Climate change

How do high tunnels extend the growing season?

Despite cold and snowy winters, farmers in the northern states can grow cool season vegetables year around! The structures that help them do this are referred to as hoop houses or high tunnels. These plastic-covered tunnels are an economical way that allows farmers to grow food for nine or ten months of the year. While it’s freezing outside, some very tasty vegetables including spinach, kale and carrots grow inside high tunnels.

Large green leaves of squash plants over yellow flowers in a high tunnel
Squash grown in high tunnels can be provided fresh to local markets well beyond the normal growing season. Credit: Vicki Morrone

High tunnels started out as simple structures made from PVC pipes. The pipes were bent, and both ends placed in the ground. This formed the “hoop” that is then covered with plastic. The structures keep the wind out and hold on to warm air. On sunny days, the warmth is trapped by the plastic. High tunnels can be homemade or purchased from companies that make different sizes, and various qualities.

The houses do not need heaters or lights. They get light and heat from the sun shining through the plastic, onto the plants that are grown in the ground. The sun takes care of two of the things needed to grow plants: sunlight and warmth. The missing ingredients are water and nutrients. Since the high tunnels are covered in plastic, no rain can get to the plants.

Semi circle hoop house used for late season farming
High tunnels have different farming challenges than open fields, but allow farmers to extend their growing season significantly. Credit: Vicki Morrone.

Farmers water with irrigation from drip tape or by hand watering. Plants need water for growth and as a way for nutrients to be carried to the plant. Water also plays an important role in maintaining soil health in a high tunnel.

Farmers often apply manure, compost and chemical fertilizers to provide nitrogen and phosphorus, the most important nutrients needed for plants to grow. Adding fertilizer or manure not only feeds the plants, but also adds salt to the soil. This type of salt is not the same as table salt, which is sodium chloride. Salts from fertilizer take various forms, like calcium phosphate. This is an unfortunate side-effect of fertilization that needs to be managed.

Salt is what is left-over after the plants use the nitrogen and other nutrients from the fertilizer, regardless of type of fertilizer used. You may have seen this happen if you have grown house plants and saw a white salty crust on the surface of the plant. Too much fertilizer will cause problems, as salt builds up in the soil and this is generally harmful to plants. Salt problems can be prevented by leaching soil (a type of draining) of these salts by occasional deep watering or removing the plastic from high tunnel for year will allow rain to do this work and a chance to grow cover crops, a good solution for farmers with more than one high tunnel.

To improve soil health in a high tunnel and reduce the problem of salt-buildup, it’s recommended to use other sources of nitrogen besides manure and chemical fertilizer. One of the most important and innovative ways to build soil fertility and health is to grow it in place, by growing plants called green manures or cover crops.

It is important to keep in mind that too many nutrients can be just as bad for the crop and the environment as too little. So high tunnel farmers have the challenge to ‘feed the soil to feed the plant’ without building up salts and without losing nutrients into the air or nearby water ways.  This is why farmers are exploring environmentally-friendly ways such as growing green manure crops to provide nutrients to crops grown in high tunnels.

Although high tunnels have different farming challenges, they allow farmers to extend their growing seasons. Having extended season, fresh produce grown close to home can be an environmentally-friendly option for shoppers in our northern states.

By Vicki Morrone, Center for Regional Food Systems at Michigan State University

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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