Late September through early October is the best time to get your lawn back into good, healthy shape.
Our hot summers are hard on lawn grasses. If your lawn has more weeds than grass or obvious bare spots, try to identify the probable causes for grass die off before you take on the task of re-establishing a vigorous lawn.
Over- and underwatering and over- and underfertilizing lead to rampant weed growth (weeds thrive in conditions that kill lawn grass) and dead or puny lawn grasses. It's easy to correct these improper cultural practices. Adjust irrigation timing as the weather changes; water deeply but infrequently to encourage strong root growth; feed regularly and lightly during the grass's growing season.
One of the most common, but invisible causes of poor growth in any type of lawn grass is compacted soil. Irrigate your lawn for several minutes, then take a screwdriver and poke it into weedy, bare, or raggedy-looking spots in your lawn. If the soil is hard or dry (indicating that water is not penetrating), the soil is compacted and needs aeration to improve drainage.
A foot-driven aeration tool works well for small areas; you might want to rent an aeration tiller attachment for larger areas. The aeration tools or attachments pull out small dirt plugs. Let those dry out on the soil surface then break them up and rake them back into the lawn. Heavy traffic areas will need annual, even bi-annual aeration to prevent re-occurring compaction.
Thatch is a buildup of dead grass that forms a waterproof mat at the base of the grass blades. Scrape up a portion of grass and soil in spots where grass is not growing well. If there is more than a half-inch of thatch, rake out as much as possible, then aerate using the same tools and techniques described above.
Before reseeding, you'll need to remove rock, roots and dead grass and kill existing weeds. Post-emergent herbicides will kill the weeds, but be careful of drift onto nearby healthy grass and plantings.
Use a cardboard shield to protect desirable plants. Do not spray herbicides when temperatures are above 90 degrees and do not irrigate until the herbicide is dry to prevent runoff into our water supply.
After the weeds are dead and debris is removed, till the soil to a depth of six inches. Herbicides also kill soil-dwelling beneficial microbes. Amend the soil (with compost or products containing beneficial microbes if you've used an herbicide), level it, then spread seed evenly, cover the seed with a light topping of compost or a lightweight mulch, and water well. Keep the soil consistently moist until the new grass reaches its normal height.