Three useful starting groups
Broadleaf weeds have leaves that differ from narrow turfgrass blades; dandelion and clover are familiar examples. Grassy weeds can resemble the lawn itself, so leaf collars, stems, seedheads and growth habit matter. Sedges can also look grasslike; a triangular stem is a useful clue in many common lawn sedges, but one feature alone is not a complete identification.
Take photographs that can identify a plant
Take one photo showing how the weed grows in the lawn and another showing leaves and stems with an object for scale. Include the collar where a leaf meets the stem, roots or underground structures where practical, and a seedhead if present. Record location and season. A picture of a distant brown patch cannot identify a weed.
Look past the familiar nickname
Crabgrass is a summer annual grass. Not every coarse clump is crabgrass: desirable turf out of place and perennial grassy weeds can require different responses. Dandelions and clover are broadleaf examples, while nutsedges need a sedge-specific identification. Use a regional weed key or local extension office to confirm the plant.
Decide what needs changing
A few unwanted plants may be removable by hand. Repeated weeds in the same spot can point to thin turf, wear or growing conditions that need attention. If considering an herbicide, confirm both the target weed and lawn species on the current label. A general “lawn weed killer” description is not enough.
Weeds and disease are different questions
This page previously mixed weed identification with disease advice. If the problem is discoloration, damaged leaves or expanding dead patches rather than an unwanted plant, use the brown-patch diagnosis guide. For known annual weeds, see germination and prevention timing.
Sources and local guidance
University guidance supports the species facts and care principles here. Rates and dates in a regional source apply to that region; they are not national prescriptions. Use your local extension office for cultivar, soil-test and treatment decisions.