Of all the gardening projects I will undertake this year, the most rewarding one will be setting up a low tunnel for cool-season crops. Low tunnels push spring four to six weeks forward by increasing air and soil temperatures and protecting plants from mild to moderate freezes. They also speed plant growth. When I fill a low tunnel with lettuce, cabbage and other cool-season vegetables in March, many are ready to harvest before my last frost date rolls around in early May.
But the most remarkable thing about using low tunnels for cool-season crops is that they help plants grow better. Numerous studies have found that low tunnels enhance vegetative growth while improving water and nutrient use efficiency, so that vegetables mature faster. Under blustery spring conditions, low tunnels protect plants from wind, hail, and pounding rain. On occasional hot, sunny days, low tunnels reduce plant stress by filtering intense light. Later in spring, they become barriers to aphids and cabbage white butterflies, whose caterpillars are so troublesome for cabbage family crops.
How to Set up a Low Tunnel
Mobile, temporary and easy to assemble, low tunnels have three parts: support hoops, a cover, and weights to secure the edges. For good wind resistance, low tunnels need to be low and tight, which requires a good matchup of the pieces.
After trying different support hoops, my recommendation is to go with 9 or 10-gauge stainless steel wire, cut to 76-inch (2m) lengths. This length provides maximum versatility in adjusting the width and height of low tunnels. Do not waste money on painted or coated hoops, which flake and rust after a few seasons. Plan to place hoops no more than 5 feet (1.5m) apart, with crossed double hoops at the ends for added stability.
As for the covers, for this use I suggest horticultural fleece with 70-85% light transmission. Also consider width, which needs to be 12 inches (30cm) greater than the length of your hoops. With a proper match, when the hoops are pushed 6 to 8 inches (15-20cm) into the ground, you will have at least 12 inches (30cm) of overhang on both sides of the tunnel. More overhang is better, so go wide when buying fleece when you can.
Options for securing the edges of the cover include burial, bricks, sandbags or boards. Burial is messy and unreliable, and though I do like bricks for the ends of low tunnels, when used along the edges they provide too many opportunities for uplift from strong winds, which is also an issue with sandbags. Heavy boards work better because they leave no gaps, and you can wrap the fabric’s edges around them to get a tight fit. Best of all, with board weights you can open or close the tunnel for maintenance in a few seconds.
Low Tunnel Maintenance
Once a tunnel is up and running, it’s important to check its wind resistance on breezy days and make any adjustments needed. I use wood clothespins to secure covers on the windward side of my spring tunnels by pinning the cover to the hoops low down, close to the ground, where lift problems usually begin.
Fabric row covers admit rain, but plan to water crops grown under cover during dry periods. Try to water early enough in the day so the plants’ leaves are dry before nightfall.
Weeds grow as eagerly beneath low tunnels as do vegetables, but you can maintain order with short sessions with a sharp weeding tool. This must be said: On a lovely spring day, it is pure joy to free one edge of a low tunnel, turn it back, and behold the robust life that has come to be. What’s a little weeding when your lettuce is looking so beautiful?
Cool-Season Crops for Low Tunnels
Here are 28 vegetables that are great candidates for growing under a low tunnel in spring: Beetroot, pan choi, broccoli, chives, cabbage, cardoon, carrot, celeriac, celery, chard, Chinese cabbage, cress, endive, escarole, bulb fennel, kale, kohlrabi, leek, lettuce, mache, onion, parsley, pea, radicchio, radish, spring onions, shallots, and spinach.
When frost risk has ended but nights remain cool, move your low tunnel to where it can benefit cucumbers, summer squash, or even melons, which love growing under low tunnels until they start blooming and need pollinators more than protection.