Spring arrives in the polytunnel four to six weeks earlier than it does outside, and so spring is when the benefits of tunnel growing will be most obvious to you. Outside things are struggling to get going, wind-swept and frost-burnt, but inside the tunnel it already feels like summer. As soon as the days lengthen noticeably, the tunnel will be up and running even though conditions outside might leave a lot to be desired: so make a start with early sowings and enjoy harvesting your autumn-sown plants, while your neighbours are still waiting for their soil to warm up.
Ventilation can increase as the days grow longer, but nights can still be very cold so make sure that the doors are closed at least an hour before sunset and are opened again at first light. Fleece (floating row cover) protection for tender plants can be removed, but replace it if heavy frost is forecast and take it off the next morning.
When daylight hours begin to stretch in February, the soil in the polytunnel will warm up much earlier than the soil outside. As soon as you can leave your fingers in the soil comfortably (or for the scientifically-minded, as soon as the soil temperature at 8cm depth reaches 8C/46F) then plant some first early new potatoes. These will be ready around the end of April, providing a welcome new flavour in the ‘hungry gap’ and leaving the space clear for hot weather crops.
Once things really start warming up in the tunnel in March you can push on with sowing as you would outside, except four weeks earlier. Start to sow summer salad leaves now, such as lettuce, rocket, radishes, spinach, sorrel, spring onions and lambs lettuce, and also salad roots such as carrot and beetroot. Aim to sow salad crops in small patches every two to three weeks throughout the growing season for a succession of tender young leaves at their very best; you need never settle for limp salad again.
Legumes such as peas and broad beans are well worth giving some space to since they will be ready at least a month before their outdoor relatives, as will early short varieties of sweetcorn and even parnethocarpic varieties of courgette (Zucchini), which need no pollination. These can be lifted in time to make space for winter planting, but make sure that you leave yourself room for your summer crops such as tomatoes and aubergines. These slots need not be empty, as you can use them for quick-maturing crops such as radish and pea-shoots – an extravagance if you buy your seed, but much less so if you save your seeds yourself – or the space can be occupied by a suitable green manure plant such as buckwheat, rye or phacelia. Tunnel soil should never be left empty for long during the growing season, as it dries out incredibly quickly and the vital soil life perishes. If you don’t want to use green manure, mulch the area with organic material such as straw or grass clippings.
You can also use the tunnel as you would a greenhouse, for sowing and bringing on seedlings for outside planting later in the season – except that with a polytunnel you have much more room than in a small greenhouse, so you can make much greater use of ‘sowing under glass’. Large seeds such as peas or pumpkins this can prove a great temptation to mice, so plant them in trays or modules and place them in a mouse-free area (such as a suspended shelf) until they are well established, or be prepared to suffer some losses.
Some plants that will occupy the tunnel during the hottest part of the summer need to be planted in spring in pots or modules, such as tomatoes and tomatillos, chillis, melons and watermelons, cucumbers and aubergines. Getting as early a start as possible with these plants is vital if they are to set and ripen fruit properly in time to come out for winter planting, so if necessary get them started in a heated propagator or sunny windowsill, and give them as much light as you possibly can.
Keep an eye on soil moisture levels and increase watering accordingly; as soon as the risk of hard frosts recedes reinstall any automatic watering systems that were stored for the winter. Spring clean your tunnel by washing all equipment and staging down, and clean the cover inside and out using a plant-safe organic detergent such as Citrox. Remove all weeds from the tunnel and check under low plants for slugs; combined with removing any visible slugs of any size shortly after dark once or twice a week will help to keep the tunnel relatively slug-free. Because polytunnels crop so intensively it is important that you support the living soil as much as possible, so top-dress any beds that were not done in autumn or winter with 7.5cm (3″) of compost or well-rotted manure. Feed all the beds with a dressing of fish, blood and bone meal or a similar all-purpose slow release organic fertiliser, and water it in well.
Globe artichokes, autumn-planted peas and broad beans, radishes, parsley, spinach, chard, salsify, early potatoes, endive, scorzonera, rhubarb, strawberries. Winter salad plants will start to grow again, then bolt as light levels increase. With regular cropping of leaves and pinching out of flowering stems, harvesting can be prolonged until new plantings become ready.