In addition to extending your summer harvest, cool weather means you can plant a second crop of vegetables that specifically like cooler weather.
If you have areas of marginal sun that work okay for summer, but are beginning to be shaded more of the day as the sun moves lower in the sky, plant things that appreciate a bit of partial shade.
If you're fairly new to gardening, you may think August is the best time to plant your fall crop. However, to get the best mileage from your seed and soil, it helps to plan your succession of planting in the spring when your first seeds and transplants go in the ground. Some plants require a long period to mature, so need to be planted in the spring for a late fall harvest. Others, like lettuces and arugula can be planted at regular intervals, weather permitting, for harvest all three seasons until winter freezes put them out of business.
Germinating and transplanting in summer can be tricky. The heat and dry air can retard germination and kill young plants, so direct sow sprouted seeds in well-moistened soil. Cover with a moist vermiculite and peat mix to prevent crusting. Alternately, plant seeds that tolerate transplanting in moist flats in a partly shady corner of the garden. Be careful to make sure they kept consistently moist, but not wet. Once you transplant young seedlings use a row cover to protect them from bright, hot sun for a few days until they become acclimated.
The best vegetables for fall in most areas include
Arugula—Lovely in fresh salads, arugula is a gourmet treat that you can easily cultivate. With rich, well drained soil and good sun, you can harvest arugula up to the first freeze. Arugula also tolerates a bit of light shade, so take advantage of that as the days shorten. Direct sow and space closely 1" apart. Harvest baby plants to thin.
Bush beans—Tolerant of many soil conditions as long as it has good sun, your bush bean will produce as long as soil temps hover around 60°F. A raised bed or container in a sunny location may produce fresh beans almost up to the first frost.
Beets—Famous for both their tender greens and luxurious root, beets are often tolerant of a light frost. They like to be cool so starting beets in summer often results in a second crop. Once daytime temps drop below 80°F, direct sow seed about 1/2" deep and about 3" apart. Each seed is a cluster and produces multiple plants. Soil should be very loose and rich. Harvest at about 45 days.
Broccoli—If you have sandy loam, sun, and a bit of water, you can grow broccoli. Straight from the garden, fresh broccoli is superior to any you'll find in the market. Direct sow into a prepared, premoistened bed about 1/4" deep and space about 24" apart. Time sowing when soil is warm to spur germination, usually in August. Alternately, plant in flats then transplant at about 6 weeks. Broccoli is a relatively heavy feeder so plan to feed fish emulsion at regular intervals.
Cabbage—A hard frost won't stop your cabbage, which makes it a wonderful vegetable for fall and winter gardening. Like broccoli, it's a heavy feeder so plan regular doses of fish emulsion fertilizer. When soil temps hover around 75°F, direct sow seed 1/2" deep, every three inches. Thin to 12" between plants. Harvest when heads are large and firm.
Spinach—In late August or early September, weed a bed in full sun and moisten, then broadcast your spinach seed. Cover lightly with soil. Keep damp. Thin young plants as they grow, but keep an eye out for birds. Leaves will be ready for harvest in about five weeks depending on your climate.