Every lawn will eventually sport some weeds, but not every lawn will be taken over by them. The weeds most likely to kill out grass are also the least likely to become a threat to your lawn if you practice a comprehensive, regular maintenance schedule.
Perhaps the most visible and well-known lawn weed, dandelions (Taraxacum officinale) are prolific growers and seed makers. They quickly spread to fill in bare patches in a lawn, especially if the grass is not regularly maintained. Unlike most perennials, dandelions produce flowers during their first year of growth and flower from early spring through late fall, with seedlings developing all season long. They have a deep tap root and will re-grow if even a small piece of their root is left in the ground. Control dandelions by hand-pulling, spraying with a selective herbicide specifically for broadleaf weeds, and by maintaining a thick, healthy lawn through regular mowing, watering and fertilizing.
Inhabiting disturbed land in urban and rural areas, broadleaf plantain (Plantago major) grows even on compacted soil where nothing else grows. It produces a basal rosette of lance- or egg-shaped leaves, and produces a leafless spike of small, inconspicuous flowers. Plantain is a prevalent weed in lawns that are overwatered or poorly maintained, especially if the soil is compacted. Treat plantain with a selective broadleaf herbicide along with improved turf management. Reducing the amount of irrigation, aerating the soil and mowing consistently all help prevent plantain from killing out grass.
An annual grass native to Eurasia, smooth crabgrass (Digitaria ischaemum) is commonly found in lawns, landscapes and gardens. It is a low-growing grass that spreads by rooting at the nodes and by seed. Even when mowed as short as ¼ inch, crabgrass can still produce seed. Young leaves are smooth, light green and stand out in the lawn. The mature leaves are smooth, darker green and up to 1/3 inch across. It often forms patches, or clumps, in lawns. Control crabgrass with a pre-emergent herbicide in early spring to prevent the seeds from germinating and a post-emergent herbicide in summer when crabgrass is in active growth. Cultivating a vigorous lawn will also reduce the possibility of crabgrass taking over. These cultural practices include selecting the best variety of grass for your area, yearly overseeding to keep the lawn thick, mowing to the correct height for your grass variety, proper irrigation and fertilizing at the correct time of year.