Controlling weeds in summer lawns
Summer annual grasses continue to be pervasive weed problems in many turfgrass areas throughout the Lehigh Valley. The most common summer annual grasses in turf include crabgrasses (Digitaria spp.) and goosegrass (Eleusine indica). Satisfactory control of these weeds can be obtained by cultural methods, provided the life cycle of the plant is understood.
Two species of crabgrass, the hairy or large (Digitaria sanquinalis) and the smooth or small (Digitaria ischaemum), are commonly found in Pennsylvania. Both are true summer annuals. Their seed germination period ranges from mid-spring to midsummer, and all plants are killed by frost in the fall.
Flowering and subsequent seed set take place from midsummer to early frost and are the means of perpetuating the species. Seed can be produced at mowing heights as low as one-quarter inch. Abundant quantities of seed are produced, varying in number depending on the general health and vigor of the plants.
Once established, crabgrass plants tolerate high temperatures, compact soils, and dry soils better than most turfgrasses. They do not survive shaded conditions produced by buildings, trees and shrubs, or dense turf.
Crabgrass control cannot be completely accomplished in one growing season because of the great number of viable seeds in the soil from previous years of infestation. The basic principle of crabgrass control is to prevent re-infestation through seeding. If seed production is controlled for several years, the viable seed supply in the soil will diminish until it is no longer a serious threat to the lawn.
Goosegrass, also known as silver crabgrass, is commonly found in the Lehigh Valley and is often mistaken for crabgrass. Like smooth and hairy crabgrass, it is a summer annual, but it germinates four to six weeks later than crabgrass.
Goosegrass is characterized by fibrous roots and very flattened sheaths that have a silvery-green color, especially near the center of the plant. It has fingerlike seedheads bearing seeds with a zipper-like appearance on the seed stalk. Goosegrass grows well on heavily compacted soils.
Any management practice that increases the density and vigor of desirable turfgrasses tends to discourage competition from weeds. Cultural practices for the control of summer annual grass weeds are aimed at shading and crowding the young weed seedlings by producing a dense sod.
Effective cultural control measures include the proper selection and establishment of turfgrasses, adequate liming and fertilization, proper mowing practices, judicious watering, and insect and disease control.
Turfgrasses that are not adapted to the environmental conditions and intended use of the turf may become weak and result in a thin stand. When there are voids in the turf, weeds have an opportunity to grow and compete with the desirable species. The use of proper establishment procedures helps ensure a dense turf that will compete with germinating weed seedlings.
Inadequate liming and fertilization lessens the competitiveness of turfgrasses, resulting in reduced density and subsequent weed invasion. Complete soil testing is the key to proper liming and fertilization. Soil testing can provide guidelines for fertilization and liming to establish and maintain turf grasses.
Adequate nitrogen should be supplied to favor the desirable species in the stand. Phosphorus fertilization increases seedling vigor and is one factor in reducing weed infestations in newly established turf. Liming keeps the soil from becoming too acid.
Improper mowing is one of the most common causes of weed invasion. Mowing heights that are too short result in weakened turfgrasses. Most lawns should be cut at least two inches or higher.
Improper watering also contributes to summer annual weed invasion. Frequent light watering encourages shallow rooting and promotes weak turf, which becomes susceptible to insect and disease attacks as well as damage from traffic.
Frequent light watering also encourages germination and development of crabgrass and goosegrass at the expense of turfgrasses. Watering deeply (four to six inches) just before the turf begins to wilt is a practical approach to a sound watering program.
Summer annual grass weeds are extremely opportunistic, filling in voids in turf caused by diseases and insects. Diseases can be controlled by cultural practices and with fungicides.
Insect damage can be reduced by maintaining a healthy turf and using biorational means of control, such as using endophyte-containing ryegrasses and fescues that discourage leaf- and stem-feeding insects.
There is pre-emergent and post-emergent chemical control. Chemical weed control with herbicides can help you produce a quality lawn.
It should not be undertaken unless accompanied by an adequate management program designed to prevent re-infestation. To use herbicides safely and successfully, read the manufacturer's label carefully and follow directions.
"Growing Green" is contributed by Lehigh County Extension Office Staff and Master Gardeners. Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-746-1970.