Crabgrass easily swamps newly planted lawns. Ample water and a lack of competing turf spurs crabgrass growth. The warm-season annual basks in early summer temperatures that stress fescue or bluegrass. Unfortunately, most crabgrass killers will also kill your newly seeded lawn.
Do not use pre-emergent crabgrass killers when planting grass seed. Most pre-emergent herbicides kill germinating seeds, whether the they are turf seeds or weed seeds. The exception is siduron, a pre-emergent crabgrass herbicide that does not affect bluegrass, ryegrass or fescue.
Broadcast post-emergent herbicides are most effective on small, young crabgrass patches but also harm new turfgrass seedlings. Wait until you have mowed the new grass three times, or spray individual crabgrass clumps with a broad-spectrum herbicide. After the weed produces two to five tillers, or grassy branches, it is mature enough to shrug off most crabgrass killers.
New crabgrass plants stop emerging in late summer, and existing plants focus on setting seed before early frosts kill the annual. In contrast, late summer is an ideal time to plant new lawns, as cool-season turf grasses thrive throughout late summer and early fall. Seeding your lawn in fall avoids crabgrass and allows you to use a pre-emergent in spring.
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