Using high tunnels for growing season extension

Growing in a high tunnel or hoop house can be very rewarding and help improve vegetable yields. Both are non-permanent, unheated greenhouse structures typically covered with plastic, and their names are interchangeable.

Growing plants in one of these structures has a very different set of rules compared to growing plants outside. 

In a high tunnel, plant growers can start as early as February or March and garden as late as December. If growers are late getting a garden in, they can still get a crop of tomatoes with a high tunnel. To some degree, it can protect from frost and offer protection from small hail. 

Fertilizer considerations

What works in an outside garden can be very damaging inside of a high tunnel. High tunnels have a different set of rules to follow. Regardless of growing direct in the soil, raised beds or containers, growers need to take a holistic approach to soil and water management.  

It’s important to develop a fertilizer plan with low nitrogen inputs of around 10 percent nitrogen and keep records of what and when fertilizer is placed. 

Never use Miracle Grow or manures in a high tunnel or greenhouse. Miracle Grow is the wrong type of nitrogen. Plants will get very leggy, have insect problems, little fruit and lots of leaves.

 Never use manures in a high tunnel. Manures are low in nitrogen, but very high in salts and can create soil problems. 

Greenhouse temps

Soil in a high tunnel heats up fast, which is great for tomatoes, peppers and other warm season vegetables. However, it causes organic matter (OM) to decompose quicker compared to outside garden soil – a loss of 1.5 to three percent OM per year isn’t unusual. 

After each season, growers need to add OM back to the soil. Grass clippings, leaves, old hay, peat moss or non-manure-based composts are best. It’s important not to work manures into the soil as they will raise soil salt levels and cause irreversible soil damage. 

Keeping temperatures below 95 degrees Fahrenheit inside the high tunnel is very important.  Pollen loses its viability at 90 degrees Fahrenheit and sticks together if the humidity is too high. Most vegetable plants will drop blossoms and stop growing at temps above 95 degrees Fahrenheit. 

Air circulation is also very important. Rolling up the structure’s sides work best for achieving this. If the sides don’t roll up, utilizing box fans and opening windows can help with air circulation.   

Growers do not want high humidity in their high tunnel. Humidity fosters fungal, bacterial and insect problems, which will be next to impossible to get rid of. It’s important to always keep growing structures on the dry side in an effort to avoid turning them into a petri dish.

General rules for these structures include increasing air circulation, decreasing humidity, never using overhead watering systems and keeping the area clean of dead plants and weeds.  

Other important considerations include never using manures or Miracle Grow, and all diseases become worse with over watering and high humidity.  

Catherine Wissner is the University of Wyoming Laramie County Extension horticulturist. She can be reached at cwissner@uwyo.edu or 307-633-4480.