Is there some secret to growing good fall crops of broccoli and other cabbage family crops? I start the seedlings indoors, but as soon as I set them out they get eaten by bugs or wither in the heat. If I wait for cooler weather, they won't have time to mature before frost.
As you have learned, the most challenging aspect of growing fall broccoli and other brassicas is getting them through the last weeks of summer. Try to have broccoli seedlings ready to plant 10 weeks before the first frost date. In Zone 7, either purchased or homegrown seedlings should be set out in mid-August, when hot, dry conditions often prevail. To be successful, you will need to use floating fabric row covers to protect the seedlings from army worms, cabbage loopers and other leaf-eaters.
In addition, a thick mulch will go a long way toward keeping the soil cool, and you can maintain moisture by using a soaker hose. For best results, use stakes or hoops to hold the row cover above the plants, which will keep them from getting overheated. The row cover will keep out the moths whose offspring become leaf-eating caterpillars, but you will need to check beneath the cover every week or so for clusters of green cabbage aphids, which sometimes emerge and feed on these plants. You usually can control them with insecticidal soap.
Broccoli, Brussels sprouts, cabbage, cauliflower and kohlrabi that mature in cool weather are remarkably tender and sweet, so growing a fall crop is worth extra trouble. The flavor of these vegetables is improved by a few light frosts, so don't think of your first frost date as the end of your fall garden. These cabbage cousins will continue to grow until hard freezes become frequent.