Repair the Lawn -- In cooler regions (Zones 6 and colder), September also is an excellent month to reseed and repair lawns. You'll need to water as often as daily until the seed has sprouted and established. In warmer regions where daily highs are still regularly into the 80s F, wait to plant grass seed until October in warmer regions, when there are cooler temperatures and rain.
Whack Your Weeds -- Although this time of year it's tempting to forget about weeding, keep up with it. There's an old saying about weeds that one year's seeding means seven years' weeding.
Deadheading 101 -- Even now, keep deadheading! You'll have more flowers longer, not to mention a nicer-looking garden.
USDA Zone Maps -- In Zones 3 and colder and at high elevations, your first frost is likely to come this month. Stay tuned to the television and newspaper forecasts to find out exactly when. Prolong the growing season by throwing a sheet or other non-plastic material over your annuals and vegetables. In fact, for vegetables, you can cover them indefinitely with any other very light landscape fabric and anchor the corners with bricks or stones. It lets in sun and rain, but prevents light frosts from doing any damage.
Bird Feeding -- Fall is the time overwintering birds establish their food sources. If you haven't already, put out your bird feeding equipment.