Coastal and Tropical South
Crape myrtles and dogwood have begun to show "fall color", but the change in color and leaf drop may not be truly seasonal. Waterlogged roots cause plants to stress; leaves concentrate their nutrients back into the stems for survival, color up, and then drop. Drought stress sends the trees into a similar survival mode. Prune the affected plants back a bit next spring and, if leaf spot has also been a problem, spray new growth with a fungicide.
Rake and mulch
Garden sanitation takes center stage once the garden is secure from potential hazards. Cut down the weeds and pull them out, rake leaves as they fall, and rogue out the invasive plants like Jewels of Opar that you can't stand to kill completely. Tuck the beds in with a new layer of mulch around shrubs and perennials. When you're ready to add overwintering annuals and bulbs, it will be easier to plant through the mulch than to wait and mulch around the small plants.
Weak trees and shrubs
0-20-20 is a fertilizer formula we don't use enough, but should consider, especially in early fall. If trees have weak limbs, or just aren't growing, if shrubs haven't put on much new growth for several years, try fertilizing the root system. Consult the label for amounts, usually given in terms of the caliper of the plant's trunk. It does not take much, but a no-nitrogen formula can nourish roots without pushing top growth. When new growth does emerge, it benefits from the stronger root system.
The glory that is glossy leaf hibiscus can get a bit ragged this time of year. Plants may be overgrown, have a few bugs, or even stop blooming. Cut them back by about one third right now, fertilize with a flower formula, and watch for new leaves now and flowers by November. If whitefly or other piercing and sucking insects are present, spray thoroughly with Neem now and again in 8 days.
Transplant new lettuce and mesclun mix plants and sow another round of lettuce seeds now. By staggering the planting times over several weeks, you're sure to have fresh plants all winter. Pick a spot in the flowerbed or vegetable row that isn't in full sun. Work the soil well and rake very smooth, then sow seed on top. Press in, but don't cover. Use a floating row cover for best results. Sow a few pots of lettuce or mesclun, too, for clever holiday gifts.