Cole crops are anything in the family brassicaceae (cruciferae) and include cabbage, kale, collard, broccoli, raab, cauliflower, bok choy, horseradish, mustards, cress, brussels sprouts, rutabaga, arugula, radish, wasabi, tatsoi, mizuna, broccoliflower, napa and the much maligned kohlrabi. I have read that these were all bred/selected out of European wild kale varieties over thousands of years. They are very nutritious and provide the eater with vitamin C, fiber, iodine and phytochemicals. Many are also high in calcium and the trace element boron.
Humans have made very good use of these plants for many millennia. In olden times cabbage was “Lacto-Fermented” and turned into sauerkraut. (carrots were also lacto-fermented) Before the advent of the Ice Box or refrigeration, it was more easily stored in this form and the fermentation process actually increased its nutritional value. Kraut “Juice” was once considered to be healthful delicacy.
Crucifers and Brassicas will grow well in any well-drained moderately fertile soil. They are however susceptible to boron deficiency. If you are having issues with them perhaps a soil test is in order. They prefer average moisture and full sun, for best results, weeds must be kept to a minimum. For smaller cabbage heads we recommend spacing them at about 12″ to keep them from becoming small planetoids. The only time nowadays that one would desire a very large cabbage head is if one is root cellaring them (leave them on the stem to hang in the root cellar.) or for making Kraut!
Again, for small cabbage heads, we recommend 12″ spacing, for larger heads, 18-24″ is advised. Broccoli is most often spaced at 18″, Brussels sprouts need elbow room so we recommend 24-30″ depending on variety. The long maturing ones need more room. Kale, collard, mustard and the like are most commonly spaced at 18″. The distance between rows should allow for you to walk and maneuver to maintain weeding and other crop maintenance, 36″ is usually good unless you grow giants.
Insects can be an issue, the yellow sulfur moth lays eggs which hatch into cabbage worms. A light row cover from transplant to harvest works great for cole crops. Thrips are incredibly small sucking insects and can damage cabbage. Flea beetles can decimate leafy green brassicas, leaving the plants looking like window screen.
Top recommendations: Fertile soil, lite row cover from transplant till harvest, space plants accordingly, keep weeds to a minimum, soil test every 2-3 years if plants fail to perform as expected. Water as needed during times of low rainfall.