Since the dawn of horticulture, growers have found ways to extend the length of time their crops could be produced. Thousands of years ago, this was done by selecting protected production sites with favorable microclimates. Hundreds of years ago, cold frames and hot beds with bottom heat from decomposing manure were used. Then came glass greenhouses. Over the past few decades, there’s been an explosion of season-extension options for growers to choose from, most of which rely on plastic in one form or another, from mulches and row covers to plastic-covered greenhouses. Most of the season-extension structures that fall between ground-level covers and permanent greenhouses can be referred to as tunnels.
Tunnels vary widely in size, shape and purpose. Unlike mulches or floating row covers, tunnels have some type of rigid support for the plastic cover. Tunnels, unlike greenhouses, are not equipped with poured foundations and/or elaborate heating and cooling systems. Typically, tunnels rely on solar energy for heat and passive ventilation, while greenhouses usually have a furnace and may have mechanical as well as passive ventilation. However, there isn’t a clear line between where tunnels end and greenhouses begin.
This hoophouse is a simple structure about 6 feet tall and 14 feet wide. It has steel bows, a single purlin down the ridge for structural stability, roll-up sides for passive ventilation and drip irrigation.
There are several distinct types of tunnels that can be differentiated using the terms high tunnel, hoophouse, caterpillar and low tunnel. This terminology is not always consistent, in part because characteristics may be mixed and matched across the types. For example, while hoophouses are usually unheated, there are plenty of exceptions to that, especially in northern locations. So, take these definitions with a grain of salt, they are useful only if they help you understand the different kinds of options that are available for growing crops under cover.
High tunnels and hoophouses are very similar (some might say the same), as they generally have steel pipe frames set into the ground, are covered with greenhouse-grade plastic and employ roll-up sides for passive ventilation. They may be Quonset shaped (half-circle frame) or Gothic shaped (peaked frame). Gothic frames are better suited to shedding snow, and they usually come with more internal bracing, so they make sense where severe winter weather is a concern. Whatever their shape, a strong frame and secure anchoring help structures stay in place during high winds.
This walk-in tunnel, or caterpillar, is a low-cost structure that uses plastic bows to support a covering that is anchored with ropes. In northern climates, the cover must be removed before winter to avoid collapse from snow load.
Photo by Becky Sideman, University of New Hampshire.
Hoophouses and high tunnels both tend to employ drip irrigation, though sometimes hand-watering or micro-sprinklers are used. The greenhouse film used to cover hoophouses and high tunnels may be single layer or double layer with an inflation fan. Hoophouses are generally unheated, while high tunnels can go either way. When they are heated, high tunnels usually have supplemental heat rather than the large furnaces typical of greenhouses. High tunnels are also more likely than hoophouses to have HAF (horizontal airflow) fans for mixing the internal air.
Hoophouses tend to be smaller in height and width than high tunnels, with a simple frame of hoops (bows) plus a central purlin (perhaps several) running the length of the tunnel to add structural strength and rigidity. Very small hoophouses may rely only on end wall openings for ventilation. Roughly speaking, a hoophouse may be 12 to 17 feet wide and 8 to 12 feet tall at the ridgeline.
High tunnels are typically Gothic shaped, 17 to 40 feet wide and 12 to 20 feet high. The length of these structures varies from about 24 feet to several hundred feet, though 48 feet and 96 feet are common lengths. They typically have multiple purlins as well as cross braces for added structural stability. Some high tunnels have a ridge vent at the top of the frame to optimize passive ventilation; others have gable ends with large openings such as garage doors, peak vents or other openings to enhance access and ventilation.
Multi-bay structures are hoophouses or high tunnels constructed side-by-side with “gutters” at the roof seams between each bay. Typically, multibay hoophouses, such as Haygroves, are covered only during the growing season, and the cover is rolled up or removed during the winter so that snow load and wind are not an issue. Multi-bay high tunnels that remain covered all year must have enough structural strength to bear the anticipated worst-case snow load, and adequate gutters between the bays to capture and remove rain and melting snow. The structures may need to be heated to melt snow or ice if it accumulates.
Caterpillars and low tunnels differ from hoophouses and high tunnels in that they are put up for only part of the year and then taken down. They have frames that are much less durable, and they are much more portable.
Caterpillars, sometimes called walk-in tunnels, are narrow single-bay structures, often barely tall enough to stand up in. They have a segmented appearance, thus the caterpillar moniker. They are less expensive than hoophouses and high tunnels, costing about a quarter of the price per unit of area covered. They provide less cold temperature protection than high tunnels and hoophouses, but more than low tunnels. The width of caterpillars ranges from about 8 to 18 feet wide, and the length may be up to several hundred feet, depending on the beds they are intended to cover and the size of available covers.
Caterpillars can be covered with greenhouse film, spun bonded row cover, shade cloth or other materials suitable for the crops being grown. The supports, or bows, can be made from PVC pipe, electrical conduit or steel pipe. These are placed over ground stakes made of rebar or steel pipe, or set into the ground about a foot deep. Bows are spaced 6 to 10 feet apart, depending on the expected wind exposure. A heavy-duty (.25-inch) rope is tied from bow to bow to form a peak purlin; this rope must be attached to heavy-duty ground stakes at both ends of the caterpillar.
The caterpillar cover is held in place by .25-inch ropes over the top of the caterpillar, which are secured to stakes or earth anchors in the ground. The edges of the plastic are left loose, with at least 2 extra feet on each side. When it’s windy, the edges can be held down with rocks or sandbags. At the gable ends, the plastic is bunched together using rope, and the rope is tied to a secure stake. Caterpillars have to be manually ventilated. During cold periods, some edges of the cover can be held up with short Y-shaped props. When it’s warmer, the covers can be rolled up all along the tunnel, using clamps or taller props to hold it in place. The sides must be rolled down if it’s going to get windy. These temporary structures will not withstand heavy snow or wind.
Low tunnels are smaller still, from 1 to 4 feet tall. They are essentially large row covers on some kind of large hoop. The size of the hoops and the covers can be as wide as needed to cover the beds to be protected. If used outside, low covers need to have the covers firmly secured with rocks, soil or sandbags. If the cover freezes to the ground in winter, it is difficult to harvest crops inside a low tunnel. Low tunnels are also used inside a hoophouse or high tunnel for added thermal protection of winter-harvested crops, such as greens, or to help overwinter tender crops.
Given the right market for the crops they produce, tunnels have proven to be a good investment for many growers. Everything from cut flowers to raspberries is being grown in tunnels. Obviously, returns vary with prices, labor costs, growing conditions and management. For example, well-managed leafy greens or tomato crops have been shown to net $1 to $3 per square foot annually, or more.
For more information on tunnel enterprise budgets, suppliers of tunnels and tunnel management in general, see the Cornell High Tunnel site, www.hort.cornell.edu/hightunnel/, and the Northeast High Tunnel Manual, www.uvm.edu/sustainableagriculture/high tunnels.html.
The author is vegetable and berry specialist with the University of Vermont Extension based at the Brattleboro office. He can be reached at email@example.com. Comment or question? Visit www.farmingforumsite.com and join in the discussions.