Winter Growing Guide | Part 2: Production in the High Tunnel

Advice from a Winter Grower

“We ease into winter: some crops go under low tunnels in fall, and then we move into the high tunnels as it gets colder. That way we use all our space well and save the most expensive space for when we truly need it.

“Choosing the right varieties for winter is key. Watering is another big challenge; it's easy to give too much or too little in winter, so tools like a moisture meter really matter.

“High tunnel plants need fresh air in winter just as much as in summer. Without good airflow and daytime venting, you can end up with fungal or aphid problems. Managing heat is also harder. It's tempting to think, 'It's freezing out, so I'll check the tunnels around 8.' Then you find the sun has been up for two hours and it's gone from 25° to 75° inside and still climbing, which greens and lettuce do not like.

“Plenty of organic matter in the soil also matters, because it keeps more air pockets in the soil instead of a solid frozen block.

“Keep in mind that in winter, high tunnel space is the most expensive you have, so weigh every cost against what winter crops bring in.”

—a four-season vegetable grower in southern Illinois

A tunnel that will hold up under snow, catch as much sun as possible and let you control heat and humidity has to follow basic structural design rules. To learn more, connect with your local Cooperative Extension office, colleges and regional grower groups, which offer forums, workshops, online resources, grants and other programs.

Winter Blankets

At northern latitudes, a proven way to protect crops in the coldest months is to lay one or more layers of row cover over them inside the tunnel. Row covers of different weights and fabrics are used in many different ways depending on region, farm and microclimate. Some growers leave light covers on all the time. Others cover crops at night and uncover them on warmer days once the tunnel has heated up, which lets in more sun and vents the extra moisture that encourages winter tunnel diseases such as downy mildew on spinach and lettuce.

Some crops can carry row cover laid right on them; others need support, especially under several layers or heavier fabric. Conduit hoops, wire wickets, cables or metal frames can all hold covers up and make it quicker to pull them back each day to manage heat and humidity.

Acclimating Plants to the Cold

From planting through winter, plants have to adjust gradually to cold and to swings in temperature, or they can suffer shock and tissue damage. It works much like hardening off seedlings in spring, only in the opposite direction, toward colder conditions. Expose plants to temperatures near freezing, 32°F (0°C), for longer and more often over time. Watch temperatures closely so you know when to pull back row cover, roll up the tunnel sides or both, and if you roll up the sides, keep an eye on the forecast for wind, rain, sleet or snow that could harm the crop.

Going into winter and coming out of it, keeping the tunnel warm matters less than avoiding big temperature swings. Aim for temperatures that stay as steady as possible, or change gradually, to limit stress on the plants. Without good daytime airflow and venting, disease can take hold.

Once properly hardened, the cold-hardy plants can handle a hard freeze. Just be sure they have thawed completely before you harvest; on harvest days you may need to wait for the tunnel to warm or add some heat.

Watering & Fertility

Water at sowing and transplanting and keep checking soil moisture, but winter plantings usually need little more water until days are back to 10 hours long.

Water thoroughly to get crops established; after that they need very little until day length returns to 10 hours and growth picks up again. Water early on sunny days so plants and soil can dry, which helps prevent disease, and use a moisture meter to avoid watering too much or too little.

If you need to add fertilizer, choose mild, gentle sources. Because rain can't flush the soil inside a tunnel, salts can build up. Some growers flush salts with overhead irrigation; others leave the tunnel fallow and uncovered now and then so rain can wash them out.