Many cool-weather vegetables planted in spring and harvested before summer also can be planted for harvesting in the fall. When planting in July and August, the plants will need to be at least twice as deep in the soil to keep the germinating seeds cool and protect them from summer heat.
Determine the average date of the first fall frost in your area. This information is available from your County Extension Office or in virtually any seed catalog. Add a week, then subtract the number of days it takes for the vegetables to mature (found on the seed package). This will tell you when to plant so that the vegetables have ample time to mature before frost. Many areas enjoy several weeks of frost-free weather after their first fall frost. If you cover your plants when frost is expected, they will not freeze and will continue to grow, taking advantage of several weeks of warm days before freezing weather arrives to stay.
Plant slow-maturing vegetables in mid-July in most areas. These are varieties that take up to 90 days to reach maturity. Vegetables such as rutabagas, parsnips and carrots should be direct-seeded by this time. Start seeds of Brussels sprouts, cabbage or cauliflower in early July and transplant to the garden by the end of the month.
Plant vegetables that take about 60 days to mature in early- to mid-August in most areas. These include turnips, kohlrabi, early cabbages, collard greens, broccoli, Swiss chard and kale. Plant head lettuce, such as romaine, and daikon radishes at this time, too.
Plant fast-maturing vegetables by the end of August or early September in most areas. Fast-growing cool weather crops such as radishes, leaf lettuce, mesclun, mustard greens, arugula and spinach all will reach harvestable size in 30 to 45 days.
Give cool-weather, fall-harvested vegetables at least 1½ inches of rain per week. If your area experiences an extra-long hot spell, shade the seedlings with cloth or potted plants set among them so they don't wither in the summer sun.