Updated: June 25, 2019
Brassicas are a great group of plants to start your greenhouse adventures with—they’re easy to care for and they’re strong performers in the fall and winter, provided you can keep them from freezing. For greenhousers, this means very little heat input and big results. Broccoli, cauliflower, Brussels sprouts, cabbage, collards and kale are just a few of the delicious members of this very special vegetable family. The Brassicas go by other names; you may have heard someone refer to them as cole crops, cruciferous vegetables or mustards, but at the end of the day, they’re the same group. Collectively, they like cool weather and are easy for indoor gardeners, since they don’t require hand pollination or special treatment to produce inside. The parts of the Brassicas we typically eat include the leaves, immature flowers and roots.
Meet the Brassicas
Generally speaking, you can grow any Brassica in a medium with a pH of 6.0 to 7.0, provided you keep the soil moist. Plants perform best at temperatures between 50 and 68 degrees Fahrenheit (10 to 20 degrees Celsius), though some seeds will need to be heated above these temperatures to get a good germination rate. Choose a light, well-draining medium for your greenhouse-grown Brassicas and keep an eye on the temperatures in your building—midday temperatures can rise surprisingly high if your area gets a lot of winter sun.
Some common Brassicas and how to them from seed
Arugula – One of the many greens in the Brassica family, arugula adds a peppery flavor to salads and grows quickly under well-controlled conditions. You can get a continuous supply of young leaves going if you make new plantings every two or three weeks. Sow arugula seeds 1/4 inch deep, spaced at one inch intervals—they germinate at temperatures from 40 to 55 degrees Fahrenheit (4 to 13 degrees Celsius) and emerge in about a week.
Broccoli – Broccoli plants need to be maintained around 60 degrees Fahrenheit (16 degrees Celsius) when growing, but seeds will sprout in about a week in a wide range of temperatures, from 45 to 85 degrees Fahrenheit (7 to 29 degrees Celsius). Space seeds about three inches apart, about a 1/2 inch below the surface of the soil. Broccoli does well in containers, provided they hold at least three gallons of medium and drain well—wider containers are better for broccoli’s shallow roots. Start seeds intended as transplants six to eight weeks before your last frost date.
Broccoli Raab – This plant looks a lot like a cross between broccoli and kale, with both edible leaves and small edible flowers. Broccoli raab is flexible and will germinate at temperatures between 45 and 85 degrees Fahrenheit (7 to 29 degrees Celsius) in about a week. Sow seeds about an inch apart and 1/2 inch below the soil’s surface in a flat, thinning to about four inches apart before transplanting to three-gallon pots or taking them out to the garden after all threat of frost has passed.
Brussels sprouts – Brussels sprouts are one of those vegetables that gets better in the cold, but you should still protect your seedlings from frost. Many gardeners start Brussels sprouts in the summer for a Christmas harvest and again in the winter for a spring harvest. They’ll germinate in a week at temperatures from 45 to 85 degrees Fahrenheit (7 to 29 degrees Celsius) and are ideal for areas with three growing seasons. Time your seedlings so they’ll have about four months to grow between transplanting and harvest or keep individual plants at about 65 degrees Fahrenheit (18 degrees Celsius) in a three-gallon pot in the greenhouse.
Cabbage – The seeds of these leathery balls of leaves are easy to start in the greenhouse, germinating in about a week at temperatures between 45 and 85 degrees Fahrenheit (7 to 29 degrees Celsius). Sow them three inches apart and about 1/2 inch deep. Once seedlings have emerged, keep the temperature around 60 degrees Fahrenheit (16 degrees Celsius) to discourage splitting and bolting. You can transplant them into the garden at four to six weeks of age, provided the ground isn’t frozen. They also do well in a three-gallon pot in the greenhouse.
Chinese cabbage – If you prefer the crinkly texture of Chinese cabbage to the standard fare, you can just as easily grow these plants in your greenhouse as their cousins. Chinese cabbages are slightly pickier about low temperatures, though, so never let the seedlings experience temperatures below 50 degrees Fahrenheit (10 degrees Celsius) or they may prematurely bolt. You can start them in the greenhouse at temperatures between 50 and 80 degrees Fahrenheit (10 and 27 degrees Celsius) and expect emergence in about a week. Sow them 1/2 inch deep, about three inches apart, four to six weeks before transplanting. Pots work great if you select something with about a 14-inch radius—Chinese cabbage benefits from tight spacing.
Cauliflower – Although the cousins of the cauliflower are flexible and generous, cauliflower needs extra care, especially if you want to grow the white types. Look for a self-blanching variety if you’ve never grown white cauliflower before or stick to the colored varieties—cauliflower commonly appears in green, orange and purple. Germination will take place at temperatures between 45 and 85 degrees Fahrenheit (7 to 29 degrees Celsius) in about a week, but once seedlings emerge, a steady 60 degrees Fahrenheit (16 degrees Celsius) is ideal. Start transplants four to six weeks before you intend to plant, but keep them in the greenhouse until the soil is consistently above 50 degrees Fahrenheit (10 degrees Celsius). Cauliflower can also be grown under shades in three-gallon pots in a cooled greenhouse—high temperatures, bright sunlight and too much nitrogen fertilizer will ruin heads.
Collards – These tough cabbage greens are great for a Brassica novice. They tolerate both heat and cold and germinate from 45 to 85 degrees Fahrenheit (7 to 29 degrees Celsius) in just a week. Sow them 1/2 inch deep about three inches apart, six to eight weeks ahead of the last frost if you intend to transplant them to the garden. You can move the transplants out as soon as the ground can be worked, or plant them in 18-inch pots in the greenhouse for year-round harvesting. Keep your collards moist for best flavor.
Kale – People love kale for various reasons—ornamental kale is a popular landscape plant in the fall and winter, but the leaves of these plants can be eaten, too. When harvesting kale, choose young leaves for salads and boil or stir-fry older, tougher leaves. They grow exactly like collards, a very close cousin. Sow seeds 1/2 inch deep, eight weeks before the last frost for transplants—like collards, you can move them outside once the ground can be worked. Indoors, plant kale in 18-inch pots.
Kohlrabi – It looks nothing like collards or kale, but kohlrabi is a close relative of the two and should be treated the same way. If you plant them 1/2 inch deep, they’ll germinate in about a week when temperatures are 45 to 85 degrees Fahrenheit (7 to 29 degrees Celsius). The swollen stem of this strange-looking plant is ready to harvest in as few as six weeks after transplanting, but they are cold-sensitive, so keep them inside at about 60 degrees Fahrenheit (16 degrees Celsius) until all threat of frost has passed. This ornamental Brassica does well in three-gallon containers indoors and out.
Pak Choy: This is one of the easiest cabbages to grow indoors, since it can tolerate wider temperature fluctuations than other cabbages, provided it’s kept moist. Germinate seeds in about a week at temperatures between 50 and 80 degrees Fahrenheit (7 to 29 degrees Celsius), but keep seedlings around 60 degrees Fahrenheit (16 degrees Celsius) for best results. Sow seeds about 1/2 inch deep, one inch apart and four to six weeks before the last frost if you intend to transplant. Bok choy has been bred into a range of sizes perfect for indoor production, so choose a pot appropriately sized to the bok choy you’re growing—small varieties may do well with several plants in an 18-inch pot. Bigger varieties may need as much as three gallons of medium.
Radish: My favorite Brassica, the lowly radish can be found in a rainbow of colors and most varieties are ready for harvest in only a month. They do lovely indoors and out, but don’t try to transplant them. Instead, sow seeds in a tray deep enough for the radish you’ve chosen or in a patch outside. Seeds germinate at temperatures from 55 to 85 degrees Fahrenheit (7 to 29 degrees Celsius) in just four days and grow like mad in cool weather. No need to space radish seedlings out or cover them, instead sow them thickly every two weeks and thin the young radish greens into your salads as they grow, ultimately leaving two to three inches between plants.
Rutabaga: Brassica root vegetables are all pretty easy to grow, but none transplant easily. If you want to try rutabagas in your greenhouse, sow seeds into biodegradable pots at temperatures between 45 and 85 degrees Fahrenheit (7 to 29 degrees Celsius). Once seedlings emerge a week later, keep the temperature steady at 60 degrees Fahrenheit (16 degrees Celsius). As seedlings develop their true leaves, move two or three to a single large container, or plant individuals in eight-inch pots. They tolerate cold temperatures moderately well, but don’t let your rutabagas get overheated.
Turnip: Grow turnips just like rutabagas, protecting them from heat and keeping the greenhouse on the cool side. Sow seeds into biodegradable pots or directly into containers; they’ll germinate in about a week at temperatures between 45 and 85 degrees Fahrenheit (7 to 29 degrees Celsius). If you really like turnips, consider sowing seeds every two weeks through the cool weather for a more continuous supply. Keep your turnips moist and cool and you’ll be rewarded for your hard work.