A polytunnel is a tunnel made of polyethylene, usually semi-circular, square or elongated in shape. The interior heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure.

Thanks to Local Food scheme money we have two more polytunnels as of May 2009. From this view , you can also see the new Physic Garden demonstrating a wide range of traditionally useful medical and culinary herbs.
View of all three polytunnels looking south west.
Tomatoes well established and starting to flower. Morning glory on right just for a bit of colour.
October 2013: Final defoliation of tomato plants to aid ripening and reduce risk of disease. To keep the humidity low and prevent fruit split, the crop has not been irrigated for several weeks.
Building the first polytunnel, February 2005. Fixing rails to polytunnel, onto which the plastic cover is attached.
It's crucial at this point that the wind doesn't blow!
A few minutes later the polythene 'skin' is spread over and ready for attaching to the rails and tensioning.
Early August scene: Aubergine, courgette, butternut squash, cucumber, climbing beans and purple basil.
Arranging irrigation system for tomato plants. Flexible 'spaghetti' pipe pegged in to each plant's moat. The water comes on automatically at night when there is maximum water pressure and minimum evaporation.
Same crop late summer. These plants yielded over 200 kg of fruit. The yellow flowers are french marigolds used to deter pests.
Autumn turnaround - overwintering crops replacing summer ones. On view here: chinese cabbage, mizuna (just been cut) sessantina mustard green, mooli radish, spring cabbage.
October. Tomatoes and cucumbers gone and being replaced by over-wintering varieties of lettuce. Claytonia, coriander and spinach about to be sown along back row.
Midwinter view with heavy snowdrifts outside, but plenty of harvestable salad crops inside the polytunnels.
Digging in compost to make a fertile seedbed for brassica seedlings in the Spring. The bed will be regularly watered to encourage weed seeds to germinate so they can be hoed off to reduce competition before we sow.
Early (non-frost hardy) celery grows well in the polytunnel, and can be harvested in time to replant area with winter salads.
Organic standards require rotations in polytunnels, and recommend regular use of green manures. Pictured here is a strip of buckwheat, which works well in polytunnels during the Summer/Autumn. Fast growing and weed supressing as well.
Spring: raising seedlings in the small propagation polytunnel.