Many types of vegetables can be planted earlier in the spring or grown later into the fall with protection from low tunnels. Some, like spinach or cilantro, can even be reliably carried over through the entire winter with protection from a poly low tunnel. Winter harvests are sweeter due to concentrated sugars, and cold temperatures enhance red colors in greens.
Recommended Crops | Construction | Growing in Low Tunnels | More Information
Crops for Low Tunnels
Not all crops can handle being frozen without tissue damage. Planting frost-hardy crops is the first step to success.
Spinach and cilantro are the hardiest. Onions can be planted in the fall for earlier bulbs in the spring. Most other cold-hardy crops are best when harvested from low tunnels by the end of December. Lettuce salad mix and single-cut salad heads can sometimes survive the winter but are more damaged by long, deep cold.
Crops that can handle frost and are good candidates for low tunnels early or late in the growing season include:
- Lettuce
- Kale
- Parsley
- Onions & Leeks
- Cilantro
- Carrots
- Beets & Beet Greens
- Cabbage
- Broccoli
- Cauliflower
- Brussels Sprouts
- Kohlrabi
- Radish
- Chicories
- Spinach
Low Tunnel Construction
Poly Low Tunnels are comprised of hoops covered in plastic secured to the ground. Typical low tunnels are the length of the garden/row several feet wide and 2 or 3 feet tall at the center.
Hoops can be made by bending a 10 foot piece of EMT conduit. PVC conduit can be used, but it reacts with plastic covers, causing the plastic covering to age rapidly and tear in a couple of years. To bend conduit into hoops, use pre-made jigs available for purchase (~$100) or jigs created out of scrap wood or other materials. Hoops can either be pushed into the soil, or they can fit over rebar that has been driven into the ground.
Connect each hoop along the row using strapping, rope, or conduit to keep the hoops upright. If rope or strapping is used, it should be secured to the ground on each end to maintain tension.
Cover the hoops with UV-resistant 6-mil greenhouse poly. The poly sheeting should contact the ground on each side of the hoop along the entire length. At either end of the row leave extra length (about 5 to 6 feet) to gather the plastic and secure it to a stake. This closes off the ends of the row.
Attach fasteners at ground level on either end of each hoop to connect ropes or bungee cords that stretch over the tunnel and keep the plastic from billowing in the wind. Finally, weigh down the edges of the plastic with sandbags or other weighted material. Securing the plastic well allows for a greater chance of the low tunnel withstanding high winds.
Growing in Low Tunnels
Light
Low tunnels should be sited in full sun. Shade at any time of the day means colder temperatures and less survival.
Venting & Temperature Management
Venting will be needed to prevent heat and moisture build-up. Open tunnels end and/or roll up tunnel sides to vent when conditions are sunny and above 40°F. Venting for heat is often unnecessary for small tunnels unless the outside temperature is above freezing for a long time.
Low tunnels should be closed up at night unless overnight temperature will remain above 40°F. Monitor the weather closely so tunnels can be closed up before temperatures drop into the mid to low 30s°F.
Watering
Monitor soil moisture as plants under tunnels will not receive water from rainfall. Irrigate when the soil is dry.
Planting
Crops must be full-sized by the beginning of November to successfully produce well into the winter season. In central Iowa, this means planting from mid-August to late September, depending on the crop. Due to rapidly shortening day length, planting later by a couple of days in the fall can mean maturity is pushed weeks later.
Below are planting dates for select cool-season crops that would grow well in low tunnels.
- 8/15 – Chard, Kale, Collards, Scallions
- 9/1 – Dill and Parsley
- 9/7 Bok Choy, Salad turnips
- 9/15 Cilantro, Spinach, Lettuce Heads, Radishes, Beet Greens
- 9/21 Lettuce Salad Mix or Brassica Salad Mix
Harvesting
Cool-season crops cannot be harvested while frozen. Wait for plants to thaw. Typically this means waiting to harvest until late morning after a night with temperatures below freezing.
More Information
- All About Cold Frames
- Tips for Earlier Yields in the Home Vegetable Garden
- Early Spring Planting of Vegetables
- Fall Planting of Vegetables
- Frost and Freeze Dates in Iowa
- How to Protect Plants from Frost and Freeze
Photo credits: Dan Fillius