens channels that allow for the reintroduction of oxygen and water into the root zone. Simply adding an annual aerification program will increase plant health and rooting and reduce the occurrence of common weeds that thrive in compacted soils such as annual bluegrass, annual sedges, goosegrass, spurge, knotweed and broadleaf plantain.

Fertility

There are fourteen elements that are often referred to as mineral nutrient elements and are generally obtained in the soil via root extraction. The quantity of most of these nutrients contained in the soil is high compared with the requirements of turfgrass. However, inadequate or excessive nutrients affects weed pressure. The two most common nutrients that influence weed populations are nitrogen and phosphorus.

Infertile soil due to low nitrogen:

When fertilizer has not been added for several years, black medic, clovers, and moss tend to dominate by pushing out the turf, and spreading readily. A single nitrogen application would encourage turf growth and reduce weed pressure.

White clover is an obvious indicator of an infertile soil or under-fertilized area. Clover is unique in the fact it fixes its own nitrogen, allowing it to outcompete an insufficiently fertilized turfgrass species. White clover is recognized by its trifoliate leaf with three rounded leaflets and a white flower that blooms through much of the growing season.

Excessive Nitrogen and phosphorus:

In contrast, overfertilization can lead to turf decline and increased presence of annual bluegrass, bermudagrass, chickweed, ryegrass, and purslane. Annual bluegrass is the most notorious invader of high nitrogen, high phosphorus soils and continues to be one of the most difficult to eradicate in managed turf.

Weeds are generally more adapted to adverse growing conditions than desirable turf species. By understanding the underlying causes, monitoring weed species and correcting cultural practices, it is possible to discourage weeds and increase the quality of your desired species. Any factor that will improve the overall health of the desired species will reduce weed pressure. Correcting the maintenance issues will not eliminate all weeds, but will go a long way in reducing weed pressure.

When it comes to weed control, it really is true that an ounce of prevention is better than a pound of cure. First and foremost, it is a good idea to prevent weedy pests from sprouting in the first place. The easiest way to do this is to ensure that your lawn is thick and full, which makes it very difficult for weeds to thrive. By understanding which weeds you have, and listening to what they are telling you, you can potentially diagnose and solve your own weedy turfgrass challenges.