Small-leaf lawn weeds can quickly overtake an unhealthy lawn, crowding out grass or creating unsightly bare spots. Annual weeds only return the following year if allowed to go to seed. Perennial weeds return every year and spread, creating a larger problem. Treat both proactively before resorting to toxic herbicides.
Lawn weeds fall into two general categories--grasses, such as crabgrass and broad-leaf perennial or annual weeds. Many broad-leaf weeds actually have very small leaves, but aren't classified as such. Annual broad-leaf weeds include chickweed, black medic, purslane and knotweed. These weeds spread by producing seed; otherwise, they leave bare spots in the grass that promote run-off and more weed growth. Perennial broad-leaf weeds, such as clover, ground ivy, clover and speedwell are tough to eradicate because they spread through underground runners.
Knotweed forms mats of coarse, grass-like stems with sparse leaves. Common chickweed also forms a mat and produces small, star-shaped flowers. Clover produces clusters of three leaves and purple or white blooms. Ground ivy has heart-shaped leaves that grow on runners. Speedwell produces white or blue four-petaled flowers and opposite leaves, while violets produce tiny, white, blue or yellow flowers and heart-shaped leaves.
Almost all broad-leaf weeds can provide insight into the growing conditions and potential problems of a lawn. For example, knotweed is usually found in hard, compacted soils, while an abundance of clover may indicate low fertility, according to the University of Illinois Extension. Ground ivy, otherwise known as creeping Charlie, thrives in shade and poor-draining conditions.
A healthy lawn is the best defense for lawn weeds, according to Cornell University. Plant a lawn species appropriate for your soil and garden conditions and aerate the soil annually to relieve compaction. Water deeply, but irregularly, to encourage deep roots. Mow the grass high to conserve moisture and crowd out weeds.
Control small weed infestations by hand weeding, advises Cornell University. After addressing any soil or fertility problems, treat broad-leaf weeds with herbicide if necessary. Herbicides are best applied in the fall, according to Cornell, when weeds temperatures are cool and weeds are storing up food for the next year's growth. Herbicides are most likely to circulate throughout the plant at this time, reaching and killing the roots. Use a selective herbicide that won't kill grass and follow all manufacturer's directions carefully.