Carroll County Times Articles
High Tunnels for Agriculture
by Bryan Butler – March 19, 2006
The Basic High Tunnel
A high tunnel is a simple, inexpensive structure similar to a greenhouse that provides a great deal of season extension versatility. High tunnels offer the opportunity for the grower to get a crop in early in the season, to stay in production later in the season, and, possibly to produce a crop through the winter. High tunnels also provide protection from rain and hail and can reduce disease and pest stress to plants.
In a high tunnel system, the tunnel is large enough for the grower to plant, monitor and harvest the crop from inside the structure. The standard tunnel is 21 feet wide, 96 feet long, and 9 feet 6 inches tall at the center. The quonset frame consists of metal bows made by bending steel pipe or tubing.
During periods of cold weather, the sides are lowered in the afternoon to hold heat, and then raised in the morning to vent before temperatures inside get too high. The floor of the structure is covered with a layer of 6 mil black plastic. This helps to raise the temperature inside the house, control weeds, and prevent evaporation of soil moisture. Excess moisture will raise humidity in the tunnel and may lead to disease problems. Humidity of the air will increase at night as the air cools down. Venting in the morning will allow drying of any condensed water.
High tunnels can reduce the incidence of some diseases, particularly if trickle-irrigation tubing is used underneath the black plastic mulch. No water (rain or irrigation) gets onto the foliage to transport spores or otherwise encourage disease development.
Benefits for Production
The use of high tunnels for crop production creates a microclimate that can increase quality. Plants are grown in a structure covered with one layer of plastic, and therefore the plants do not get wet, thus reducing the incidence of diseases. The soil does not become excessively wet since the only water supply to the plants in the tunnel is trickle irrigation. The single layer of plastic reduces light levels only about 10%. Therefore photosynthesis is not reduced. Temperatures inside the tunnel are usually warmer than outside, providing season extension. The floor in the house will not freeze during most winters, allowing work to be done with soil amendment incorporation or the growing of a cover crop during the coldest part of the winter.
The roll-up sides provide passive ventilation to cool the structure and to dry the foliage, again helping to reduce disease incidence. These sides can be lowered in the evening to hold in heat and to protect tender plants from blustery conditions. The structure also provides a foundation for the use of plastic netting for support, shade cloth, and row covers for increased plant protection on cold nights.
High Density Tunnels
The High Density Tunnel (HDT) is a name that has recently been coined here in Maryland to help identify the high tech version of the common high tunnel. There is a tremendous opportunity to add to the utility and profitability of a high tunnel by adding a few options.
The HDT uses a series of six-inch industrial gutters on stands called trees, which hold eight gutters each, with crops grown in a soilless mix in the gutters. Each gutter is irrigated individually with trickle tape. There is also a high pressure line for each tree that allows misting for cooling, but can also be run though the injector for pesticide application.
This system increases growing space and the ability to produce multiple crops in a season. Plants are often started in a small greenhouse and transplanted into the gutters to reduce the crop turnover time in the HDT.
HDT's are covered with a double poly to improve insulation. A reliable source of heat is also necessary. Air circulation is still primarily provided by the roll-up sides.
Basically, the HTD will cost approximately three times as much as a basic high tunnel; however, the potential for increased production can go up significantly.
Potential Problems
The use of high tunnels does require an increase in both the level and the amount of management required to grow the crop. The sides must be raised and lowered to regulate temperature and humidity. Plants must be irrigated regularly and fertilized as needed. Unless supplemental heat is provided, the tunnel may not be able to provide adequate protection to the plants after the November/ December timeframe.
The microclimate created for the plants is also ideal for insect growth. Integrated pest management must begin when the plants are set out. Screening the sides to exclude insects, and the use of floating row covers will prevent many insects from reaching the crops.
Pollination for many crops is provided by air movement from side to side. However, bumblebees or honey bees may be required to maximize production in the early and late part of the season. Maryland researchers are currently examining the use of a honeybee hive placed at the end of a tunnel with the opposite end open during the day.
With good planning, variety selection, and close management, this low-cost system can add another dimension to a vegetable or small fruit operation, extending the conventional growing season.
About the Author
Bryan Butler is County Extension Director and Extension Educator, Agriculture & Natural Resources of the Maryland Cooperative Extension, and an advisor for the Carroll Technology Council, a non-profit organization dedicated to educating businesses, government and residents about technology issues. Questions are welcomed and may be addressed in future articles. Email advisors@carrolltechcouncil.org or go to www.carrolltechcouncil.org for a list of Advisor categories.
