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- High Tunnels Extend Season and Allow Winter Production
- By Bryan Butler, Harry Swartz, David Lankford
University of Maryland
- 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 such as greens through the winter. High tunnels also provide protection from rain and hail and can reduce disease and pest pressure.
In Maryland, the seasonal weather patterns vary greatly from one year to the next and even with these unheated tunnels it is difficult to confirm exactly the best timing and choice of crops. However, with good planning, variety selection, and close management, this low cost system can add another dimension to a vegetable or small fruit operation.
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 14 feet wide, 96 feet long, and 7 feet, 6 inches tall at the center. Tunnels should be no wider than 30 feet, for good cross ventilation and reduction of snow accumulation on the roof in the winter. The quonset frame consists of metal bows made by bending steel pipe or tubing and potential stresses caused by the weight of snow or heavy wind must be considered.
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Each tree has a high pressure irrigation line for fogging and a low pressure line for trickle irrigation of each gutter with shut offs for each line.
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Metal pipes are driven into the ground approximately two feet deep and set every four feet of the high tunnel length, providing support for the quonset frame. The bows fit into the ground pipes and are attached by bolts. The ends of the structure can be plastic or wood on a wood stud frame, but should be removable to allow access for tillage equipment and to increase ventilation in the summer.
The structure is typically covered with a single layer of six-mil polyethylene with provisions for rolling up the sidewalls. The poly is secured onto a batten board on each side of the high tunnel about 3.5 feet above the soil line. A vertical sidewall helps to keep rain out of the tunnel and when rolled up, provides ventilation. A pipe is then attached to the loose bottom end of the plastic along the length of the structure. A “T” handle on the end of the pipe is used to roll the plastic onto the pipe to open the sides. Cross ventilation is assisted by wind and has proven to be very efficient.
The key to successful use of the high tunnel is to spend the time laying out and preparing the site for construction. The better the tunnel is constructed, the easier the roll-up sides will work and the easier it will be to ventilate.
Crops are grown directly in the soil inside the high tunnel and productivity can be maintained into or beyond the conventional growing season as long as the side vents are carefully managed.
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 six 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 actually 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 provides the opportunity to increase quality. Since the plants are grown in a structure covered with one layer of plastic, the foliage, flowers, and fruit do not get wet. This can reduce the incidence of many diseases. The soil does not become excessively wet since the only water supply to the plants in the tunnel is trickle irrigation. Proper water management will also help to reduce the incidence of certain root rotting diseases. The single layer of plastic only reduces light levels about 10% as compared to growing outside. Therefore photosynthesis is not reduced except in shaded parts of the plant canopy.
Temperatures inside the tunnel are usually warmer than outside temperatures, providing the environment for season extension. The floor in the house will not freeze during most winters. This allows work to be done with soil amendment incorporation or the growing of a cover crop during the coldest part of the winter. Since the floor receives no rainfall, if irrigation is done carefully the area between the beds becomes too dry for weeds to germinate.
The roll up sides that truly make the structure a high tunnel 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 can protect cut tender plants from blustery conditions. The structure will also provide 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. Maryland Cooperative Extension and cooperating growers are trialing the HDT developed by Davon Crest Farms on the Maryland’s Eastern Shore. Preliminary data has indicated 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. The stands are 3 1/2 feet tall with a peaked support to which the gutters are secured with screws. There are four six-inch-wide gutters per side and the gutters are mounted on a 45 degree angle about four feet off the ground. The crops are grown in a soilless mix in the gutters. Each gutter is irrigated individually with trickle tape so watering regimes can be customized. 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 grower’s ability to turn multiple crops over 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. A heater that can maintain a low temperature of around 38oF will be adequate. Air circulation is still primarily provided by the roll up sides but additional fans and vents may be desired depending on the grower. This will be especially important in areas with very hot summer months.
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. Pay back period on the investment is currently under investigation. The number of crop rotations per year and the types of crops grown will vary greatly depending on market demands. This house will provide excellent early and late season protection. However, it is not intended to be used for year-round production of any crops other than those that are relatively cold hardy.
The configuration of the HDT interior will vary based on crop needs. Gutters can be used in the entire house so that a 30 x 96 foot house could have 3,600 linear feet of rows, or a little less than half an outdoors acre of rows at five feet between row spacing, in one fourteenth of an acre. Other configurations may leave some floor space for potted plants to be brought in for crops such as raspberries, blueberries, tomatoes, or squash. These larger plants may not fit in the gutters and also may not need to be inside all year.
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 fertigated as needed. Plants can be grown on raised beds covered with plastic or landscape fabric with the rows in between bare dirt, or the entire floor can be covered with landscape fabric. Unless supplemental heat is provided the tunnel may not be able to provide adequate protection to the plants after the November/December time frame, depending on the year.
Disease problems may occur in the protected environment; management of the environment is critical. Ventilation to avoid high temperatures or high humidity is very important. Maryland’s unpredictable weather in spring and fall will make management intensive. Powdery mildew is one disease that may be favored by the high tunnel climate and should be monitored closely.
Insects will find the microclimate created for the plants to be favorable to their growth also. Without a doubt, IPM scouting must begin when the plants are set out. The use of beneficials may be the most practical way to deal with some insect and mite problems. However, season extenders can actually be used as physical barriers to keep insects off the plants. For example, screening the sides to exclude insects, and the use of floating row covers that have the edges secured will prevent insects from reaching the crop. Further research regarding pollination is planned.
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