Additional contributing authors: , professor, University of Florida Department of Horticulture Sciences.
Usage
High tunnels are passively ventilated and heated structures that farmers use to extend a tender crop's growing season outside the normal window for production. Emery Emmert, a professor of horticulture at the University of Kentucky, generally is credited with developing the concept of the unheated, plastic-covered greenhouse in the 1950s. While they gained significant traction in Europe and Asia, their use in the United States was limited for many years. By the turn of the 21st century, Asia and the Mediterranean regions had more than 1.3 million acres of plastic greenhouses and high tunnels in production (Papadopoulos & Demers, 2003). High tunnels gained traction in the United States from the 1980s onward, as growers in more northern climates sought to extend their market windows. In a 2007 survey, Maine and Vermont were estimated to have approximately 700–1000 and 500 tunnels, respectively (Carey et al., 2009). That same survey reported roughly an acre of high tunnel production in Georgia at the time, with Florida reporting between 50–100 total acres. Throughout the 2000s, high-tunnel usage steadily increased throughout the United States, aided by the growth of small farms offering fruits vegetables for local sales, as well as programs such as the USDA Natural Resource Conservation Service (NRCS) Environmental Quality Incentive Program (EQIP). Between 2010–2020, NRCS helped fund more than 26,000 high tunnels across the United States (K. Jacobsen, personal communication).
Structures and Siting
Figure 1. A range of high tunnel structures, from inexpensive tunnels made from PVC pipe to more typical standalone steel tunnels and wood-framed tunnels.
High tunnels can come in many forms, such as simple homemade structures made inexpensively with PVC, stand-alone metal-framed unheated greenhouses, large multibay complexes, and nearly everything in between (Figure 1). While most tunnels are made from galvanized steel, there also are wooden-framed structures. Because wooden trusses may have a larger surface area than a comparable steel bow, growers should be cautious that wooden structures don't cause too much shading over the plants. Organic growers should be aware that treated lumber may not be used for baseboards or any part of the structure that contacts the soil in high tunnels where crops are grown in the ground (Taylor et al., 2013). While it's possible to encase the treated lumber to ensure that it has no contact with the soil, growers typically utilize nontreated woods or steel instead.
Anchoring your high tunnel also is an important consideration. Some tunnel kits come with sidewall posts that have augured bases that act as anchors in the soil, while others are simply steel pipes driven into the soil. In areas with sandy or shallow soils, or if the site is exposed to high winds, ground posts may be anchored with concrete. High tunnels have a large surface area and can act as a wing when subjected to high winds. The amount of uplift on a high tunnel can be substantial and producers should think through how their structures will be anchored (Figure 2).
Figure 2. This high tunnel was destroyed after winds pulled it from the ground.
Though most high tunnels are stationary, there are several options for growers that utilize movable tunnels (Figure 3), which can allow for improved crop rotation. The intensive year-round production in tunnel structures can lead to increased soilborne pathogens such as nematodes and diseases such as southern blight, caused by the soilborne fungus Sclerotium rolfsii, or white mold, caused by Sclerotinia sclerotiorum. Movable tunnels may be anchored using portable tree/vineyard anchors or similar. Another advantage of movable tunnels is that a crop may be started outdoors in late summer and then a tunnel can be moved over the crop when temperatures drop. This avoids exposing the young plants to high temperatures often encountered in tunnels in the late summer.
Figure 3. Moveable high tunnels use a rail system with rollers or a skid system that allows them to be pulled through a field. Photos courtesy of Krista Jacobsen. For more information on movable high tunnels, see the Jacobsen et al. “Movable tunnels” entry under Additional Resources.
High tunnels can be covered with one or two layers of plastic. When two layers of plastic are used, an inflation fan is often needed to create the air space between the layers of plastic. This air space provides extra insulation and can help trap heat during the winter months. In the north Georgia mountains this extra insulation can be helpful, but it isn't typically required throughout the rest of Georgia and Florida. Plastics for high tunnels are typically 4 to 6 mil thick. While the 6-mil plastic often has an expected life span of 4 years, keep in mind that there are many variations on plastics. Infrared (IR) plastics hold heat better than others, while some plastics are made with anticondensate, which reduces dripping onto the crop below. Woven plastics are heavyweight (9 to 12 mil) coverings with longer life spans that are built to withstand extreme weather events, such as hail. Growers should consider washing or cleaning plastic annually because dirt and dust can build up, leading to significant shading. Measurements taken in tunnels with old and dirty plastic showed a 20%–30% reduction in light intensity compared to ambient conditions (Figure 4).
Figure 4. Exceptionally dirty plastic covering a high tunnel.
Siting a high tunnel is also an important consideration. While many growers must make do with space limitations, high tunnels ideally should be located on a north–south orientation to reduce shaded areas in the tunnel. If you plan to expand over time, with multiple tunnels adjacent to each other, a north–south orientation is even more critical. If adjacent tunnels are oriented parallel and east to west, the southernmost tunnel will shade tunnels located north of it. While this is less noticeable during midsummer when the sun is highest above the horizon, the effects are quite pronounced during winter months when the sun remains lower to the horizon. If movable tunnels are a consideration, growers must also account for the extra room needed to move the tunnel.
Ventilation and Cooling
While growers in cold climates often focus heavily on the temperature protection from cold weather that high tunnels offer, in warm weather states such as Georgia and Florida adequate ventilation of high tunnels is critical. Our farm trials have shown 3 to 10 °F increases in nighttime low temperatures in high tunnels depending on the type and quality of the tunnel. Daytime temperatures in the summer in those same tunnels often can exceed 125 °F. Even in cold-weather months during clear sunny days, high tunnel temperatures can easily exceed outside temperatures by 40 °F or more. During warmer months, temperatures in poorly vented tunnels easily can kill crops.
Sidewalls that roll up or down are essential for high-tunnel growers in the southeastern U.S. Growers may also use ridge vents in tunnels to provide maximum cooling, though the additional costs associated with the vents may be more than many can afford. Because high tunnels typically are passively ventilated (no fans), evaporative cooling systems generally are not employed. Studies conducted at 海角官方首页 evaluating the efficacy of misting/fogging to reduce temperatures in high tunnels suggested that without active ventilation, the cooling effects of fogging systems were negligible while also leading to higher humidity and soil-moisture levels (Laur et al., 2021). Although commonly employed in greenhouses, evaporative cooling methods may be less effective in high tunnel structures. Excessive moisture in the soil coupled with little air movement also can cause significant fog and condensation inside tunnels, which can lead to disease (Figure 5).
Figure 5. Fog in a high tunnel in late fall.
The type of high tunnel structure also can result in improved or reduced air flow during warm periods of the year. Tunnels with static end walls and low sides may hinder air movement compared to those that have large open ends with high peaks. Shade cloth is routinely used for high tunnel production in the southeastern U.S. Research conducted at 海角官方首页 suggests that 30%–50% shading is appropriate for production of crops from late spring to early fall in high tunnels in Georgia. While black shade cloth has traditionally been used by growers, silver-reflective shade cloth results in a greater reduction in temperatures because it reflects more light (Hohenstein, 2012). Growers in some situations use other colors of shade cloth, including red and green, but black and silver are the most common.
Because of the prevalence of insect pests and their ability to serve as virus vectors in crops such as tomatoes, some growers use insect netting over side and end walls. Research conducted at Florida A&M University, Auburn University, and the USDA utilizing tunnels covered only with insect netting have shown promising results. While some insects inevitably make it into the tunnels, the netting can reduce insect pressure and allow for the use of organic products or predatory insects for control of some insect pests in high tunnels. In semitropical environments, insect netting may allow for cooler temperatures compared to only using plastic-covered tunnel structures.
Planting, Pruning, and Trellising
One of the biggest decisions that growers will need to make regarding organic tomato production in high tunnels is what type of tomato will be grown—indeterminate or determinate—and the spacing of these plants. While most high tunnel tomato growers choose a closer spacing than in the field to maximize profit potential and yields, the type of tomato production system can significantly influence spacing. In general, when growing indeterminate varieties that are recommended for greenhouse production using a trellis system, plants can be spaced closer together as they usually are pruned to one or two central leaders (Figure 6). Researchers at 海角官方首页 have grown multiple successful crops of organic indeterminate tomatoes pruned to a single leader with between-row spacing of 4 ft and within-row spacing of 18 in. This equates to roughly 6 square feet per plant. Conventional greenhouse hydroponic tomatoes may be grown using tighter spacing (4 square feet per plant), but keep in mind that this production system often has greater air movement and nutrient availability. As plants in a trellis system become tall and reaching a trellis wire, air flow can dramatically decrease within the tunnel (Fatnassi et al., 2009).
Indeterminate high tunnel tomatoes grown using a string-spool attached to a wire trellis (A, B, and C). An internal support trellis (D) can also be used. Tomato vines are shown “laying down” in a typical hydroponic operation (E). Determinate tomatoes are shown growing in a high tunnel using traditional staking and stringing (F)
Indeterminate tomato plants grown using a trellis system are often “laid down” in a traditional hydroponic production system (Figure 6). However, this method of production requires significant labor to manage plants as well as a commitment to keep a plant in production for long periods of time (10 months or more). Because of the summer heat in high tunnels in Georgia and Florida, growers may find it better to “top” plants when they reach a certain height, usually the height of the support wire in the tunnel. This reduces labor requirements and moves production out of the tunnel and into the field for the hottest summer months.
Growers may also choose to grow indeterminate tomatoes in high tunnels using a traditional stake-and-weave method, similar to what is used in field production. This method requires less initial infrastructure than a trellis system with strings and hooks but will require frequent stringing to keep plants upright.
High tunnel producers may also choose to plant field-type determinate tomatoes. Because these plants may only have a few suckers removed near the bottom of the plant they tend to be bushier than indeterminate varieties pruned to a central leader. Determinate varieties shouldn't be pruned to a central leader as their yields will suffer since they are not meant for this type of production. Because of the bushier habit of determinate tomatoes, they may need to be spaced further apart. Trials and on-farm observations suggest a spacing of 7.5 to 9 square feet per plant for these types of tomatoes in high tunnels. Research conducted at 海角官方首页 indicated that for a short spring season (March–July), determinate tomatoes had comparable yields to indeterminate types per 100 row feet. However, because of a concentrated fruit set, harvests of the determinate types took place over a much shorter period (four harvests for determinate vs. nine for indeterminate). Nonetheless, labor was significantly less because the determinate types were staked and strung using a Florida weave system, which requires less effort than a vertical trellis. Determinate tomatoes may be a better option for growers simply looking to extend their market a few weeks on either side of the field season. Indeterminate varieties may be a better choice for growers looking for longer production periods in the tunnel and a greater variety of specialty type fruit than usually are available (Figure 7).
Figure 7. Indeterminate tomatoes often provide more specialty types of fruit.
Planting dates will vary widely with the type of structure used and the location within Georgia and Florida, which covers a distance of nearly 700 miles north to south. While some single-layer tunnels may only provide 2–3 °F of cold-temperature protection, on particularly cold nights an additional layer of row cover can be provided to protect plants. On-farm data shows that this cover within a tunnel can provide another 2–3 °F of protection. In southern Florida, where temperatures rarely dip below freezing, high tunnels may be used to provide a greater accumulation of growing degree units during the day and reduce rainfall on plants. First planting dates for tomatoes in Georgia in the field range from March 1 in southern Georgia to April 15 in north Georgia. In most well-constructed high tunnels, growers should be able to plant at least 1 month earlier than in the field.
Light levels also impact tomato planting. This is much more pronounced in the fall than in the spring. Tomatoes need adequate light to set fruit. While light levels remain high through September, average daily solar radiation drops to 50% of peak summer levels by November in central Georgia (Dunlap et al., 1994). While temperatures may remain adequate for tomato plant growth, light levels in late fall drop enough to significantly reduce fruit set. Because of this many growers have decided to focus on high tunnel tomatoes in spring, when light levels rapidly increase from February to April, and plant leafy greens crops that require lower light levels in the fall.
Grafting
Figure 8. Grafted tomato transplants at the nursery ready to ship to growers.
Vegetable grafting is a technique that is widely used around the world (King et al., 2008). Use of rootstocks with resistance to soilborne diseases, including root-knot nematodes, effectively has managed these plant pathogens in high-tunnel-grown tomatoes (Frey et al., 2020; Rivard et al., 2010; Thies, 2021). While breeding a variety with high levels of disease resistance and high-quality fruit may take many years—if ever—grafting can significantly shorten this process by physically uniting two genotypes (i.e., scion and rootstock) to integrate desirable above- and belowground traits in a new plant (Figure 8). Because of restrictions on using chemical pesticides in organic production, grafting is an even more important tool for organic farmers (Figure 9). In addition to disease resistance, grafting with selected rootstocks also can increase the vigor of plants in some cases and help improve nutrient and water-use efficiency.
