Excessive rain can lengthen the germination period for crabgrass
seed, and it can speed the chemical and microbial degradation of the
herbicide. Also, a thatch layer over a half-inch can interfere with
the performance of pre-emergent herbicides. Thatch is the
accumulation of sloughed off stems, rhizomes and roots that is the
natural by-product of grass as it grows. You can find it between the
soil surface and the crowns of the grass plants.
activity that disturbs the herbicide barrier – digging, core
aeration, dethatching – will also decrease the effectiveness of
pre-emergent herbicides. Environmental conditions, especially cool
soil and drought, can delay the germination of crabgrass seed beyond
the effective residual of the herbicide. Once the rain quit around
mid-July, we did have an extended period of hot, dry weather.
Core aeration helps when done once or twice a year
Finally, the best defense against all kinds of weeds is a dense,
thick lawn. Proper fertilization, irrigation and mowing can have a
huge impact on weed populations.
Allowing your lawn to grow at least 3 inches tall can help shade out
germinating weed seeds, and it also encourages the grass to have a
deeper, more extensive root system. Managing insect and disease
problems helps keep the lawn from thinning out, which keeps the
weeds from moving in.
Spot-seeding your lawn
Mulching leaves when mowing
September is lawn month