All weed species choke off the available resources lawn grass needs to grow well. Weeds use the sunlight, water and soil nutrients that grass plants also need, often overtaking the grass growth in the process. Grass that has been subjected to drought or excessive chemical fertilizer application is especially susceptible to choking weed infestation. Identifying weed species is essential to developing weed management practices. There are several keys to begin identification of lawn weeds.
Weeds such as dandelion, chickweed, buttonweed and clover have broad leaves with one vein from which smaller veins branch. Their seedlings develop into two opposite branching cotyledons (first leaves) with two true leaves that emerge next. Their growth habit is either prostrate, like knotweed, or upright, like knapweed. Their leaf forms are either in opposite pairs, whorled, single leaf or rosettes.
Sedge weed leaves are narrow and arranged in sets of three. Stems are triangular in cross-section. Sedge weeds such as purple nut sedge, globe sedge and watergrass grow in moist soil, often in thick clusters. They spread rapidly through rhizomes, tubers, basal bulbs or wind-born seed. They can grow up to two feet in height and often spread very aggressively in a lawn.
Grass weeds such as goosegrass, crabgrass, yellow foxtail and barnyard grass are more difficult to identify as weeds because they are similar to turf grass plants. Maintaining a dense turf grass is the best preventative practice for eliminating grassy weeds. Grassy weeds are identified by narrow leaves arranged in sets of two, with rounded or flattened stems. Their hollow stems have visible bulges at the joints where the leaves attach. They have either a bunching or spreading growth pattern by rhizomes or seeds.
Regular mowing with the blade set at three to four inches high is an important deterrent to weed growth. Mowing prevents many weed types from setting seed in the fall. Fertilization in late autumn and early winter is a key to interrupting weed growth cycles. Fertilizing in spring is not recommended because emerging weeds also benefit from the fertilizer.
A lawn established on high quality topsoil has thick growth which prevents unwanted weed germination. Weeds develop on soil with poor nutrient content or insect infestation. Turf grass fed with homemade or commercially available organic compost is able to resist weed infestation. Weeds also occur were grass is stressed from drought. Once the choking lawn weeds are identified they can be hand-picked or eradicated with a pre-emergent herbicide in early spring.