- Inspect and reinforce your structure: Walk around your high tunnel and look closely at bows, purlins, and bracing. Tighten bolts and replace rusted parts. Confirm that ground posts are secure and cross-braced; add temporary supports if heavy snow is forecast.
- Assess the plastic cover: Check for holes, tears, or brittleness. Replace old film before winter. If your tunnel is not rated for snow, remove the plastic after the final harvest and store it securely. Leaving brittle plastic up invites tearing, leaks, and eventual collapse. Brush snow off promptly and check slopes during storms.
- Maintain drainage and site conditions: Clear gutters, swales, and ditches so rain and snowmelt flow away from the tunnel. Raised beds and mulches absorb more heat and reduce cold injury.
- Plan ventilation and humidity control: Service roll-up sides, end-wall doors, and fans. Use fans to circulate air when vents are closed.
- Prepare row covers and low tunnels: Have floating row covers ready for cold nights. Choose weights that balance insulation and light transmission. Lightweight covers can stay on during the day if light levels are adequate; heavy covers should be removed when the sun is up.
- Schedule winter plantings: Sow winter greens 5–6 weeks before the Persephone period, roughly late September in Nevada, or start them in trays while summer crops finish. Plant legume cover crops after the fall harvest to maintain living roots and soil fertility.
- Monitor and record: Place thermometers and hygrometers at crop height to track conditions. Keep a notebook of weather events, crop responses, and ventilation actions. The more you measure, the less you guess, and data help refine your strategy from year to year.
With this checklist in mind, let’s explore why each step matters and how research guides management decisions.
How high tunnels behave in winter conditions
Many high tunnel users, especially first-timers, discover that a high tunnel behaves differently in winter than it does in spring or summer; it warms quickly under sunshine but offers limited protection at night. Research carried out at the University of Nevada’s Desert Farming Initiative shows that high tunnels raise daytime air temperatures in spring, autumn, and winter but do little to increase minimum temperatures and sometimes even record colder nightly minima inside than outside. Because the structure raises daytime heat without buffering night-time cold, crops inside the tunnel experience larger daily temperature swings than those grown outdoors. Vapor-pressure deficit patterns follow the same trend; humidity inside the tunnel is lower by day because of higher temperatures. Wind speed, on the other hand, is substantially lower inside the tunnel, even when vents are partly open. Therefore, successful winter management requires active decision-making rather than assuming the structure will protect itself.
Understand the limits of the winter microclimate
A winter high tunnel functions as a solar collector: it converts sunlight to heat but loses warmth quickly after sunset. Measurements from western Nevada show that while maximum temperatures inside unvented tunnels are consistently higher, minimum temperatures remain nearly identical to outdoors, meaning crops are still exposed to freezing stress. Near the soil surface (16 cm), nighttime temperatures may be slightly warmer but still routinely fall below crop tolerance thresholds. As a result, crops inside the tunnel still face freezing conditions and may need additional protection such as low tunnels or floating row covers. Furthermore, the research notes that high tunnels reduce solar radiation compared with outdoor conditions; in winter, the reduction is small enough to allow photosynthesis, but growers should avoid additional shading.
You should also expect larger daily temperature swings inside the tunnel. Because the structure warms rapidly under sunlight but cools rapidly at night, the diurnal temperature range tends to be wider than outdoors. This is not necessarily harmful; cooler nights reduce respiration costs, while warm days extend the crop’s time in its optimal temperature range. However, if nights fall below a crop’s tolerance, the swings can cause stress and damage. For warm-season crops such as tomatoes and cucumbers, optimum temperatures lie between 70 °F and 85 °F, and stress develops above 85 °F day/75 °F night. Cool-season crops like leafy greens can tolerate 60 °F to 75 °F and are better suited to winter conditions.
The “Persephone period” and light limitations
Plant growth slows dramatically when day length drops below 10 hours, a period known as the Persephone period, according to the University of Minnesota’s high-tunnel research, which further noted that in Minnesota, it begins in early November. In northern Nevada, this period occurs around the same time. Crops like spinach and baby leaf Brassicas should be seeded 5–6 weeks before the Persephone period so that plants reach a harvestable size before light becomes limiting. Planting winter greens too late leads to stunted growth and poor yields. Growers can also use their high tunnel for curing onions, garlic, squash, and other storage crops when cool, dry weather is required; this should be done with the doors open and fans running to maintain air flow.
Site selection and orientation
A poor site can undermine winter performance. An interdisciplinary study distributed by the University of Vermont Center for Sustainable Agriculture recommends orienting single-bay tunnels east–west so the long side faces south to maximize winter solar gain. At the same time, the tunnel should face prevailing winter winds on one end wall to minimise wind pressure. Purdue University’s high-tunnel series echoes this advice and notes that an east-west orientation provides more light in late fall and winter. However, longer shadows from hipboards or end walls can create shaded zones; growers should weigh orientation against other factors such as wind exposure and site layout. When using multi-bay (gutter-connected) tunnels, ridges should run north–south to distribute light more evenly and reduce fixed shadows.
Shading from buildings, trees, or adjacent tunnels is another hazard. The NRCS guide on controlling high tunnel environment suggests placing structures at least twice as far from an obstacle as the obstacle’s height. In winter, shadows can be more than twice the height of the object, so a site that allows your high tunnel to receive the maximum possible sunlight. Good soil drainage is equally important; high tunnels placed where water pools can develop saturated soils, leading to root rot and mud.
Roof slope and snow load
Nevada rarely sees deep snow, but occasional storms can collapse lightweight tunnels. Engineering research emphasises that the roof slope should be at least 10 ° and preferably 20–30 ° to shed snow and improve solar transmission. Flat or shallow roofs reduce structural strength and increase condensation drip. Any unheated tunnel that can accumulate snow is vulnerable; the University of Arizona’s engineering review advises removing the plastic cover after the final harvest and before winter storms. A similar warning comes from a Vermont case study on Haygrove tunnels: these multi-bay structures are not designed to support heavy snow loads, so the polyethylene film should be pulled off the hoops and stored whenever snowfall is possible. Growers are advised to wrap the plastic in black film to prevent UV damage and remount it in spring. This practice eliminates the risk of collapse and extends the life of the plastic.
Structural reinforcement and anchoring
Even when plastic remains on, reinforcing the frame can prevent damage from wind and wet snow. The engineering literature recommends a minimum 2-ft transition between the sidewall and roof to avoid weak points and to allow a sufficient roof slope. Strong anchoring is crucial for your high tunnel, which keeps the structure from lifting in high winds. For single-bay tunnels, ensure that ground posts are set deeply and braced with diagonal supports; inspect bolts, purlins, and cross-braces for signs of corrosion or looseness before winter. Replace weak or rusted components and don’t rely on appearances alone; a frame that looks sturdy may have hidden weaknesses.
