Polytunnel Gardening

Early Spring

• Start planting seeds and seedlings as soon as the ground is thawed in a new polytunnel or in late winter in established polytunnels. Choose varieties that tolerate cool temperatures. Seedlings should be acclimated to expected overnight temperatures. If a seed packet says "as soon as the ground can be worked," that variety is suitable for spring seeding in a polytunnel.

  Polytunnels collect warmth on cloudy as well as sunny days. Check the temperature inside and open the ends, sides or doors if it will be too warm for the growing plants.

Summer

• Polytunnels provide additional heat in cool summers and overnight. Measure the air temperature at plant level. Avoid temperatures above 85 F when possible, to ensure good pollination. Provide adequate circulation to discourage excess moisture problems such as fungi and blights. Bees and other pollinators need access to the plants on nice days but aren't active during rain, at which times the doors and sides may be closed.

  Be sure to water well. Your plants won't be getting rain. You'll need a minimum of 1 inch of water per week. Heavy water users such as tomatoes will need 1.5 to 2 inches per week to provide for adequate growth and help prevent blossom end rot.

Autumn

• Close the polytunnel late in the afternoon when overnight temperatures will be below 40 F. In early September into early October you should seed and transplant the plants you want to grow into December. Choose cold weather tolerant varieties that do not need pollinators, such as Winter Density lettuce, Space spinach, boc choy, kale and mache. It's important to have the plants' roots well established before the plants stop growing in early to mid December.

Winter

• Plants stop growing when the days are short and cold. You can still pick greens all winter. In the morning the greens will be wilted. When the temperature rises above freezing the plants will thaw and look fresh. They should be cut at that time. Growth will begin again slowly in mid to late February.

  You'll need to water less in the winter than in other seasons. Moist soil retains heat better than dry soil. Don't water log the soil, and make sure the plants are dry before the temperature inside the polytunnel falls to freezing to avoid plant damage.

Bed Preparation

• Weed well, water deeply and amend the soil with compost or fertilizer between crops. There's a constant drain on soil resources because you're using the soil longer than normal. Look for pests hiding in the soil when you pull out plants.

  Place soaker hoses or drip tape in the spring and make sure they're working before you plant. Drain and put them away before they freeze in the fall.

  Keep normal plant rotation in mind when planning your layout. Pests get the same head start as plants. Diseases normally killed when the soil freezes can overwinter and become a problem early in the season.