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Why a Polytunnel Changes the Rules

A polytunnel is the single most useful piece of kit a UK kitchen gardener can own. It does not just keep the rain off; it shifts your whole calendar. By trapping daytime warmth in the soil and sheltering plants from wind, a tunnel can add four to six weeks at each end of the growing season. That means fresh salad leaves in March when the allotment is still grey and cold, and carrots pulled in early June while outdoor sowings are barely through.

The key is the soil. Under cover, the ground warms faster and stays warmer, so roots can get going long before they would outside. A tunnel also spares plants the battering of wind and heavy rain, which is often what finishes off young seedlings in a wet British spring. Get the basics right and the tunnel pays you back from February through to November, and sometimes beyond.

What to Sow First

Start with crops that tolerate cool conditions but reward protection. Hardy salads are the obvious first choice. Sow a cut-and-come-again mix of rocket, mizuna, mustard greens, claytonia and corn salad in shallow drills or scattered thinly in a seed tray of compost. These will germinate at low temperatures and give you leaves in three to four weeks.

For hearted lettuce, choose cold-tolerant types such as 'Winter Density' or 'Arctic King'. A row of these sown in late January will be ready to cut as proper heads by April. Early carrots are the other great tunnel crop. 'Amsterdam Forcing' and 'Early Nantes' are bred for speed and sweetness, and a sowing in late February will give you tender roots in June.

• Late January: hardy salad mixes, winter lettuce, spinach, spring onions.
• February: first carrots under cover, radish, more salad for succession.
• March: beetroot, turnips, kohlrabi, transplanted lettuce and brassica seedlings.

Ventilation Is the Job You Must Not Skip

This is where most tunnel growers come unstuck. On a bright February day, the temperature inside an unvented tunnel can hit 30°C by midday, then crash to near freezing after dark. That swing stresses plants and invites disease.

Get into the habit of opening doors and vents by mid-morning on any sunny day, and closing them again by mid-afternoon so you trap the warmth for the night. Aim to keep daytime temperatures below about 25°C. If you are at work during the day, fit automatic vent openers or leave the door ajar on a timer or a simple prop.

The real enemy is condensation. Still, damp air sitting on leaves overnight is what drives botrytis, downy mildew and damping off in seedlings. Ventilating properly clears that moisture. Water in the morning rather than the evening, keep foliage dry where you can, and remove any yellowing or mouldy leaves promptly rather than letting them sit on the soil.

Watering and Soil Care Under Cover

Remember that rain never reaches tunnel beds. You are the weather now. Check the soil every couple of days by pushing a finger in to the second knuckle, and water thoroughly when it feels dry at that depth rather than little and often.

A drip line or soaker hose on a timer takes the guesswork out of it and keeps foliage dry, which reduces disease. Mulch beds with a couple of centimetres of compost or well-rotted manure to hold moisture and feed the soil. Because the tunnel is warm and sheltered, weeds grow fast too, so a quick hoe every week saves a lot of grief later.

Rotate your crops between beds each year. Aphids, whitefly and red spider mite all thrive in still air and build up quickly under cover, so watch the undersides of leaves and act early. Damping off in seedlings is usually a sign of cold, wet compost and poor airflow, not bad luck.

Carrots Under Cover: Getting Them Right

Carrots need a deep, stone-free bed. Fork the soil over to at least 30 centimetres and remove any lumps, then rake it fine. Never add fresh manure, as this forks the roots and encourages splitting.

Sow thinly in shallow drills about one centimetre deep and keep the bed evenly moist until germination. Thin the seedlings to three or four centimetres apart once they are big enough to handle, and use the thinnings in the kitchen. Carrot fly is less of a problem under cover, but keep the tunnel door closed on windy days in late spring to be safe. Keep the bed watered steadily; dry spells followed by heavy watering cause roots to split.

Succession Sowing and the Shoulders of the Season

The tunnel is at its best in the shoulders of the year, but do not let it sit idle in summer. Sow little and often, roughly every two to three weeks, so you always have young salad coming on. Use the tunnel to raise seedlings for outdoor beds in spring, then move them out and replant the space with tomatoes, peppers, cucumbers or aubergines once the risk of frost has passed.

In autumn, clear spent crops promptly and sow another round of hardy salads and winter spinach for cutting through the coldest months. A layer of fleece over the rows inside the tunnel on hard frost nights will keep leaves tender and ready to pick. Treated well, a polytunnel gives you something fresh to eat almost every week of the year, and that is worth far more than the space it takes up.