At a cost of $1to $2 per square foot, a high tunnel can add low-cost growing space for season extension or plant protection. Growers and researchers are finding many innovative ways to utilize these structures to supplement their greenhouse operations.
By definition, the high tunnel is a walk-in, hoop or gothic-shaped, pipe frame structure that is covered with a single layer of film plastic. It generally does not have electricity and the only heat is provided by the sun. Ventilation is by rolling up the sidewalls and opening the doors. Irrigation water is provided by piping from another building. Plant production can be in the soil or in containers on top of a weed mat. A multi-bay tunnel is available from Haygrove Tunnels, 116 Trail Road , North Elizabethtown NJ , 17022 .
These structures have their roots in Europe where they have been used for growing off-season vegetables. Development work in the U.S. has been done at the University of Hew Hampshire , Penn State University , Cornell University and several other states.
In addition to their use for early spring tomatoes, peppers, and cucumbers. Plants are started in a growth chamber or propagation greenhouse and transplanted into the tunnel about a month before they could normally be planted outdoors. This allows the vegetables to be available for market when the price is the highest.
A relatively new use of high tunnels is in the production of specialty cut flowers. Most flowers that do well outdoors will do better indoors with a modified environment. Eliminating of wind and rain usually improves quality and shortens the growing season. Some of the most popular cultivars are aster, coneflower, dianthus, delphinium, coneflower, snapdragon, sunflower and zinnia. Sales generally go to florists, roadside stands and farm markets. In some areas, pick-your-own have become popular.
In the nursery, high tunnels are common in northern climates for the winter protection of nursery stock, perennials and herbs. Container plants are placed, pot to pot on gravel or weed barrier mat. The tunnel, covered with white poly provides wind and temperature protection. Irrigation may be needed if the weather turns warm for an extended period of time. Ventilation is accomplished by opening the endwall doors. A layer of foam insulation may be placed over low-temperature sensitive plants to provide additional protection. In Connecticut over 2000 acres of plants are protected this way each winter.
Some growers have used the high tunnel in place of cold frames for hardening off early season crops such as pansies, snapdragons and vegetable transplants. In place of adding additional expensive greenhouse space that may only be needed for a month in the spring, the high tunnel can provide the space needed at a much lower cost.
Another application for high tunnel technology is in the production of small fruit. Strawberries, raspberrries and other small fruit do very well as the modified environment increases yield as well as earliness. The choice of cultivars, planting time and spacing are critical to getting a good crop.
Site selection is important. As solar heating and natural ventilation are the means of temperature and humidity control, it is important to locate the tunnel away from buildings and trees. If the crops are to be grown in the soil, organic matter and amendments should be added and the soil should be tested. A swale should be installed around the greenhouse to drain rainwater away.
Steel tubing or fence pipe is the standard material used for the hoops. Posts are driven into the ground about 2' deep to support the hoops. To get a higher ridge and therefore better ventilation, the ground posts are sometimes extended 2' to 4' above grade. A 2" x 12" pressure treated lumber baseboard is attached to the posts with bolts to help make the frame rigid. The endwall frame and doors are made of 2" x 4" lumber. Doors large enough to allow access for a small tractor in are desirable if the ground will be tilled. If the crops will be grown in the soil, the structure should be installed so that it can be easily moved from one plot to another.
Widths of 14' to 20' are most common but some manufacturers make structures as wide as 30'. Although the length can be any multiple of the 4' hoop spacing, a 48' or 96' length will utilize the 100' sheets of plastic better. It is best to utilize the stronger 6-mil greenhouse grade of polyethylene that will give 4-years of service rather than the agricultural grade which usually deteriorates in less than one year. In windy locations, a woven polyethylene has given better service. A hip board, attached to the frames, 3' to 4' above the baseboard will hold the plastic when the roll-up sides are open. The plastic is attached to the baseboard and hip board with aluminum extrusions or a double furring strip for a tight seal and easy replacement.
Except for the multi-bay, gutter-connected style which ventilates by opening up the plastic roof, cooling is by roll-up sides and endwall doors. Ken-Bar, Inc., Reading MA makes small manual and automatic vents that can be inserted into the roof to reduce the temperature about 5ºF if installed on a 20' spacing. If crops are to be grown during the summer, a shade cloth could be placed over the tunnel to reduce the inside air temperature.
To gain extra growing time for crops such as tomatoes, cucumbers and peppers, a root-zone heating system could be installed. This system utilizes a propane gas hot water heater, circulating pump, ¾" diameter poly pipe placed 12" below the soil surface and remote bulb thermostat to maintain a 70 to 75ºF soil temperature in the root zone. Electricity will be needed to operate the pump. A standby portable propane heater could also be used for cold nights.
Irrigation water is needed for the plants. If zoned properly, several high tunnels may be supplied by a 1" poly pipe line. Where the tunnels are located a distance from water, a trailer mounted tank could be used for the water supply. For crops grown in rows or containers, a drip system will reduce the amount of water needed.
Although high tunnels can't provide the same environment as a greenhouse, they can offer a grower the opportunity to have additional growing space during the spring and fall for crops that are not as temperature sensitive. They do require more attention and maintenance.John W. Bartok, Jr.
Natural Resources Mgt. & Engr. Dept.
University of Connecticut , Storrs CT