A wide variety of techniques have been used by growers to extend the growing season. More recently, growers have employed row covers, low tunnels and high tunnels because of their simplicity and effectiveness in protecting crops from low temperatures.
Row covers and tunnels don’t offer the precision of conventional greenhouses for environmental control, but they do sufficiently modify the environment to enhance crop growth, yield and quality. Although they provide some frost protection, their primary function is to elevate temperatures a few degrees each day over a period of several weeks. There are the additional benefits of protection from wind and rain, soil warming and in some instances control of pests and diseases. Row covers and tunnels have sufficient versatility to make them useful in various cropping systems.
There are two basic types of row covers: hoop-supported and floating. Hoop-supported covers (or low tunnels) generally cover a single row, whereas floating covers may cover multiple rows. Cover materials are usually either clear polyethylene plastic or spunbonded polyester/polypropylene, available in varying thicknesses, weights, widths, lengths and ventilation configurations.
Hoop-supported covers were first researched in the 1950s and ‘60s and were supported above the crop by wire hoops. The ends of the hoops are inserted six to eight inches into the soil on each side of a row. There are many variations of the dimensions used, depending on crop size. For most crops grown with hoop-supported covers, black plastic mulch is used to control weeds as well as improve crop growth.
After the hoops are set, the plastic cover is applied and secured by burying the edges with soil. The cover should be taut to prevent flapping in the wind. An alternative way of securing the cover is to insert an outer wire hoop over the inner hoop with a solid (non-perforated) plastic cover wedged between the two hoops. With the double hoop technique, the edges of the plastic are not buried so the cover can be raised and lowered for ventilation.
Another cover used is plastic with either slits or perforations (3/8-inch holes). Generally, daytime opening is not required, although at night there is a greater loss of heat. The advantage over solid plastic is minimal, if any, and labor is required for manual ventilation. Daytime opening is necessary for crops such as tomatoes and peppers if the temperature under the cover is expected to exceed 90º F for several hours. Cucurbits are more tolerant of high temperatures.
Depending on the crop and the environmental conditions, the covers are left in place for three to four weeks and then removed. For crops requiring insect pollination, the covers are removed about the time of the appearance of the first female flowers. For wind-pollinated crops (tomato, pepper, eggplant), time of removal is somewhat temperature dependent, since temperatures should not exceed 90º at the late bud to open flower stage for more than a few hours.
For most crops, floating covers require no support because they’re lightweight. They “float” (or lie directly) over the crop. Materials include perforated plastic or spunbonded polyester/polypropylene; they do not impede seedling emergence or subsequent growth of the crop. To secure the cover against wind, the edges are buried or weighted down.
Perforated polyethylene and spunbonded polyester or polypropylene are available in several weights. Perforated polyethylene has a uniform pattern of 3/8-inch holes for ventilation. However, the holes allow for heat loss at night and are entry points for insects.
Spunbonded covers feature a thin mesh of white synthetic fibers which trap heat and serve as barriers to wind and insects. Water from rain or overhead irrigation freely passes through. The weight of these covers ranges from 0.3 to about 2.0 ounces/square yard. The lightest covers are used primarily for insect exclusion; the heaviest are for frost protection. The most common weights are 0.5 to 1.25 ounces/square yard. With covers under 0.5 ounces there is minimal heat retention at night; over 1.75 ounces, there is a significant reduction in light transmission. The heavy covers are used for nighttime frost protection only since they don’t transmit sufficient light for optimum crop growth.
Floating covers require much less installation labor than hoop-supported covers. The wider and longer the covers, the less labor required per unit area, since only the edges are secured. These covers vary in width from six to 50 feet and can be up to about 800 feet long. One way to evaluate labor needs is to compare the time it takes to apply one piece of cover 50 feet wide by 200 feet long versus the time it takes to apply five pieces 10 feet wide by 200 feet long.
Insect control is effective with spunbonded covers when all of the edges are completely sealed. For example, maggots in radish (and other crops in the cabbage family) are controlled when a cover is applied at seeding with a complete seal around the edges. Adult flies will be unable to get under or through the cover. A continuous furrow of soil along the edges is probably the best method.
It is feasible, however, that row covers could actually increase insect damage on some crops. The environment under the cover is pretty “cozy” for insects, so it’s important that all seedlings to be covered are free of insects at transplanting. To prevent a heavy population of Colorado potato beetles under cover, don’t plant potatoes in the same place in successive years and use a cover the second year. Overwintering adult beetles could emerge from the soil under the cover and lay eggs, which would hatch into a dense population of larvae.
One disadvantage of floating covers is weed pressure under the covers. Between strips of plastic mulch or with crops where plastic mulch is not used, weeds grow rapidly and competitively. There is also some inefficiency in covering the space between strips of plastic, but the trade-off is reduced installation labor of wide covers versus narrow covers.
A final material used in protecting a crop is a thermal blanket (like Aluminet). This is a lightweight, reflective metalized, HDPE knitted screen for use in high tunnels to preserve heat and protect crops from cold damage. It’s generally applied in the late afternoon and is suspended just above the crop canopy. It’s removed the following morning, as light does not penetrate this material.
The high tunnel growing system fits somewhere between row covers and traditional greenhouses. They’re relatively inexpensive, permitting entry into high tunnel crop production with limited capital. This system is particularly appealing to both established and new growers who utilize retail, direct marketing or CSA channels.
Dr. Otho Wells (now retired from the University of New Hampshire) started researching the use high tunnels for the production of tomatoes in 1983. He collaborated with Ed Person of Ledgewood Farms and Greenhouses on developing a high tunnel design. Person developed the roll-up side currently utilized in many high tunnels. High tunnels are usually covered with one layer of greenhouse-grade polyethylene and are ventilated by manually rolling the sides up and down. There is no permanent heating system, although it’s advisable to have a standby portable propane unit to protect against unexpected below-freezing temperatures. The only external connection is a water supply, usually for operating a drip/trickle irrigation system.
Plastic mulches, drip irrigation, row covers and thermal blankets can all be used in high tunnels. Current research on the use of grafted plants and other engineering modifications to high tunnels are being tested, as well as different plastic coverings which influence the environment inside the tunnel.
High tunnels can either be stationary or moveable and range in size. I believe the moveable high tunnels have a role in maintaining soil health by allowing the tunnel to be moved and the soil to be flushed out and subjected to natural freezing and thawing. I believe we’ll continue to see even more improvements in the engineering and application of high tunnel technology in the future.
A tremendous amount of research has been done and is ongoing on the use of high tunnels for the production of a wide variety of vegetables, berries and myriad cut flowers (and even tree fruits, such as the research on sweet cherry production conducted at Michigan State University).
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