Weeds as Indicators

Question: Are weeds killing my lawn?

Answer: Weeds are not killing your lawn. Weeds are plants that are taking advantage of the open space created by weakened turf grass. Weeds, like all plants, vary in their tolerance of specific site conditions. Shade, compacted soil, wet soil, dry soil and soils with low pH or fertility can severely limit the growth of turfgrass. If you have an abundance of a particular weed, it could be providing clues about the site conditions challenging your lawn. Please see Table 1 for a more complete list of indicator weeds.

Shade

Turfgrasses vary in their tolerance of shade. A once beautiful lawn will begin to thin out, as established trees expand the area of shade cast. Some weeds are very tolerant of low levels of light. If speedwells, violets, ground ivy or common chickweed dominate your landscape, it may be time to consider switching to a more shade tolerant turf grass. For more information on selecting a turfgrass and growing grass in the shade refer to HGIC 1223, Turgrasses for the Carolinas and HGIC 1202, Growing Grass In Shade.

Field speedwell.
Chris Evans, University of Illinois, Bugwood.org

Common violet.
Karan A. Rawlins, University of Georgia, Bugwood.org

Compacted Soil

Compacted soil impedes the growth of your lawn by restricting the amount of oxygen available for the roots of your turfgrass. Types of lawn grasses vary in their tolerance of compacted soil. Centipede does not tolerate compacted soil very well and can thin out in high traffic areas. Weeds like goosegrass, annual bluegrass and prostrate spurge can indicate that compaction has become a problem in your lawn. For information on remediating soil compaction, please see HGIC 1226, Turfgrass Cultivation and HGIC 1200, Aerating Lawns.

Annual Bluegrass.
Ohio State Weed Lab, The Ohio State University, Bugwood.org

Red Sorrel.
Theodore Webster, USDA Agricultural Research Service, Bugwood.org

Low Ph

The presence of red sorrel in the landscape indicates that the soil is acidic. Take a soil sample and apply lime as directed by your results. Turfgrasses vary in their tolerance for acidic soils and have different pH requirements to maintain healthy growth. For more information on taking a soil sample, please see HGIC 1652, Soil Testing.

Wet Soil

Wet soils inhibit turf grass growth by restricting oxygen to the root system of the lawn. Poorly drained or over-irrigated soils tend to be dominated by yellow nutsedge, mouse-ear chickweed, annual bluegrass and Virginia buttonweed. Over-irrigation is a common problem in home lawns. Established lawns only require one inch of water a week. To determine how much water, you are applying through your irrigation system please see, HGIC 1802, Landscape Irrigation Management Part 3: How Much Water? For other information on irrigating the lawn see, HGIC 1207, Watering Lawns and HGIC 1225, Conservative Turfgrass Irrigation.

Yellow nutsedge.
Howard F. Schwartz, Colorado State University, Bugwood.org

Florida pusley.
Rebekah D. Wallace, University of Georgia, Bugwood.org

Nematodes

Nematodes are microscopic worms that damage the roots of turfgrass. A severe infestation can cause the lawn to thin in areas. The presence of prostrate spurge, Florida pusley and prostrate knotweed could indicate that nematodes have become a problem for your grass. The Plant and Pest Diagnostic Clinic can test a sample of the soil from your lawn and provide a count of the nematode species present to determine if they are limiting turfgrass growth in your landscape. For information on submitting a nematode assay, please follow the link https://www.clemson.edu/public/regulatory/plant-problem/nematode-assay-lab/index.html

The most efficient means of controlling weeds in the lawn is by using good maintenance practices. Correcting environmental factors, such as compacted soil or over-irrigation, can lead to increased turf grass vigor.

Table 1. Weeds as indicators of Soil Conditions

For more information on managing weeds see HGIC 2300, Grassy Weeds; HGIC 2301, Broadleaf Weeds; HGIC 2312, Nutsedge; HGIC 2322, Lespedeza; HGIC 2324, White Clover; and HGIC 2325, Annual Bluegrass.