Turnips are a fairly easy and reliable crop to grow in your polytunnel. If you choose the correct varieties, you can be growing turnips almost all year round. There are early and maincrop varieties. Early turnips can be pulled small, when they are around golf-ball sized if you want to eat them raw, around the size of a tennis ball if you want to cook with them. Larger, maincrop varieties can be pulled from October onwards. Turnip greens also offer a bonus crop, in addition to the roots.
Sow early varieties of turnip between March and June and maincrop varieties between Mid July and Mid August. Early varieties will take only around six - eight weeks to reach an edible size for salads and so can be planted as a catch crop to take advantage of space left by a slower growing crop. Early turnips will be up and away before the space and nutrients are needed by the other crop. Early turnips also work well for successional sowings, so you can enjoy a few turnips at a time rather than getting a glut. Sow turnips thinly where they are to grow. Thin out the seedlings until they are around 15cm apart for early varieties, or double that for your maincrop turnips.
Turnips will need a cool, moisture retentive soil in order to thrive. In your polytunnel, you may find that turnips will do better when shaded in summer by a taller companion plant. Watering is a key concern and you should always be sure to water well, especially in hot weather. Mulch will also help turnip roots to hang onto the water they need. If they dry out or overheat, turnips can be prone to going to seed. As soon as the turnip seedlings are around 12cm or so high, apply a rich organic mulch of well-rotted organic matter, compost or leaf mould to a depth of around 5cm.
Turnips are a brassica and will do well planted alongside other members of the same family. There is evidence to suggest that peas and other nitrogen fixers may also be beneficial as companion plants.
If you have sown early turnips in succession then you can expect to harvest early turnips from May right through to September. Simply pull the turnips as they are needed and remember not to discard the greens as these can also be cooked and make good eating. Maincrop turnips are usually ready to harvest from mid October onwards, so you can be enjoying turnips right up until the end of the year and perhaps even beyond. Turnips still in the ground can be protected from frosts in colder regions with a mulch of straw or bracken held down with netting or horticultural fleece.