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Extending the Season with High Tunnels, Gardening in a New World


Terrance T. Nennich, University of Minnesota Extension Educator

The gardening season in Northern Minnesota is brutal and harsh to say the least. Lack of heat units, freezing temperatures in early June and late August, very cool nights and high winds are very challenging to even the most experienced and patient gardeners. The long period of times that plants are wet from dew or prolonged rain can make disease control nearly impossible some years. Gardeners in Northern Minnesota are usually very optimistic people, continually telling themselves that next year things will be much better and the weather will be much more cooperative to help produce that lush, bountiful harvest that we all hope for. Then about every five years, somewhat ideal conditions come together and that super abundant crop is produced. And so the cycle goes.

High tunnels can help gardeners produce that great crop every year with little risk. High tunnels can lengthen the growing season as much as 5-6 weeks in the spring and also in the fall. What are high tunnels?

Photo 1: High tunnels are simple structures that can dramatically extend the growing season in Minnesota. Terry Nennich


While high tunnels resemble greenhouses in appearance, this is the only similarity.  High tunnels do not use electricity, do not use artificial heat (except in emergency situations), use only a single layer of plastic, and achieve ventilation from natural airflow by rolling up the sides instead of using electric fans.  Drip irrigation is used to water the crops.

Crops in high tunnels are typically grown in the ground, as is the case for typical garden crops; however container gardening in high tunnels is very possible.

Photo 2: Plants are typically planted directly in the soil in high tunnels in a way similar to a typical garden bed. David Zlesak

Compared to typical garden-grown crops the yield and quality of produce and flowers are usually far superior in high tunnels.  Additionally, Minnesota research has indicated that high tunnels have greatly aided in the control of diseases and in reducing common vegetable and flower pest problems.  High tunnels provide an excellent tool for organic production in Minnesota since diseases and other pests can be controlled without chemical intervention.


High tunnels can either be permanent or on heavy skids and movable. Some gardens use high tunnels to start early flower beds and pull the high tunnel away in early summer, allowing flowers to bloom several weeks earlier. Most vegetable gardeners prefer permanent structures as they are more weather resistant, especially in high winds.

Photo 3: A minimum size is needed for high tunnels in order to efficiently take advantage of solar radiation. David Zlesak

While high tunnels can be constructed in many shapes and sizes, they must be of a minimum size to be effective and utilize solar radiation efficiently. While the jury is still out on an exact minimum size, I recommend at least 10-12 foot wide,6-8 ft high at the peak and about 20 ft long.


Photo 4: There are often less disease problems when growing plants within high tunnels because high tunnels minimize free moisture by shielding plants from rain and reducing dew. Terry Nennich

Before constructing a high tunnel, gardeners should raise the soil level at least 4-6 inches so the floor of the high tunnel is above the surrounding ground level. This will allow excess water from heavy rains to flow away from the tunnel.

While is it recommended to use a plastic that is six mil and UV treated greenhouse clear,  that will last four to six years, it is possible to start out with a inexpensive four mil clear plastic that will usually last only one year. However to get that very early spring start it is recommended to have that plastic already in place.  Therefore, it is recommended to leave the plastic on the entire year.

Photo 5: Trellising and other space saving growing techniques used outdoors can also be used in high tunnels. Terry Nennich

While high tunnels have many great advantages, gardeners must be aware that there is a learning curve to using high tunnels. Management concerns include: not letting the tunnels get too hot, supplying enough soil fertility, supplying timely irrigation through the drip tape and not letting the weeds get out of control.


High tunnel web sites that I recommend include . This is a national site and has some simple designs using PVC and other materials.  is the University of Minnesota High Tunnel Web Site and contains the online version of the Minnesota High Tunnel Manual, along with several recent powerpoint presentations and a list serve.  Gardeners interested in purchasing a hard copy of the Minnesota High Tunnel Production Manual may do so by calling 763-434-0400. The cost is 25.00 plus shipping.

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