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Six critical points in high tunnel production

By Carol Stender

Date Modified: 02/19/2013 1:43 PM

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ST. CLOUD, Minn. — Terry Nennich doesn't view high tunnel production as a technique.

The University of Minnesota Extension high tunnel specialist calls it a growing system with many parts.

He focused on six critical production points for high tunnel growing at the Minnesota Fruit and Vegetable Conference.

"There could be more, but these are some of the critical points in areas of management that can lead to serious and drastic consequences if not recognized and properly regulated," he said. "You are dealing with a piece of ground that is a fifth of an acre. You need to get as much out of it as you possibly can."

Soil structure and texture top his list. The texture, the soil particle, can range from sand to clay but not gravel, Nennich said.

Heavy soils can lead to compaction and lodging of water in the soils, he said. Lighter soils can cause difficult water and fertility management. Adding organic matter isn't necessarily a silver-bullet fix, he said.

Organic matter should be added annually with soils monitored for possible hardpan. Deep-rooted crops, such as alfalfa, will help break up the soil structure. If that's not possible, consider some type of deep tillage.

Use common sense in the high tunnel, Nennich added. If it's flooded, stay out of the building.

If possible, keep organic matter to 5 percent to 6 percent.

The second critical point is soil and plant nutrition. It's something that needs to be monitored regularly, he said. Soil nutrients don't run out all at one time. The supply must be ample and constant, balanced and supplied at the end of harvest.

Growers usually have good production the first year using high tunnels, he said.

"Then the second year I get this call that they aren't doing as good," he said. "When you start out with the soil test that is low, you try to make it up through a drip tape. You can't do that. You have to know what's there before you start. You need to do a soil test."

Water management is the third critical point.

Since high tunnels receive no external rainfall, a drip tape will supply the plants' water needs. Over watering, however, is very critical with young plants. It can result in root rots, slow growth and a reduced growing season. Soil nutrients may not be available and may be leaching from the soils.

Last summer, growers found that running the drip tape at noon and in the evening helped with growing conditions.

A water monitor can be set to turn the water on and off.

Temperature management is the fourth point Nennich mentioned.

"The ability of high tunnels to capture solar heat is the main reason for our success," he said.

But it can easily become an enemy if it isn't controlled and monitored on a regular basis.

Warm-season crops may like 80 to 90 degree temperatures, but when it reaches 95 degrees, problems can abound. There may be lower yields caused by pollen sterilization or fruit abortion. Tomatoes that flower at high temperatures could result in a production decline 40 to 45 days later.

"One of the basic rules of management is that you can't control that which you can't measure," he said. "You must get some monitors in there."

The more detailed the measuring, the closer it can be managed, Nennich said.

Shade cloths or shade paints can offer relief in buildings. While shade cloths may not drop the temperature much, it keeps sunshine from falling directly on the plant.

Humidity and ventilation combine for the fifth critical control point.

"There is no more explosive problem in a high tunnel than high humidity," he said. "You have free water for over 12 hours combined with high temperatures —that can cause a severe problem with disease."

He suggests putting in gable vents that can slide back and forth. It's best to place them on each end of the high tunnel to help with ventilation and to get the humidity out of the tunnel's ceiling.

Rolling up the sides is another way to ventilate the high tunnel. Automatic rollups are available.

His final critical point focused on daily insect and disease management.

"It is your job to be out there everyday, walking through the tunnel," he said. "Look at the leaves to see if there is discoloration. See if there is anything chewing whether it's spider mites, white flies or aphids. Look on the tops and bottoms of the leaves."