COLUMNS


High Tunnels for Longer Seasons

By Vern Grubinger


Photos by Vern Grubinger.
This simple high tunnel is used to gain a few weeks of growing on either end of the season, and to improve crop quality by protection from the elements. The tunnel has a single layer of covering, roll-up sides, black plastic mulch, drip irrigation and not much else. In colder climates, a small heater may be a wise addition since nighttime heat loss will be high from a long narrow tunnel with a single layer of poly.

Season extension has long been practiced by growers using tools like cold frames and green-houses. A more recent innovation is the high tunnel, which has become a remarkably popular structure on diversified vegetable farms in northern growing climates.

Much of the credit for the high tunnel revolution goes to people such as Dr. Otho Wells, professor emeritus at the University of New Hampshire, who studied and promoted the use of low-cost plastic structures throughout his career. Steve Moore, a farmer from Pennsylvania, now with North Carolina State University, has been called the guru of passive solar greenhouses and high tunnels. Moore is one of the authors, along with Ted Blomgren and Tracy Frisch, of “High Tunnels: Using Low-Cost Technology to Increase Yields, Improve Quality, and Extend the Season,” a manual funded by Northeast SARE. The information below is adapted from that, which includes six case studies of farms and their enterprise budgets for different high tunnel crops. The manual is available online at www.uvm.edu/sustainableagriculture/hightunnels.html.

High tunnels are simple, plastic-covered, tubular steel structures that rely mainly on the sun’s energy to warm the soil and air. Their name comes from the fact that they are high enough to stand up in.

Typically, high tunnels do not have mechanical systems such as heaters, fans and lights, so they are less costly to build than greenhouses. However, their frames are often identical to those used in greenhouses. Because high tunnels are less capital-intensive than greenhouses, it usually takes less time for them to pay for themselves.

Preparation

Setting out to build a high tunnel is no different from any other capital project. The steps prior to construction—planning, financing and site preparation—are often the most difficult. The actual building project is relatively simple. In shopping for a high tunnel, you’ll need to select from an array of designs, sizes and materials.

Structure shape

The shape of a tunnel affects its performance. Shape has an effect on lighting (and shading), energy gain, growing space and ventilation. Single-bay high tunnels come in two primary shapes: quonset and gothic arch. The quonset shape is relatively short and squat with a rounded roof and sloped sides, while the gothic has a high pointed peak and straight sidewalls. Unheated quonset structures can also serve as cold frames for overwintering nursery stock. Multibay high tunnels, including the Haygrove system, are usually a series of interconnected quonset-shaped tunnels.

Gothic-type tunnels have several advantages compared to quonset models. In many circumstances, these advantages easily offset their greater cost. A gothic-shaped structure readily sheds snow because of the steep pitch of its roof. Quonsets, especially those with PVC bows (rather than steel), need to be swept free of snow to prevent collapse. When snow threatens, some growers set up 2x4s as temporary props under the ridge pole, purlins or bows of their quonset-shaped high tunnels. PVC tunnel owners should remove their plastic for the duration of the snowy season.

Match your structure’s design load to local conditions of snow and wind. It’s not uncommon to hear about collapsed high tunnels in areas with strong winter storms. Some greenhouse suppliers will help you select the design most appropriate to your area.

The taller sidewalls of gothic tunnels offer more usable space along the sides for crop production and growth and for working comfort. For trellised crops like tomatoes, gothic-style tunnels provide adequate height both for the interior and perimeter rows. The headroom over the edge beds in a quonset tunnel may be so low that even a short person is uncomfortable when using a walk-behind seeder, for instance.

So-called “passive solar” tunnels or greenhouses can grow crops all winter (or at least keep them alive) without supplemental heat, even in cold areas. They feature a shape that minimizes heat loss (length-to-width ratio of around 2:1), well-sealed covering and leak-proof vents, and the use of additional layers of covers inside the greenhouse to capture heat from the soil around plants at night. Site preparation to prevent surface water from flowing through or under the greenhouse is also key.

The gothic shape also contributes to better air exchange and moisture control, and thus a superior growing environment. The greater height of gothic tunnels allows for better ventilation through higher gable-end vents. Gothic arch roofs tend to have enough of an angle to help shed water that condenses on the interior, instead of dripping on the plants below.

In quonset tunnels, since the whole structure is curved, opened roll-up sides expose some of the crops growing along the side to precipitation and other adverse weather conditions. This defect can be partially alleviated by purchasing extended ground posts.

Structure footprint

High tunnels come in many sizes, in widths from 14 feet to over 40 feet, and in incremental lengths. A width-to-length ratio of 1:2 is ideal for passive solar (or limited reliance on a heating system). Narrower tunnels have more heat loss than wider tunnels because of their perimeter-to-growing area ratio. A 10-by-90-foot tunnel has a 200-foot perimeter and a 900-square-foot growing area, whereas a 30-by-70-foot tunnel also has a 200-foot perimeter, but a much larger 2,100-square-foot growing area. That’s 133 percent more growing area, and less than half the ratio of perimeter to growing area (meaning less heat loss potential). Wider tunnels also tend to be taller, with better ventilation and interior air circulation.

Under northern growing conditions, even 30-foot-wide high tunnels can be sufficiently ventilated with roll-up sides and large gable-end vents and doors. In warmer climates, in tunnels with tall, dense crops like tomatoes, narrower tunnels (20 to 26 feet wide) may more effectively reduce stale air in the middle of the structure without mechanical ventilation.

Tunnel size

How big should your high tunnel be? Consider how much growing space you need now, as well as what you’ll want in a few years. A well-built tunnel will last at least 20 years. Another important factor in determining the size of a high tunnel to purchase is the amount of additional workload you are prepared to take on. Moore estimates that a 30-by-96-foot high tunnel that is intensively planted with multiple crops can take 10 hours or more of labor each week after initial set-up and skill development. Marketing time is additional. When a farm uses a simple cropping pattern-regular management time can be greatly reduced.

To get the most from your investment, it might make more sense to start with a short, wide house, like a 30-by-48-foot one, and add on later, rather than buying a 16-by-96-foot high tunnel that may quickly outgrow its usefulness.

When deciding what size tunnel to build, take into account how the structure will fit on your property. What are the site possibilities? How much land is available and what is its topography? It is critical to have enough room around the tunnel for easy access with vehicles and equipment, snow removal, water drainage and ventilation; to avoid shading; and to allow for future expansion.

Heat-activated vents are inexpensive and can improve ventilation, especially in quonset-shaped tunnels that lack a peak vent on the endwalls; they can also help where roll-up sides are inadequate due to tunnel design or a dense crop canopy that limits air flow

Frame materials

Historically, wood was commonly used to frame greenhouses, but it is has gone out of fashion because of its relatively high maintenance cost and the availability of steel greenhouses. The best greenhouse structures are made of high-tensile strength steel covered with a good galvanized coating to prevent rust.

An alternative to steel for the structural members of a high tunnel is polyvinyl chloride (PVC) pipe. Price is PVC’s only real advantage compared to steel. Mainly it is used in farmer-built tunnels. Tunnels whose bows are made of PVC pipe are more prone to collapse under snow and wind. Only narrow high tunnels with a quonset shape and smaller walk-in or caterpillar tunnels can be constructed of this weaker material. PVC also has a negative environmental impact during manufacture and disposal.

• Baseboards and endwalls—Most high tunnels use wood for baseboards, hip boards and endwall framing. For the endwalls, another framing option is steel. While more costly, it will not need to be replaced and is easy to work with. For the baseboard, recycled plastic lumber is rot and insect-resistant, and relatively inexpensive. Finding lumber that is affordable, durable and sustainably harvested is a challenge and trade-offs are inevitable. For example, redwood is expensive and unsustainable. Organic farmers are prohibited from using arsenic-containing pressure-treated lumber.

End-wall coverings can be made of plywood (painted is best), twin-wall polycarbonate sheets, other structured sheets or low-cost polyethylene film. Of course, opaque materials like plywood prevent light transmission.

• Twin-wall polycarbonate—Twin-wall polycarbonate is an extruded ribbed high-tech plastic with double walls for added insulation. It is sold as structural sheeting. For high tunnels, it has an application for the gable-end walls. As a hoop house covering, it compares favorably with both polyethylene and glass. It transmits up to 83 percent of light (more than two layers of polyethylene film), and insulates 40 percent better than glass, but weighs one-sixteenth as much and won’t shatter. It is durable and cuts easily with a saw. It is far more expensive than poly, but costs less than glass.

• Plastic Film—Made of polyethylene, plastic film is by far the most common cover for high tunnels. UV-resistant greenhouse-quality polyethylene is far superior to common construction-grade polyethylene. It transmits light better; is more resistant to wind, heat and yellowing, and has a longer life.

You should be sure to select a high tunnel structure that can withstand the worst-case winter weather in your area.

It is important to replace poly film as recommended. After four years, standard 6-mil plastic loses about 15 percent of its ability to transmit light. This is particularly significant during winter production, especially in cloudy climates.

Greenhouse film treated with anti-condensate additives prevents condensation drips. Infrared re-radiant (IR) materials are added to film to reduce overnight heat loss. Anecdotal evidence suggests that the frost forming on the inside of the plastic on a high tunnel is an excellent reflector of infrared radiation. Moore thinks that it may be equal in value to special infrared plastic in unheated structures. In heated structures (where the interior frost is not present), infrared plastics undoubtedly retain heat better.

New types of greenhouse films continue to enter the market. Some use infrared blockers to reduce excess daytime heat and scorching while also helping to minimize heat loss at night. Films can also be treated to increase light diffusion and thus photosynthesis and yields.

When preparing a tunnel for inground growing, pay attention to the soil physical condition as well as nutrient status. Take care to avoid soil compaction with equipment and excessive foot traffic. Then, test soil and add necessary fertilizers and amendments, such as compost. If soil on the site is of poor quality, consider making permanent raised beds filled with a good growing medium.

How many layers?

Moore experimented with several types of plastic film over a multiyear period on two adjacent high tunnels. He compared double layers (inflated) of the standard 6-mil four-year IR film and anti-condensate to single-layer 7.8-mil film with similar characteristics and an eight-year life-span claim.

In south central Pennsylvania, the two- layer high tunnel was warmer by an average of over 6 degrees Fahrenheit during the winter, and had superior plant growth compared to high tunnels with the single layer of high-performance plastic. Moore suspects that this difference in thermal performance between the two types of film would be less significant in a warmer climate or under late fall or early spring conditions.

A double layer of poly film with inflation between the layers provides insulation and reduces heat loss by 40 percent, according to Aldrich and Bartok (see the NRAES publication “Greenhouse Engineering”). Along with increasing heat retention, the second poly layer reduces the light level by about 10 percent, so a balance must be reached. Low light levels cause plants to become weak and leggy, and slow down growth. An alternative to double poly layers with an inflation fan is to use multiple layers of floating row covers, but this drastically decreases light transmission unless the covers are removed during the day.

The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.