Hoop-Supported Covers (low tunnels).

These plastic covers (1.0 to 1.5 mil thick) are supported above the crop by wire hoops made of No. 9 wire cut to lengths of 65 to 72 inches. A simple way to make pre-formed hoops is to purchase a coil of wire and use bolt cutters to cut the hoops at the desired length. When installing the hoops in the field, the ends of the hoops are inserted 6 to 8 inches into the soil on each side of the row so that the width of the hoop at the base is 24 to 36 inches and the height at the apex of the hoop is 14 to 18 inches. The hoops are spaced about 4 feet apart in the row. There are many variations of these dimensions, depending on crop size. Hoops made from a coil of wire are installed by hand; but for machine-installed hoops, the wire has to be straight to properly feed into the machine. For most crops grown with hoop-supported covers, black plastic mulch is used for weed control as well as for improved crop growth.

After the hoops are set, the plastic cover is applied and secured by burying the edges with soil. The cover should be taunt 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 cover are not buried so that the cover can be raised and lowered for ventilation.

Another method of covering is to use 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 through these openings than through solid plastic. The advantage of slitted over solid plastic is that a minimum, if any, labor is needed for manual ventilation. Daytime opening is necessary for crops such as tomatoes and pepper if the temperature under the cover is expected to exceed 90oF for several hours. Under such conditions, the easiest way to ventilate quickly is to make long slits at the top of the low tunnel; but this should be done in anticipation of the high temperatures, not after the fact. Cucurbits (melons, cucumbers, and squash) are more tolerant of high temperatures.

Depending on the crop and the environmental conditions, the covers are left in place for 3 to 4 weeks and then removed. For crops requiring bee 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 F at the late bud to open flower stage for more than a few hours.

Floating Covers

For most crops, floating covers require no support because they are lightweight. They "float" or lie directly over the crop, whether direct seeded or transplanted. Materials include perforated plastic, spunbonded polyester, and spunbonded polypropylene; they do not impede seedling emergence or subsequent growth of the crop, with an exception for tomatoes and pepper. To secure the cover against wind, all the edges are buried or weighted down with sandbags, stones, or other materials.

There are two types of floating covers: perforated polyethylene that is about 1 mil thick and spunbonded polyester or polypropylene that is available in several weights (rather than in thickness).

Perforated polyethylene has a uniform pattern of 3/8-inch holes (74 holes/sq ft) for ventilation. However, the holes allow for heat loss at night and may be an entry point for insects. Overall, many growers have found these covers to be very beneficial for growth enhancement.

Spunbonded covers are comprised of a thin mesh of white synthetic fibers that entrap heat and serve as a barrier to wind, insects, and varmints. Water from rain or overhead irrigation freely passes through without causing undue weight on the crop. The weight of these covers range from 0.3 to about 2.0 oz/sq yd (10 to 68 grams/sq meter). The lightest covers are used primarily for insect exclusion while the heaviest of the covers are used for frost protection. The most common weights are 0.5 to 1.25 oz\sq yd (17 to 42 grams/sq meter). With covers under 0.5 oz there is minimal heat retention at night; and over 1.75 oz, there is a significant reduction in light transmission. The heavy covers are used for nighttime frost protection only since they do not transmit sufficient light for optimum crop growth.

Spunbonded row covers in the 0.5 to 1.25 oz range provide 2o to 4o F frost protection in the spring. In the fall, there is more protection because there is a larger reservoir of heat in the soil in the fall than in the spring. These covers perform very well in protecting late-season tomatoes and pepper from early frosts.

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 6 to 50 feet and up to about 800 feet long. One way to evaluate labor needs is to compare the time that it takes to apply one piece of cover 50 feet wide x 200 feet long (500 perimeter feet) versus the time it takes to apply 5 pieces 10 feet wide x 200 feet long (2100 perimeter feet). Regardless of width, the cover is secured by weighting down the edges (sides and ends) with soil, sand bags, stones, or long pins. In extremely windy areas, additional weighting in the middle of the cover is advisable.

Insect control is effective with spunbonded covers if the covers are installed at planting and if all of the edges are completely sealed. For example, maggots in radish (and in other crops in the cabbage family) are controlled when a cover is applied at seeding with a complete seal around the edges. When the adult maggot flies search out the young seedlings for egg-laying, the 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 may increase insect damage on some crops. The environment under the cover is conducive to insect development, so it is important that all transplants under the cover be free of insects at transplanting. To prevent a heavy population of Colorado potato beetles under a cover, do not cover potatoes that have been in the same place for more two years in succession. Overwintering adult beetles could emerge from the soil under the cover and lay an abundance of eggs that would soon hatch into a dense population of plant devouring larvae.

There is at least one important disadvantage of floating covers: that is, the weed pressure under the covers. If crops grow better with covers, so do the weeds. Between rows of plastic mulch or in crops where plastic mulch is not used, weeds grow rapidly and competitively. If herbicides are used, generally the highest labeled rate is needed to adequately control weeds. If herbicides are not used, more cultivation or hand weeding may be required, which means that the cover has to be partially removed to facilitate these operations. Make frequent weed checks under covers.

Row Cover Crops

Most all vegetable crops as well as strawberries, raspberries, and cut flowers have been grown with row covers. Although the primary crops for row cover use are high value crops such as melons, tomatoes, pepper, summer squash, sweet corn, and strawberries, many growers find row covers to be valuable for a number of crops for varying reasons -- earliness, higher yields, overwintering protection, and insect and varmint control. Crop selection and cover selection are important to the economical success of the extra inputs. Crop distinctions such as temperature sensitivity, pollination methods, and growth habit dictate the type of row cover that is best to use.

Even though most crops can be grown without damage under floating covers, tomatoes and pepper are an exception. If the spunbonded material is not supported with wire hoops for these crops, flapping of the cover in the wind will damage the growing points of young plants. Also, with summer squash under windy conditions, many of the succulent leaves might be broken by the cover. For these three crops, a series of strategically placed wire hoops will prevent crop damage.

High Tunnel Systems

High tunnels are not conventional greenhouses. But like plastic-covered greenhouses, they are generally quonset-shaped, constructed of metal bows that are attached to metal posts which have been driven into the ground about two feet deep. They are covered with one layer of 6-mil greenhouse-grade polyethylene, and are ventilated primarily by manually rolling up the sides each morning and rolling them down in early evening. Roof ventilation kits are also available. There is no permanent heating system although it is advisable to have a standby portable propane unit to protect against unexpected below-freezing night temperatures. There are no electrical connections. The only external connection is a water supply for trickle irrigation. The ends of the tunnels are framed in, and may be made portable (removable) to facilitate easy access into the tunnel with a small tractor and tiller.

Construction is quite simple. To the metal posts an 8- to 10-inch baseboard is attached flush with the soil surface so that surface water will not run into the tunnel. A hipboard is attached to the bows 36 to 42 inches above the baseboard. The plastic cover is attached at the hipboard with batten boards, held with galvanized screws spaced 8 to 24 inches apart so that the plastic is firmly sandwiched between the two boards. (Other techniques are also used.) At the bottom edge of the plastic, which should initially extend about a foot beyond the baseboard, a one-inch metal pipe (for rolling up the sides) is laid along the edge of the plastic exactly parallel to the tunnel. The edge of the plastic is taped (every 12 to 18 inches) to the pipe with duct tape and then the pipe is rolled to the edge of the tunnel alongside the baseboard. With a sliding "T" handle at one end of the pipe, the plastic may be rolled to any height up to the hipboard. The amount of the opening on each side will depend on the desired internal temperature, the ambient temperature, and wind speed. Experience and a couple of maximum/minimum thermometers are the best tools for determining how to regulate temperature.

For all practical purposes, high tunnels are protected growing structures that should result in high returns. Therefore, they should be situated on the best soil -- soil that is well drained and that has had pH and nutrient adjustments as for a field soil. The soil should be smooth, firm, and moist at planting. A line of trickle irrigation is laid for each row (or double row) of crop, and then a one-piece sheet of 6-mil black plastic is laid over the total soil surface. (An exception to the black plastic would be with direct seeding crops such as lettuce, radish, or beets.) The black plastic is very beneficial for soil warming, reduction of soil water evaporation, weed control, and for providing a disease barrier between the soil and the plants. The plastic sheet has to be anchored along the edges with metal pins and string, rocks, or some other way to prevent the billowing of the plastic during brisk winds.

After the trickle lines and the plastic sheeting are in place, planting may commence. The specific planting design will depend on the crop, but using tomatoes as an example, the rows are 3.5 feet apart and plants are 18 to 24 inches apart in the row when the plants are supported with a basketweave (short stake) trellis system. The spacing for other crops will be somewhat similar to general field planting; however, because of the economics of high tunnel production, it is imperative that intensive culture be practiced. Because tunnel crops are protected from rain and heavy dews, the plants are drier than outdoors and will tolerate closer spacing.

High Tunnel Management

High tunnels are not automated. Consequently, for maximum efficiency, they require regular daily attention, especially in the morning and evening, and during heavy rain or strong winds. Temperature and humidity are the two critical factors that should be controlled as much as feasible. Early each morning, the sides should be rolled up to flush out the humidity and to keep temperature in check. The temperature in a closed high tunnel rises very rapidly on a clear morning! In other words, don't put off rolling up the sides. In early evening roll down the sides to entrap as much heat as possible. Close the sides each evening until the night temperature reaches about 65oF. In northern states, this could mean that the sides will be rolled down each day well into the summer.

Ventilation is best accomplished when wind moves crosswise through the tunnel (from side to side); therefore orient the tunnel accordingly. The width of the tunnel also impacts ventilation. It is hard to be specific on the maximum width; but from experience, about 20 feet seems to be the maximum width that allows for good ventilation, especially as plants grow taller and block the air flow.

Benefits of Tunnels

The primary benefit of tunnels is earliness. Tomatoes in a high tunnel mature on average about one month before field tomatoes. Earliness is the combination of being able to plant in high tunnels about two weeks earlier than in the field and faster ripening (about two weeks) inside the tunnel. Overall, the cost of a tunnel is recovered the first year when selling at retail prices. Table 1 shows a complete economic analysis of producing tomatoes in a high tunnel and illustrates annual net returns at varying yields and selling prices. At $2.00 per pound for a 3000-pound harvest, the net return is $0.86 per pound.

Another highly beneficial advantage of tunnels is disease control. The plastic cover serves as a rain shelter, the plastic sheeting on the ground is a barrier against evaporation of soil moisture, and early morning ventilation reduces relative humidity. Therefore, the leaves of crops are dry for most of the day and night, impeding the incidence and spread of disease. For example, early blight of tomatoes, a serious foliage and fruit disease on field tomatoes, is not a problem in high tunnels when the tunnels are consistently vented.

Although tunnels require more manual attention than do greenhouses, the benefits of high tunnels in a diversified farm operation have proven to be a valuable asset in overcoming a short growing season. Both cool- and warm-season crops do well in the spring and summer. With cool season crops, the season easily may be extended into early winter and even throughout the winter for some hardy crops. Fall-planted strawberries ripen the following spring about six weeks earlier than field-grown berries.