Fall harvest crops are generally cool-weather crops, the same ones commonly planted in late winter or early spring for late spring or early summer harvest. Where autumn weather tends to be warm, some fast-maturing warm-weather crops such as snap beans, summer squash, and even quick-maturing tomatoes also can be planted in late summer for fall harvest.
Leaf lettuce, spinach, mustard, Oriental greens, arugula, cress, sorrel, and kale make good fall crops. Beets, carrots, radishes, and turnips are also good fall harvest crops. All of these crops can be direct sown in the garden in mid- to late-summer.
Summer cabbage, cauliflower, broccoli, Brussels sprouts, Florence fennel, endive, escarole, and collard greens are crops for fall harvest that are best started indoors and later transplanted out into the garden. Start these crops indoors in early summer for mid- to late-summer transplanting into the garden. (These crops can also be purchased as starts from a local nursery.)
Cool-weather crops sown in summer for fall harvest often perform better than when planted in spring. In mid- to late-summer, garden soil is already warm–not warming as in spring–and seeds germinate more readily. As well, transplants get a faster start in the warm season than in the colder part of the year. Cool-weather crops prefer to mature in cool weather; cool-weather crops planted in spring often come to maturity as the weather is warming, not cooling.
Cooling and cold weather enhances the flavor of many crops coming to harvest in autumn as plant metabolisms change with cooler temperatures. Sugars accumulate in plant tissues of cool-weather crops as temperatures decline and crops become sweeter flavored. Leafy crops such as lettuce are sweeter tasting in cool weather. Brussels sprouts, kale, and parsnips are noticeably more flavorful when exposed to cooler temperatures and even frost.
Planting date. To calculate the planting dates for fall harvest crops, determine the first average frost date in your area; count back the number of days to maturity for each crop you plan to sow or transplant; add an extra week or two to the maturity time allowing for cooler temperatures and shorter days which slow plant growth. For example, sow broccoli three months before the first expected hard fall frost.
In the Northern Hemisphere, gardeners in zone 5 and north can plant cool-weather crops for fall harvest in early July, zone 6 in late July, and zone 7 in August. In zone 8 and south, cool-weather crops for fall harvest can be planted in mid- to late-August and September.
An added advantage to mid- to late-summer planting is the decline in weed growth and a decline in insect pest populations with cooling temperatures.
Planting. Direct sow seeds in summer in the cooler part of the day when water will not quickly evaporate. Seeds and transplants should be watered thoroughly after planting; seedlings should be protected from harsh summer sun by shade cloth until average daytime temperatures begin to cool.
Seedlings started indoors should be gradually exposed to outdoor conditions–hardened off–just as seedlings are hardened off in spring. Set starts out for a few hours each day, protected by shade, until they become accustomed to outdoor conditions in a week or so.
Plant crops for fall harvest in blocks instead of rows. Crops planted in blocks are easier to keep watered and as plants mature their leaves will shade the soil and slow evaporation.
Harvest. Keep a calendar or notebook with planting dates and expected harvest dates to ensure you harvest your crops at their peak.
When frost is expected, be prepared to lift crops from the garden before they are damaged or place frost protection–such as frost blankets or plastic tunnels–over crops before a freeze occurs.
Carrots, leeks, parsnips, and turnips can be left in the late autumn and winter garden until needed; these crops should be protected by a one- to two-foot layer of mulch when freezing weather arrives. These crops should be harvested before the soil freezes solid.