Imagine a cold, January day in Pennsylvania. The outdoor air temperature is 25 °F (-3.9 °C). The clouds have parted, and the sun is shining brightly. Inside a high tunnel, it is 50 °F (10 °C), a warm reprieve from the outside cold, and the ground is covered in robust greens of spinach, kale, and arugula. There are no complicated heating or light systems responsible for this verdant growth, just a simple structure framed with curved metal pipes and covered with 6-mil greenhouse-grade plastic, a high tunnel. High tunnels are a type of greenhouse but instead of having an industrial heating or cooling system, they rely on passive solar heating and ventilation. Sidewalls are rolled up to provide ventilation and closed to retain heat. High tunnels are considered nonpermanent structures. These simple structures can be purchased as kits or custom built. They are less expensive than a climate-controlled greenhouse while still being functional for season extension of summer crops and winter production of cold-hardy crops. Plants are grown directly in the ground in high tunnels rather than in pots as with greenhouses.
The high tunnel environment creates a microclimate that protects and buffers plants from winter winds and fluctuating temperatures. Cold-hardy plants can survive winter conditions in Pennsylvania when they are protected from desiccation in a high tunnel. Cold-hardy plants perform best when soil temperatures remain above 40 °F (4.4 °C) but properly acclimated plants can survive freezing temperatures and still be harvestable once thawed with little to no cold damage (Gu, 2021). Cold-hardy plants include spinach, kale, Asian greens, scallions, arugula, red and green lettuces, endive, chervil, mustard greens, radicchio, mizuna, beets, carrots, turnips, radishes, Swiss chard, claytonia, etc. These plants can be successionally transplanted or direct seeded starting in early August through late October for fall and winter harvests (Coleman, 2009). Earlier sowings will reach maturity by winter while later sowings will be less vigorous since plant growth slows dramatically in winter due to limited light and low temperatures. Crops will be most successful if they are nearly mature by the time day length becomes shorter than ten hours (Coleman, 2009). Succession plantings according to each crop’s maturity date can help reach this goal.
The summer gardening season does not need to begin after the last spring frost or end with the first fall frost. High tunnels can extend the gardening season. Summer crops can be planted earlier and kept later. The most popular summer crops grown in Pennsylvania high tunnels include tomatoes, peppers, and cucumbers but many other types of vegetables, herbs, cut flowers, small fruit, and even tree fruit can be successfully grown in high tunnels (Sánchez, 2023). Commercial high-tunnel-grown tomatoes in Pennsylvania can be ready to harvest eight weeks prior to field production season (Ford, 2023). The specific timeframe of season extension will vary from year to year within a site. Additional inputs added to the basic high tunnel system can help improve season extension by insulating and buffering from temperature fluctuations. These include raised beds, mulch, additional layers of plastic, low tunnels, cold frames, floating row cover, water bags, thermal blankets, etc.. The University of Tennessee Researchers found combining row covers, water bags, and thermal blankets in the high tunnel increased the canopy temperature for spring tomatoes by 33 °F (0.6 °C) compared to outside air temperature (Leib, 2020).
The yearly average air temperature in a high tunnel is 8.4 °F (-13.1 °C) higher than outdoors (Sánchez, 2023). A number of factors will affect the specific high tunnel temperature including time of day, time of year, weather conditions, how warm it was the day before, high quickly the temperature dropped, etc. A cloudy winter day may only be a few degrees warmer in a high tunnel compared to outside, but this could increase to 25 °F (-3.9 °C) warmer on a sunny day (Leib, 2020). Spring temperatures in the high tunnel can be 50 °F (10 °C) warmer than outside but this regularly drops to only a few degrees warmer at night (Leib, 2020). Summertime temperatures could reach up to 130 °F (54.4 °C) in an unventilated high tunnel so it is important to roll up the plastic sides for ventilation to prevent plant damage (Sánchez, 2023). Automated fans and vents can be incorporated into high tunnels to aid in cooling. The high tunnel can be heated with portable propane or kerosene heaters to extend the season, but this is generally only done in spring or fall when below-freezing temperatures are forecasted. Although many believe temperature is the most limiting factor to plant growth in winter, light is equally important. Sunlight should be at the forefront of your mind when selecting a site for a high tunnel. Ideally, the site should receive four hours of direct sunlight throughout the winter months (Coleman, 1999). Make sure the low-angle winter sun will not be blocked by trees or buildings. The location and size of the high tunnel should suit your year-round needs and activities. The high tunnel can share space with an existing uncovered garden if there are space constraints. The height of the high tunnel should be comfortable to work in. If you must constantly bend over in the tunnel, work will be less enjoyable.
Foliar diseases can be reduced in the high tunnel. The exclusion of rain in this environment and use of drip irrigation keeps foliage dry, which is a disease management practice. Outside of the high tunnel, fungal spores in the environment (often surviving on plant debris in the soil) are being splashed onto leaves when it rains. High tunnel plants are not immune to disease, unfortunately. Foliar diseases still occur in this environment, especially when humidity is high. Increasing air flow through proper plant spacing, thinning, trellising, or pruning can help with this. Many of the diseases and insects encountered in the high tunnel are similar to those experienced in greenhouse production. Insects (e.g. aphids, thrips), arachnids (e.g. spider mites), and slugs can be problematic in the high tunnel. Understanding these pests’ life cycles and using an integrated pest management (IPM) approach can help protect plants. Salt accumulation can also be a challenge in high tunnels since rainfall does not leach nutrients from the soil so carefully monitoring with soil fertility testing is recommended. Leaching excess salts with irrigation or uncovering the high tunnel can also help manage salt build-up.
High tunnels are a way to bring greenery and fresh produce into your home during the coldest times of the year while giving you more flexibility with an extended growing season. For more information on high tunnels, visit the Penn State Extension website or contact your local Extension office.
Homeowner high tunnel, Photo credit: Jennie Mazzone, Penn State Plant Pathology and Environmental Microbiology
Coleman, E. 2009. The winter harvest handbook: Year-round vegetable production using deep-organic techniques and unheated greenhouses. Chelsea Green Publishing Company.
Coleman, E. 1999. Four-season harvest: Organic vegetables from your home garden all year long. Chelsea Green Publishing Company.
Ford, T., Kime, L., Harper, J.K., Bogash, S. “High Tunnel Production”. Penn State Cooperative Extension, March, 2023.
Gu, S. “High Tunnel Farming”. North Carolina Agricultural and Technical State University Cooperative Extension (ANR-21-01). August, 2021.
Leib, B. G., Emery, Z., Zheng, M., Grant, T., Wright, W. C., Wszelaki, A., Moore, J., and Butler, D.M. “Thermal protection of spring tomatoes in high tunnels”. The University of Tennessee Institute of Agriculture (W949), September, 2020.
Leib, B. G., Emery, Z., Zheng, M., Grant, T., Wright, W. C., Wszelaki, A., Moore, J., and Butler, D.M. “Thermal protection of winter lettuce in high tunnels”. The University of Tennessee Institute of Agriculture (W948), September, 2020.
Sánchez, E. and Orzolek, M. “Extending the Garden Season with High Tunnels”. Penn State Cooperative Extension, March, 2023.