It may seem a little early to be thinking about fall gardening, but this is the time to get started on cabbage-family members for a fall harvest.
Spring-planted cabbage and broccoli will be finished by the time the hot August weather sets in. By then the new crop, which you can sow directly into the garden, will be eady for transplanting into your spring patch.
Pick an area in your garden that's weed-free. It doesn't have to be a large area -- seeds can be sown closely, since you'll transfer them later to their final home. But the weed-free part is crucial. During the next weeks, when rainfall will be scarcer and the sun hotter, young seedlings must flourish uncontested -- they shouldn't have to compete for moisture and nutrition. Too, weeds grow very rapidly at this time of year, and they'll take over completely if allowed to get a foot (or a root) in.
I remember a time long ago when, knowing very little about these things, I gaily sowed a long row of broccoli and cabbage around about now, and promptly forgot, in the busy harvest of June and July, that I'd put them in. Pretty soon the area was covered with weeds. In a fit of garden cleaning some weeks later, I tackled the area and pulled weeds with abandon, to discover the long row of brassicas, which had, despite my complete neglect, germinated and reached heights of six inches. But the plants were so weak and spindly that just the act of weeding around them did enormous damage. Still, if I'd had the presence of mind to transplant them immediately, I might have gotten something out of my efforts.
Cabbage-family members thrive when transplanted as seedlings. They need to be moved to deep individual holes where their stems, when buried, will send out additional roots, giving plants the sturdiness they need to produce handsome heads.
A fall brassica crop doesn't have to take up any more space than the spring crop. Once you've sown your new crop, which can go in this weekend or the next few into mid-July, give the seeds a good drink of water, assuming recent rain hasn't made watering unnecessary. When you sow, keep in mind how many individual plants you planted in the spring and curtail your enthusiasm accordingly. There are going to be at least 50 seeds in each package -- you don't need them all.
My first crop of brassicas is mainly for eating fresh out of the garden. By the time the fall crop comes in, there's quite a variety of vegetables to choose from, and the bulk of the second crop goes into the freezer. I've found that a couple dozen broccoli plants and a dozen cabbage plants provide all we can eat during the summer, and that includes giving some away. So unless you're particularly fond of brassicas (including brussels sprouts and cauliflower, which also can go in now for a fall crop) or you are feeding 14 people every night, don't make the mistake of sowing an entire package of each. You can save the leftover seeds for next year.
Once the young seedlings emerge a week to 10 days after sowing, your only job is to keep weeds away and make sure they get adequate water. If you plant them this weekend, the young seedlings will be ready for transplanting about August 5. At that time, just pull up the old spring plants. But if your spring crop is still producing, thin the seedlings and transplant the hardy ones to any free space in the garden. Later, if you want to get them into the broccoli patch, you can transplant them again. Or wait to sow your new crop until a little later.
It's a good gardening practice to change sites of members of the cabbage family each year to prevent disease. But it's quite safe, within a given year, to plant similar crops in the same area.
Meantime, to ensure the health of your spring-planted brassicas, spray them with Bacillus thuringiensis (BT), a commercially available bacterial agent that affects certain caterpillars. If you see large holes in the leaves, these are the work of the cabbage-worm caterpillar, the larval stage of a white moth with black spots. BT is non-toxic and loses its effectiveness after just 24 hours. So keeping on top of spraying is important, but you don't have to do it more than once a week.