In many ways polytunnels are well-suited for growing grapes. For gardeners in cooler climates, the plastic covering protects vines from cold weather. The long, tunnel shape is also a fit for capping a row of grapevines. There are, however, some concerns with growing grapevines in a polytunnel, such as adequate ventilation. With minor adjustments, a polytunnel can be adapted to the needs of grapevines, alleviating these concerns.
Choose a location to plant grapevines that is in full sun. Prepare the soil by tilling, blending compost and adding any other amendments necessary for your particular soil type. Grapes prefer a soil pH between 6.0 and 6.5. Soil test kits are available from garden suppliers and extension offices. Prepare the soil before placing the polytunnel, unless the polytunnel is large enough to work in.
Plan the length and number of rows, based on the size of the polytunnel. There is not a standard-size polytunnel, but most back garden sized polytunnels accommodate only one row of grapes. Because the flow of fresh air, which is essential for successful grape growing, is reduced in a polytunnel, rows should be 1.8 to 2.4 m (6 to 8 feet) apart to ensure adequate ventilation. Rows too close together also will block each others' light.
Cover the ground where the polytunnel will go with damp-proof membrane (DPM). The DPM, which is a polythene membrane, keeps humidity levels from getting too high inside the polytunnel.
Construct a vertical trellis system for each row of grapes. Hammer wood, bamboo or steel fence posts into the ground along each row, through the DPM. Space posts 1.5 to 2.4 m (5 to 8 feet) apart. String two to three wires between each post, starting at least 90 cm (3 feet) from the ground. Leaving 90 cm (3 feet) of space below the first vining wire will further ensure adequate airflow around plants in the polytunnel.
Cut large holes in the DPM next to each fence post, for planting the vines in the ground. Dig up the ground and plant one vine near each post.
Place the polytunnel over the row. Leave the front and back ends of the polytunnel open for ventilation. Another option is to place the polytunnel up on bricks so that air can flow underneath the walls. Run a fan or two in the polytunnel when necessary to get the air moving.