Kale, Collards, Chinese Mustard, Spinach, Mizuna, Mibuna, Horseradish, Radishes, Kohl Rabi, Turnips, and Swedes.
Except for horseradish and radish, all of these members of the cabbage tribe are subject to club root. Consequently, they should not be sown in the same ground more frequently than once every three years. Radishes left to run to seed will get club root, but when harvested at the edible stage, are not in the ground long enough. Horseradish is a perennial and appears to resist the club root. Kale, Kohl Rabi, turnips and swedes are all prone to attack by grey aphis, particularly in the late autumn when the ladybirds are dormant. Thoroughly soaking the leaves with a spray of soapy water will reduce their numbers to a tolerable level, as will a hard frost. Pyrethrum works well, though it will kill any active chalcid wasps that parasitise aphids.
Except for swedes and turnips, all require well-composted soil with plenty of calcium. Boron deficiency causes brownish stains inside the roots of swedes and turnips, so supplement your soil with seaweed, or seaweed meal if this happens. Foliar sprays of liquid seaweed and/or fish emulsion will help.
Kale and collards are hardy, over wintering loose leaf cabbages that are grown for their leaves. Kale will stand heavier frosts than almost any other garden vegetable. As well as plain green varieties of kale, there are yellow and red variegated varieties that look very attractive in the flower garden. Kale is sometimes called Borecole. The leaves are tougher than cabbage and require a longer cooking time. Sow the seed in a seedbed or flat for transplanting in late summer, or early autumn. The plants should be in rows about 750 mm apart with 500-750 mm between plants.
Chinese mustard spinach resembles silver beet, except the thick, juicy stalks are smooth, rather than ribbed. The stalks are the main reason for growing this delectable vegetable. While celery is suggested as a substitute in Chinese recipe books written for westerners, it is totally different in flavour, if not in texture. Unlike silver beet or genuine spinach, Chinese mustard spinach is low in oxalic acid and hence has superior flavour. The plants grow rapidly and will produce year round when sown every few weeks. The only time to avoid sowing is in the depths of winter, though plants sown earlier will be productive then. Sow the seed 10 mm deep and allow 300 mm between rows and plants.
Mibuna and Mizuna are Japanese brassicas grown for their foliage. They have a more delicate flavour than most European brassicas and are at their best eaten raw in salads. Sow the seed 10 mm deep and allow 400 mm between rows and plants.
Horseradish is a perennial, though it is best dug up and replanted each winter. Roots more than a year old are too tough to use in the kitchen. Be especially careful where you establish your horseradish bed, it is very persistent, growing from even the smallest pieces of root left after harvest. Plant lateral roots about 200-300 mm in length and at least 6 mm in diameter. Drop them into holes about 200 mm deep and firm the soil around them. The plants should be 300 mm apart each way. Horseradish is ready for use from midsummer on. It is a gross feeder and needs copious quantities of well-rotted manure or compost to produce tender roots of useable size.
Radishes are the quickest maturing crop apart from mustard and cress. To give of their best, they must be supplied with sufficient water for rapid growth, or they will be hot and tough. They are frequently sown with parsnip and carrot seeds to act as 'markers'. They germinate rapidly, indicating where the slower germinating seeds are going to arise, allowing early weed control. In heavy soils, they also break the crust that forms on the surface of the soil, inhibiting the weaker seeds. The radishes are harvested long before the carrots or parsnips need the growing space. Sow them year round with 200 mm between rows and 25 mm between plants.
Kohl Rabi is rarely grown or eaten which is a pity. It has fine flavour, somewhat resembling its cousin the turnip, but milder. Unlike the turnip, which is a swollen root, Kohl Rabi is a swollen stem. It matures quickly and is best eaten no larger than tennis ball size. There are but two varieties, Purple and White. Kohl rabi prefers cool weather, so it is best sown in early spring and again in autumn. Sow in rows 400 mm apart with 200 mm between plants.
The humble turnip is a sadly neglected vegetable these days. Turnips are a delicious addition to soups and stews as well as a useful vegetable in their own right. In the latter case, they should be steamed lightly and still be a little crisp when brought to the table. The varieties Gilfeather and Crimson Globe are much sweeter than other sorts. The foliage of turnips is a commonly eaten green vegetable in the USA and there is no good reason why we should not follow the US lead in this. Lightly steamed, the leaves are truly delicious. Do not take too many leaves from individual plants, or you will deprive the root of the energy input it needs. Sow them in rows 300 mm apart with 75 mm between plants.
Swedes are similar to turnips, but have firmer flesh and are much slower maturing . They should be sown a few weeks earlier to mature in cool weather. They are hardier than turnips and withstand heavier frosts. Their flavour is much improved by frost, being a little too strong in flavour without it. They keep well in the garden until the lengthening days of spring send them to seed. Swede aficionados fry them like potato chips. Sow them in rows 350 mm apart with 150 mm between plants.