Polytunnels dot the countryside of many European countries and are gaining popularity in the United States, as well. Polytunnels are permanent to semi-permanent structures covered with polythene. For most gardeners, they extend the growing season by two or three months. In some areas, gardeners grow crops year-round. Warm-season crops such as tomatoes, eggplant and melons thrive in the artificially Mediterranean climate. While polytunnels won't withstand hail or severe storms, they protect crops from wind and unpredictable temperatures.
Research your options and purchase or build a polytunnel that suits your needs. Fixed tunnels are permanent and strong, relying on a metal or PVC frame. Most are large enough to walk in and have doors. Gardeners on large lots who want to grow a considerable amount of produce, possibly year-round, choose permanent polytunnels. Smaller Spanish and telescopic tunnels are temporary covers that protect crops, but aren't large enough to walk in. These tunnels work well for home gardeners with limited space who want to lengthen the growing season and protect crops from unpredictable weather.
Select a level, sunny site with good soil for your polytunnel. Lay 3 inches of compost and manure on the soil in equal amounts and till in with a shovel or rototiller.
Start seeds indoors six to 10 weeks before planting them. Consult seed packets for the length of time needed for germination and growth. For example, in most temperate climates, warm-season crops such as melons and tomatoes can be planted in a polytunnel in April. Start them from seed in February.
Transplant seedlings into your polytunnel when the seedlings stand 3 to 4 inches tall and have two or three real leaves. Apply a polythene mulch to keep weeds down, especially if you have a small Spanish or telescopic tunnel, because you won't be able to cultivate the soil.
Install soaker hoses for the most convenient, reliable method of irrigation in a polytunnel. Check the soil frequently by placing your finger in it. The soil should feel moist 2 to 3 inches under the surface. Water regularly (usually once a week) for 30 minutes. Adapt your watering schedule in hot, dry weather to keep the soil evenly moist.
Fertilize your vegetables two or three times per season with a liquid vegetable fertilizer. Add a diluter attachment filled with fertilizer to your soaker hose. The hose spreads the fertilizer to the plants at the proper dilation.
Provide ventilation to your plants during hot weather. Temperatures in polytunnels quickly rise on sunny days and can damage or kill plants. Open doors or raise sides if possible to cool temperatures. Leave doors open during the day, as well, to allow pollinating insects in during the flowering phase of vegetable production.