High tunnels are movable or permanent unheated greenhouses erected on crop land which are used to extend the growing season. Permanent high tunnels are constructed of metal bows secured to metal posts that are driven into the ground. The ends are sometimes timber-framed to include doors that are often large enough to permit entry of a small tractor or tillage equipment. The structures are usually covered with one layer of 6 mil greenhouse-grade polyethylene plastic film or a double layer with an inflation fan that maintains a layer of air between them. Movable tunnels can be similarly constructed on top of skids or wheels to allow for rotation of land usage by dragging the structure using people or a tractor power.
High-value warm-season crops such as tomatoes, peppers, cucumbers and melons can be transplanted into high tunnels as much as 6 weeks earlier than outdoors, depending on location, without supplemental heating, and can extend the harvest season by up to a month in the fall. Crop quality and marketable yields are often higher for crops grown in tunnels than for field-grown crops, in large part because tunnel coverings keep rain off crops. High tunnels can also extend the harvest season for cold-hardy crops (e.g. spinach) into the winter months. Tomato is the crop most frequently grown in high tunnels. For more information specific to tomato, please see the Greenhouse Tomato section of the guide.
Crops grown in high tunnels are often, but not always, planted in the ground. High tunnel soils are not exposed to regular leaching from rainfall, and soluble salt levels can build up over time from the application of mineral fertilizers, and mineralization of organic amendments. High levels of soluble salts, which are mostly nitrate from over application of fertilizers or organic materials like compost, can negatively affect plant growth. The most sensitive crops include strawberry and green beans, but even relatively tolerant species such as tomato can show reduced vigor in soils with very high soluble salts. Most soil testing laboratories offer soluble salts analysis for high tunnel producers. It is probably a good idea to have high tunnel soils tested for nitrate periodically as well.
Salt buildup can be prevented by avoiding excessive applications of organic materials like compost, by diligently managing soluble fertilizers, and by avoiding fertilizers with a high salt index (e.g. potassium chloride), and those high in ammonium forms of nitrogen. If salt levels are excessive, the soil should be leached with several inches of water, either by removing the plastic covering over winter, or by irrigating for a long period of time with sprinklers. If leaching is necessary, it should be done well before planting to allow time to retest the soil and apply appropriate nutrients.
High tunnel soils are often heavily amended with composts, and have very high organic matter content. Large flushes of nitrate can occur in warm soils high in organic matter and/or when the soils have been re-wetted after a dry period. This makes it difficult to manage nitrogen in high tunnels when the soil organic matter content has been greatly increased, and probably accounts for some of the salt problems in high tunnels. One solution, not backed by research but worth trying, is to monitor soil nitrate concentrations every two weeks using the PSNT. If the soil nitrate concentrations are less than 30 ppm, more nitrogen should be added, if greater than 30 ppm, no nitrogen should be added. This seems expensive, but could save money by reducing fertilizer costs and eliminating the build-up of salts. Growers who fertigate through a drip irrigation system, would have minimal risk by experimenting with the PSNT. Research on best practices for high tunnel fertilization and soil management is ongoing, and our knowledge is evolving.
Site selection and orientation
Placing tunnels on the best, most productive soil on your farm increases chances of success. Good drainage is particularly important, since tunnels are typically used during spring and fall when outdoor soil moisture is abundant. Ideally, the tunnel should be placed so that it is higher than the surrounding landscape. If this is not possible, providing a way for water to move around (rather than through) the structure is important. Permanent greenhouses are typically oriented with the ends facing North-South to prevent parts of the greenhouse that are consistently shaded by purlins as the sun moves from East-West across the sky. High tunnels used primarily for winter production should be oriented with the ends facing East-West to increase exposure to the sun, which is low in the sky during winter months.
Environmental and crop management
Temperatures can rise dramatically in closed tunnels when the sun is shining. Ventilation is important. The least expensive form of ventilation is to add roll-up sides that use a pipe along each side with a sliding "T" handle (or some other mechanism such as a crank or wheel). Sides are typically designed to roll up 4 to 5 feet. If electricity is available, vents and fans controlled by a thermostat reduces the amount of hand labor required to keep the tunnel from overheating.
A "standard" high tunnel size is 30 x 96 feet. However, there are many other variations in width and length. Cross-ventilation is essential for keeping temperatures and humidity down. For wider tunnels (e.g. greater than 20 feet), additional end vents or passive roof venting systems may be needed. Horizontal air flow fans can help increase evapotranspiration and further reduce humidity, which can reduce common disorders and diseases.
Because rain is excluded from the growing environment, drip or trickle irrigation is usually used inside high tunnels. Irrigation may be required several times a week during main season production, but little or no irrigation is required during winter months for those doing winter production.
If crops are being grown when sidewalls are closed, pollination may not take place naturally. Tomatoes, peppers and eggplants are self-fertile and pollination in the field is usually enhanced by wind. In high tunnels, mechanical pollination or bumblebees are often used to enhance fruit set, size and quality (see Greenhouse tomato for more information). Cucurbit crops also require pollination; many have both male and female flowers, and require cross-pollination. Parthenocarpic varieties of cucumber and zucchini are ones that set fruits without pollination; these varieties are best suited for tunnel production.
Pest and Disease Management
The high tunnel environment differs from the outdoor environment: it’s warmer, has less free moisture, and is more humid. As a result, different diseases are common in tunnels than are common on the same crops grown outdoors. The most effective way to prevent plant diseases in high tunnels is to manage humidity by ensuring sufficient ventilation and running fans if necessary.
The use of pesticides in tunnels presents regulatory issues. Outdoors, pesticide residues break down after application by exposure to ultraviolet radiation and rainfall. Inside tunnels, plastic coverings reduce U/V and rain, and as a result, pesticides break down differently. Within New England, each state’s pesticide regulatory agencies may have different interpretations of whether high tunnels are considered open fields or greenhouses. Because pesticides may behave differently in an enclosed environment than in a field where they are exposed to the elements, it is safest to consider a high tunnel a greenhouse from the perspective of pesticide labels.
Regarding the use of pesticides in greenhouses, pesticide labels fall into three categories:
- The label can specifically state that the product may be used in greenhouses. The label may have different guidelines for greenhouse and outdoor use, or they may be the same. Regardless, the label is the law.
- The label can specifically state that the product may not be used in greenhouses.
- The label may not mention greenhouse use at all.
The Environmental Protection Agency’s current position on greenhouse application is that, in accordance with FIFRA section 2(ee), a label does not have to specify greenhouse as a site, provided the crop is on the label, in order to use the product in a greenhouse.
If the label has multiple sections, and one of those sections is for greenhouse application, then the label must be followed explicitly for greenhouses with no exceptions. For example, the greenhouse section of a label may state that this pesticide can be used at a particular rate for tomatoes and no other "crops" besides tomatoes can be sprayed in the greenhouse with that product, even though the outdoor section of the label might state a rate for those other crops. The rate for outdoor applications on those crops is for outdoor use ONLY and CANNOT be used for those crops in the greenhouse, since those crops were not included in the greenhouse section of the label.
Growers should be aware that using a pesticide inside a greenhouse where the label does not mention greenhouse use do so with the possibility of increased risks to workers or plants. Also, growers should be aware when using a fumigant or smoke generator for an entire greenhouse, that every crop in the greenhouse must be listed on the product label. As a result we advise against applying a product in high tunnels unless the label specifically allows its use.
Other structures for season extension. Most growers in temperate climates are now using or experimenting with some form of "tunnel" on the field. Common examples include row covers over wire hoops, low tunnels or "caterpillars" made of bent metal or PVC pipes, or high tunnels, discussed above. Some use semi-permanent construction and others are modular and movable. The low tunnels and caterpillar hoops may be anchored directly in the ground with rebar or small diameter metal pipe, or may be attached to wooden side rails or skids in contact with the ground. Covers made of spun bonded material or clear plastic may be anchored to the ground with stakes along the sides and on the ends. Covers typically can be lifted from the sides for ventilation. Ropes are sometimes strung from hoop to hoop under the cover to support snow load during winter production. These structures often span two or three beds.