"High tunnel" usually refers to a hoop-shaped, walk-in cold frame. These greenhouse-like structures are erected over garden rows and can be temporary or permanent. Sometimes "high tunnel" refers to a solar greenhouse shape. Growing vegetables in a high tunnel is not too different from growing them in the open, except that the structure extends the growing season. Professional and hobby vegetable gardeners use high tunnels, which generally are erected over planted seeds and seedlings in very early spring to keep them warm. High tunnels can be taken down until early fall, when they're used to encourage continued production as the temperature drops.
Erect a high tunnel over an already established garden plot in the very early spring. A simple hoop kit with poly sheeting that spreads over the hoops is available at most garden supply centers. Sunshine will penetrate the sheeting and nourish plants. Tend the soil and plant rows as you normally would. Water the soil every day, because the poly sheeting will prevent rain from getting to the plants.
Plant seeds about a month earlier than you would without the high tunnel. The soil inside the structure will warm up more quickly than the soil outside. Take the temperature of the soil; if it is about 65 degrees and the air does not freeze inside the high tunnel at night, plant seeds.
Water the garden regularly. Take down the high tunnel, if desired, at the beginning of summer, or leave it up to act as a solar greenhouse. Monitor the temperature and humidity every day if you leave it up. Ventilate the high tunnel by opening both ends on hot days. Check the dryness of the soil at least twice a day.
Set up the high tunnel again around the beginning of fall, or if it has been up all summer, close up the ends at this time to keep it warmer inside. This will extend the time that your vegetable plants produce crops. Check for dryness and water the garden every day if necessary.