Water as needed. Even though weather is getting cooler, plants still need adequate moisture. If the fall is dry, water well. Otherwise, plants might go into winter dehydrated and stressed, making them more likely to die out by spring.
Keep weeding. Late summer and early fall are when many of the worst weeds go to seed. Let them go now, and they'll scatter thousands of seeds all over your garden. Basic cleanup in late summer and fall will prevent far more cleanup and weeding come spring.
Pitch 'em. If any annuals are struggling this late in the season, just pull them up and put them in the compost heap. If a perennial is looking shot, cut it off at ground level and discard the foliage.
Start raking. Don't worry about leaves that collect around shrubs and perennial plantings--they'll actually protect your plants. But don't allow leaves to hang out for more than a few days on lawns. They suffocate the grass.
Stock up on bulbs. October is the ideal planting time for bulbs in the Midwest, but supplies can go fast. Buy bulbs when you see them and keep in a cool, dry place (65° F or under, if possible) until planting time.
Brighten your garden with mums. Buy them now in full bloom. Choose from either florist's mums, which aren't winter-hardy but are very tidy-looking with large flowers, or garden mums, which are more wild-looking but will come back again next year. Florist's mums are great for pots indoors and out, while garden mums are good for planting in the ground for a permanent display.
Consider cool-season annuals such as flowering kale and cabbage, pansies and lobelia. They will brighten bare spots and outdoor containers for several weeks to come.
Especially in the southern half of the Midwest, fall is a great time to divide and plant most perennials and roses. Plant trees and shrubs throughout the entire Midwest—just be sure to keep well-watered if the fall is dry.
Reseed problem patches or lay new sod as days grow cooler and fall rains start. There's a saying that beautiful lawns are made in the fall!
Check or adjust mower height. If you like shorter grass, lower your mowing height to about 2 inches for cool-season grasses, such as Kentucky bluegrass, ryes and fescues. Or continue to mow at the ideal height of 3 inches. With warm-season grasses, such as bermudagrass, creeping bentgrass or zoysiagrass, keep mowing at hot-weather heights--about 2 inches.
Fertilize cool-season lawns to encourage good root growth. Do not fertilize warm-season grasses.
Don't have a compost heap? Start one! It can be as simple as a length of chicken wire, wrapped into an oval or square and held loosely in place with a few long wooden stakes. You can get away with just one, but with most gardens, three works better--each in a different stage of decomposition. In large gardens, you might have as many as a half dozen.
Keep an eye out for frost. In the northernmost Midwest, the first frost can come as soon as the last week of September. However, for the rest of the Midwest, it tends to be mid-to late October. Cover annuals and other tender plants for the first few frosts to get as much enjoyment of your plants as possible!