Plant problems can be prevented with appropriate preparation and attention. The American Horticultural Society’s New Encyclopedia of Gardening Techniques (Mitchell Beazley, 2009) gives a detailed look into planting from preparation to harvest. This discussion of cool-weather crops is taken from Chapter 4, "Growing Vegetables & Herbs."
Rutabaga, Turnips and Kohlrabi
Rutabaga and turnips are both members of the cabbage family, and should be included in the cabbage part of your crop rotation. Both are grown for their edible roots. The leaves of turnips can also be eaten. Kohlrabi is grown in the same way as turnips but the swollen stem is the edible part. Some varieties have purple coloring and are grown for their ornamental as well as culinary value.
Turnips are quick growing; both spring and fall crops can be grown in most regions. Young roots are amazingly succulent and sweetly flavored; older roots become woody and coarse flavored. Rutabaga grows slowly but is very hardy. It has harder, yellower flesh with a distinctive, sweet flavor, and its leaves usually arise from a 'neck.' Both are badly affected by drought; rutabaga in particular grows best in moist, cool soil. Kohlrabi is considered to be much less drought sensitive.
Site and Soil
These crops are usually sown where they are to grow, but where convenient they can be grown in cell packs or propagation trays, sowing three seeds per cell and thinning to the strongest seedling as soon as possible. Any seed starter or all-purpose mix can be used.
Any fertile, well-drained garden soil in full sun suits rutabaga, kohlrabi, and turnips, while light shade is acceptable for turnip tops. Dig in plenty of well-rotted compost or manure. For the best yields, rake in an all-purpose fertilizer at the application rate recommended on the package. Acid soils promote clubroot; where this disease is present liming to at least pH 6.5 is the best way of reducing damage. Resistant varieties are sometimes available, but these may not be resistant to all forms of clubroot. Cabbage root maggots may also be a problem; rotate crops to avoid a buildup of this pest.
Sowing, Cultivation and Harvesting
Sow summer turnips (see below) from late winter or early spring, initially protecting with row covers in colder climates. The earliest sowings, gathered in early summer when few other vegetables are available, are the most valuable, but later sowings in summer produce a fall crop with a flavor that sweetens after exposure to frost.
Sow turnip and kohlrabi seeds a thumb’s width apart, in drills 1⁄2–3⁄8 in. deep and 12 in. apart. Thin as soon as possible. Where crops are not doing well, apply a nitrogen-rich fertilizer. Water in dry spells and mulch around plants. Harvest as soon as they are usable, that is, from about golf-ball size. Store turnips for winter use in boxes of sand in a frost-free basement, garage, or shed.
Slower-growing rutabaga is sown from late spring to early summer in the same way as turnips, but allowing 16 in. between rows and thinning to 10 in. Water in dry summer weather to swell the roots and help to prevent powdery mildew, which is especially damaging to drought-stressed crops. The roots are ready to use beginning in late fall. Rutabaga often survives unprotected in the soil in mild regions; elsewhere, store as for turnips.
These crops suffer from the same pests and diseases as other brassicas. Cabbage root fly larvae in particular will kill young plants and tunnel into maturing roots, making them unusable. Covering initially with a row cover, and then with insect-proof mesh in summer, prevents damage. Row cover also excludes flea beetles, which can damage young plants from midspring, and aphids and caterpillars, but slugs thrive beneath row cover and mesh and usually need to be controlled.
A paperback edition of The New Encyclopedia of Gardening Techniques will be released in April 2013.
This excerpt has been reprinted with permission from The New Encyclopedia of Gardening Techniques by the American Horticultural Society, published by Mitchell Beazley, 2009.