For spring harvest, collards and kale can be started indoors up to 10 weeks before your last frost date, and transplanted or direct seeded outside 4 to 6 weeks before the last frost. For fall harvest, direct seed three months before your first frost date. Seedlings started indoors will not require a heat mat, as these cold-hardy crops will germinate in soil temperatures down to 40 degrees, but do make sure to use a grow light. When seeding or transplanting into the garden, choose a location with full sun, and prepare your soil by mixing in an inch or two of compost. Don’t plant in the same location where other brassicas have been planted within the last three years. Plant seeds ¼ inch deep and 1 inch apart, and thin to 12 inches apart once the seedlings begin to crowd one another (and eat the baby greens you thin out!) Collards and kale will begin to reach full size in about 60 days. Harvest a few leaves from the base of each plant each time you harvest, leaving at least two full-sized leaves at the top of the plant to give the plant plenty of energy to keep growing. Use a quick, twisting motion to snap leaves off close to the stem. When harvested this way, these plants will continue producing until they bolt in hot weather or are killed by a very hard freeze. Some varieties may bolt in summer while others will not (this is dependent on the weather too), but most will at least slow down growth and become more bitter tasting in the hottest months, which is why these crops are ideal for only cool season production.
Kale and collards (Brassica oleracea, Brassica napus, or Brassica carinata) are insect-pollinated biennials. In order to save true seed, they should be isolated from any other B. oleracea crops in flower at the same time by at least one-half mile. To trigger flowering, they require a cold period (called vernalization) followed by warmer weather. Different varieties have varying vernalization requirements, but most require 8 to 10 weeks at temperatures below 50 degrees, meaning you will likely need to overwinter a crop to produce seed. Ethiopian kale (Brassica carinata) is a notable exception, as it commonly bolts in hot weather after a typical spring planting. Growers in most locations can successfully overwinter kale and collards outdoors, since these hardy plants can typically survive temperatures down to 10 degrees Fahrenheit. As the plants flower in spring, they may require staking to prevent the flower stalks from falling over. Harvest the seeds by cutting whole plants once two-thirds of the seed pods have turned brown. Bring the stalks indoors to finish drying on a tarp, to catch seeds that are released from the pods as they dry. Once the stalks and pods are totally dry, thresh the rest of the seeds out by crushing the stalks underfoot on a tarp, or hitting the stalks against the inside of a clean trash can. Winnow out the chaff by pouring seed and chaff from one container into another in front of a box fan on a low setting. Store your clean seeds in a cool, dark, and dry location.