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Garden Planning, Succession & Crop RotationHigh Tunnel Crop Calendar
A high tunnel stretches the growing season at both ends, but it brings its own calendar problems: heat, ventilation, soil buildup and tight rotation.
Key points
- A high tunnel moves your season at both ends but does not remove winter.
- Plan ventilation, since sun can overheat the tunnel quickly.
- Rotate families and test soil, because salts can build up over time.
- Ask extension and NRCS about local guidance and programs.
What is a high tunnel, and what does it change?
A high tunnel is an unheated, plastic-covered structure, usually tall enough to walk in, with crops growing in the ground. It protects crops from wind, rain and some cold, which can let you plant earlier in spring and harvest later in fall.
It does not remove winter. In cold areas, growth in midwinter is slow because of short days, even with protection.
Many land-grant universities and USDA programs have published guidance on high tunnel production, so look for material written for your region.
How do you build the season calendar?
Start with your outdoor frost dates, then think of the tunnel as moving them. Many growers treat the tunnel as giving a few extra weeks at each end, but the actual gain depends on region, structure and use of extra row cover inside. Ask your extension office for local figures.
Mark early plantings, the main season crop, and the fall and winter crops on a single chart.
Cover crops can occupy a tunnel bed in the off-season, protecting soil and adding organic matter. Because rain does not reach the soil inside, some growers open the tunnel in a rainy season to let water flush salts. Ask your extension office whether that suits your area. Record the highest and lowest temperature inside the tunnel each day for a season with a simple min-max thermometer. Compared with a reading outdoors, the numbers show how much protection your structure really gives at different times of year.
- Early spring: cool-season crops and early transplants
- Main season: warm-season crops such as tomatoes and peppers
- Fall: cool-season crops started in late summer
- Winter: hardy greens with extra covers, if your climate allows
How do you manage heat and ventilation?
Sun can heat a tunnel fast, even on cold days. Open sides or vents on a schedule and watch the forecast. Overheating can damage plants, and closed tunnels with high humidity can promote disease. Venting and watering in the morning helps foliage dry.
Wind can damage a tunnel with open sides and loose plastic, so secure doors and covers. Check local building and farm-structure rules before you build.
Wind rating, snow load and local rules for farm structures differ by place, so check requirements before building or covering. A thermometer hung at plant height tells the real story, because air near the roof can be far warmer than air at the soil. Many growers also keep a note of the weather on each day they vent, so patterns show up after a few weeks.
How do you plan rotation and soil care in a tunnel?
Because the tunnel is small and valuable, the same crops tend to repeat. Plan a rotation among the beds inside the tunnel by plant family, and consider cover crops during any off-season. Tunnel soil does not get leached by natural rain, so salt can build up over time. Soil tests every year or two help you watch for it, and local advice guides amendments.
Nutrient management rules and any cost-share programs vary by state. Ask NRCS about programs that support high tunnels in your area.
Because tunnels warm quickly in spring, cool-season crops can bolt early on sunny days. Choose bolt-resistant varieties and ventilate early in the day to keep them going.
How do you schedule irrigation?
Rain does not reach the soil inside, so you control all the water. Drip lines under mulch are common because they keep foliage dry. Check soil moisture by hand and water by need, since needs change with season and crop stage.
Follow local water rules, and plan how you will drain or manage water around the structure.
What about pests and disease?
Warm, humid conditions favor some diseases, and pests can build up when crops are repeated. Scout weekly, keep tunnels clean, and remove diseased plants promptly. For identification and current options, ask your extension office.
Pesticide labels must be followed, including restrictions on enclosed spaces and pre-harvest intervals. Protect pollinators, and note that applicator licensing varies by state. Keep the plastic clean and in good repair, since dirty or torn covering lets in less light and can make winter growth slower still.
What records should you keep?
Keep a log of planting dates, harvests, temperatures and problems. Compare tunnel dates with outdoor dates to learn how much earlier and later your tunnel really lets you grow.
After a few seasons, you will have a tunnel calendar based on your own numbers.
Compare each year to the last, and adjust planting dates by a week at a time as you learn how the structure behaves in your climate.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.