Shorter days and lower temperatures mean that it's time to consider reducing the frequency and/or duration of your irrigation cycles. Runoff on driveways or sidewalks and persistant damp areas may be signs that you are irrigating more than necessary.
Stay on top of leaves and debris in your rain gutters and clean the sediment traps in your catch basins (drains) to insure that your drainage system flows freely once the rains start falling.
Remove fallen fruit and vegetables and compost or otherwise dispose of them to help prevent pests and diseases from gaining a foothold.
Fall is a great time to seed a new lawn or to repair bald spots in an existing lawn by overseeding. Lawn seed will typically germinate up through November (the cooler the weather the longer it will take) and fall temperatures make it easier to keep the seed bed properly hydrated.
Installing new plants in an existing landscape during the fall months allows them to avoid the heat stress associated with summer planting and helps them to become acclimated prior to the winter rains.
Fertilize all blooming plants with a 0-10-10 fertilizer in late fall to strengthen them before winter and help them to set buds so that they will bloom more vigorously the following spring.