Feb 8, 2016
History leaves trail of progress in controlled ag environments

For centuries a wide variety of techniques has been used to extend the growing season of horticultural crops. Glass jars, glass cloches, hotcaps, cold frames, hotbeds and greenhouses of various types have all contributed to season extension. More recently, row covers and high tunnels have become popular with growers because of their simplicity and effectiveness in protecting crops from low temperatures in both spring and fall.

Row covers and high tunnels do not 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.

In addition to temperature control, there are also the benefits of wind and rain protection, soil warming and in some instances control of insects, diseases and predators such as varmints and birds. Overall, industry advocates said, these growing systems should be considered protected growing systems that enhance earliness and higher yields, improve quality, and reduce the use of pesticides in some cases.

Row covers and high tunnels have sufficient versatility to make them useful on a wide diversity of crops and in various cropping systems. Vegetables, small fruits, and flowers are all suited to these growing systems; but the specific crops that might be grown will to a large extent depend on marketing opportunities for individual crops by individual growers.

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 by manually rolling up the sides each morning and rolling them down in early evening. There is no permanent heating system although it is advisable to have a standby portable propane unit to protect against unexpected below-freezing temperatures. There are no electrical connections.

The only external connection is a water supply for trickle irrigation. According to Penn State University Extension, Otho Wells, professor emeritus at the University of New Hampshire, was a pioneer in promoting the use of high tunnels in the northeastern United States. Wells developed the New Hampshire design and system of production that involved covering the entire soil surface inside the tunnel with a solid sheet of 6-mil thick plastic.

Penn State was the setting for the creation of re-designed endwalls so that they can be raised up to facilitate easy access into the tunnel of a small tractor and tiller and a system of production that uses 18- inch wide raised plastic mulch covered beds with drip irrigation tape buried 2-3 inches beneath the bed. The raised mulch beds are 44 inches apart, which allows four rows in a 17 -foot wide high tunnel.

More diversification

Vern Grubinger, vegetable and berry specialist with University of Vermont Extension, said season extension has long been practiced by growers using tools like cold frames and greenhouses. A more recent innovation is the high tunnel, which he said has become a remarkably popular structure on diversified vegetable farms in northern growing climates.

Grubinger also said much of the credit for the high tunnel revolution goes to people such as Wells, who studied and promoted the use of low-cost plastic structures throughout his career.

Grubinger said Steve Moore, a farmer from Pennsylvania who was 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.

Grubinger said 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.

Historically, Grubinger said, wood was commonly used to frame greenhouses, but it went 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, Grubinger said. 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.

Grubinger said 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. Endwall 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.

Early advocate

Merle Jensen, professor emeritus, Plant Life Sciences at the University of Arizona and founder of the school’s Controlled Environment Agriculture Center, was one of the early pioneers in the development of agricultural plastics for greenhouses and brought one of the first Dutch Venlo glasshouses to the United States.

Jensen has served as an academician and consultant to industry for over 40 years, developing agricultural systems for businesses, communities and aerospace application. At the age of 41, Jensen was featured in a People Magazine article in 1980, in which he described the emergence of controlled-environment agriculture as “a kind of intensive greenhouse farming never seen before. We use diesel engines to pump in available water; it doesn’t matter if it’s salty or brackish. The heat from the engines warms the water and partially evaporates it, which raises the humidity in the greenhouse. This lessens some plants’ need for irrigation with fresh water.”

Jensen also touted the benefits of the newly expanding use of trickle irrigation.

“Measured amounts of a water-and-nutrient mix are trickled directly onto the plants from a narrow hose running the length of the furrow,” he said. “This conserves precious water and you can stock the greenhouse with whatever soil is on the scene – even if it’s mostly sand – and still get results.

Jensen predicted big gains for agriculture based on the emergence of the new-style greenhouse, particularly due to their efficiency.

“Year-round growth gives yields 10 to 40 times greater than open-field production. It is a high-density operation. We use every cubic foot of space, not just ground level.”

Early in his career, Jensen did extensive research on tomato viruses and mineral nutrition for both horticultural and agronomic crops. He developed many of the cultural systems for controlled environment agriculture used in over 50 countries, including drip irrigation, growing media, solar energy, energy alternatives and conservation for greenhouse vegetable production. His programs of research served as a prototype for food support systems for aerospace application.

“Today, advanced technologies have dramatically increased vegetable, fruit and flower yields, with a whole new array of varieties available to the greenhouse industry,” Jensen wrote. “Growing greenhouse crops is one of the most exacting and intensive forms of all agriculture enterprises.

“Hydroponics/soilless growing systems, in combination with greenhouses, require high technology, and can be capital intensive and highly productive,” Jensen said.

“For success, a grower must have a clear understanding and knowledge of horticulture, plant physiology, growing media, plant pathology and entomology, as well as computer and labor relations skills and the engineering capability to provide an environment best suited for plant growth. This knowledge base, plus management skills in greenhouse vegetable production, is a must.”

Gary Pullano, associate editor




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