Southern California Coastal & Inland Valleys
Sow or Transplant Lots!
Sow or transplant two or three times the amount you would for spring harvest, as these overwintering crops will grow very slowly, and you'll harvest only a leaf or two a week from each lettuce plant.
As some herbs and flowers reseed themselves, transplant them for overwintering elsewhere in the garden to give their roots access to richer soil. Dill, especially, seems to germinate better this way, so take advantage of it.
Signal Veggies to Finish Ripening
Pinch out new blossoms and growing tips of melons, winter squashes, and determinate tomatoes to force growth into the fruits that have already set. Any that set from now on won't ripen sufficiently before cool weather comes -- unless you want lots of immature green tomatoes around Thanksgiving. Indeterminate cherry tomatoes, on the other hand, can be allowed to continue setting, as the little fruits ripen more quickly.
Plant the Last Fruit and Shade Trees
Plant new trees while the soil is still warm to encourage the roots to get established before going dormant for the winter. Trim off deadwood and watersprouts (quickly-growing upright shoots), but leave major pruning for January, when the trees are dormant.
Plant California Wildflowers
Several companies offer individual varieties and collections of California wildflowers that are grouped by color or geographic area or other characteristics such as drought resistance.