Mistake #1: Feeding the plants. Adding fertilizer too late in the season will encourage new growth that may not have time to mature before the temperature drops. These tender additions will almost certainly die with the first frost -- and could bring the rest of the plant down with them. Let your garden play out the rest of the season as-is. Feel free to hit your lawn with a fertilizer high in potassium, though, which will stimulate root growth below the surface.
Mistake #2: Maintaining a consistent watering schedule. The nights should be getting cooler and the days shorter, meaning plants need less water to maintain good health. Let nature take it from here -- slow and then stop supplemental watering as fall approaches.
Mistake #3: Forgetting to prune. Perennial plants -- especially herbaceous perennials like coneflowers, asters, and grasses -- need to be cut back each year, but when you choose to do it depends on your gardening style. Some folks like to lop it all off after the first frost, leaving a clean, mulch-sprinkled garden ready to bloom anew next spring. Others wait until the thaw, leaving plants in the ground to feed birds, insulate roots, and gracefully model the frost. A final (very active!) camp cuts back bit by bit, as plants fade and flop and generally stop looking pretty. I either trim as I go or wait until spring, but I understand the desire to get it out of the way early.
Mistake #4: Raking up the leaves. Put away that rake! Regardless of what you choose to do with your lawn, there’s no reason to remove all the leaves from your garden bed. That organic matter will break down into humus -- which is great for the soil -- and insulate roots over the winter. Leaf debris does make for a messier aesthetic, but I'd say the benefit to the plants and soil outweighs visual appeal.
Do you agree? What are your best fall gardening tips?