September starts out as an extension of summer, with mild, sometimes hot days and warm nights. Near the end of the month, however, the Fall Equinox occurs, which means equal hours of day and night, the longer night allows more cooling. At about this time it is time to shutter your tropical trees inside.
Sometime during this month or next we'll have our first frost, and our hardy trees will begin to shut down. Deciduous trees will soon show autumn colors. Give all your trees more sun now, as the days are shorter and the sun is less intense. Adjust your watering to the cooler conditions, as your trees will need less water, and there will be less evaporation.
Minor pruning of unruly growth is fine, but major reshaping should be left to late winter or spring, and any drastic root pruning to spring. This is a good time to re-pot fruiting trees.
Fertilizing can continue with low nitrogen or organic fertilizers. If you have pines, fall fertilizing instead of spring will help to shorten the needles.
This is also a good time to take hardwood cuttings and graft cedars. It's the last opportunity to sever air-layerings, as any later they wouldn't have time to establish themselves before the onset of cold weather.
Insects, snails, and slugs are still active, so stay alert. Mildew may be a problem in the fall. Lime-sulpher or a good mildew-cide will help.
This report was done by Ray Perry (edited by Andy Arnault). Reprinted from the CCBC Horticultural Almanac