USDA Zone Maps -- In all but the coldest regions (Zones 5 and colder), early fall is an excellent time to plant perennials, container trees and shrubs, and roses. This month, however, it can still be hot. Do the planting on a cool, overcast, or rainy day to prevent heat stress. See the USDA hardiness zone map.
Keep Your Lawn Looking Good In cooler regions (Zones 6 and colder), September also is an excellent month to reseed and repair lawns. You'll need to water daily until the seed has sprouted and established. In warmer regions where daily highs are still well and regularly into the 80s F, wait to plant grass seed until October in warmer regions when there are cooler temperatures and rain. Learn how to renovate your lawn.
Even though grass growth has slowed, don't let it get more than three inches tall. Apply a fertilizer and broad-leaf weed control to lawns this month. You can buy them in one formula, known as a weed-and-feed combination. Choose, if possible, one designed for fall application.
Tend to the Garden If mature plants are flopping, tie them up or use plant supports or stakes (crisscrossed like an X with ends inserted in the soil) to keep them upright and to prevent them from smothering neighboring plants.
Halt fertilizing of roses and perennials. It will only encourage tender new growth that will get zapped this winter.
Keep deadheading! You'll have more flowers longer, not to mention a nicer-looking garden.
Although this time of year it's tempting to forget about weeding, it's worth the effort to keep up with it!
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 nonplastic material over your annuals and vegetables. In fact, for vegetables, you can cover them indefinitely with any 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. Learn more about harvesting vegetables.
Attract birds to your garden by establishing their food sources now. See ways to make your garden more appealing.