Fruit & Vegetable Magazine

Features Production Research
Vegetable tunnels

March 31, 2008  By Dan Woolley

Don’t think a high tunnel for
vegetable production is the same as a greenhouse, warns Kathy Demchak,
a Penn State University senior extension specialist.

tunnelCould have important role in Canada says university specialist

Don’t think a high tunnel for vegetable production is the same as a greenhouse, warns Kathy Demchak, a Penn State University senior extension specialist.

Instead, it’s a low-cost, low technology, non-permanent production structure any producer could use for a quick start in the spring for vegetable growing, she explains, adding the principal benefit is its greater interior temperatures, which can help to lengthen the growing season in the spring and the fall. This ultimately increases the variety of vegetables a farmer can produce, which “can be a great opportunity but be sure to explore the market potential before you plant,” she urges.

Herbs, berries, tree fruit (particularly cherries), peppers, garlic, leeks and specialty potatoes do well in tunnels, Demchak says. “Sugar snap peas will grow absolutely wild,” she adds. “We have started them in January and you will need a support structure.” She rejects squash and gourds as they are difficult to manage, tend to overrun the tunnel and require insect protection.

And, while Oriental greens, okra (on plastic beds), fennel and chives grow well in tunnel conditions, Demchak warns that perennial herbs will grow large and wild.

Cut flowers are another good crop, in her opinion, but she warns that the smaller tunnels can cause flower set problems because of their high temperatures. And, while Spring bulbs will do well in tunnels, she is uncertain if they would be profitable.

“It makes sense to go for crops that are unusual so you can get a high price for them,” she says, although she adds that, in her experience, growing tomatoes in a tunnel system results in the highest returns due to a significant improvement in fruit quality and yield.

According to Demchak, most field tunnels are either single or multi-bay designs with peaked (Gothic) or arched (Quonset) roofs and their plastic coverings, which are usually about six mil thick, cost from $2.75 to $3.00 (US) per square foot with a four-year life span. Arched, multi-bay tunnels cannot tolerate snow loads, she explains, and the plastic covering must be removed before winter. The tunnel should be placed on a flat site, free of weed problems, and, due to poor wind resistance, oriented so the prevailing winds hit at right angles to their sides.

As for tunnel-growing choices, Demchak said the tunnels can be directly seeded or transplanted for an earlier crop. And, while tunnels do permit closer crop spacing, “don’t overdo it, or you will have crop loss due to high humidity in the tunnel, she warns.

Growers may want to consider added row covering for more protection and or a raised bed. For row covering, she prefers embossed plastic due to the wide range of temperatures found in the tunnels and clear black or infrared transmissible row covering to suppress weed problems when warming the soil. White plastic should be used for the cool season and floral crops. 

Since the tunnel will protect the crop from rain, irrigation is required. Most growers use trickle so “they won’t have a lot of weed growth in the row,” says Demchak. As well, growers will have earlier spring tillage and less leaching.

To feed the tunnel crop, Demchak recommends fertigation or composting with a granular fertilizer for pre-planting, as well as removing the row covering to avoid potential salt build-up and possible nitrate accumulation by leaching the soil.

Due to high temperatures and a lack of overhead watercooling, the producer will have to ventilate the tunnel manually and frequently in the summer. “When the temperature is 270C outside, in the tunnel it can reach 540C,” warns Demchak. “Basically, you will cook whatever is in there.”

Pesticide costs in a tunnel system should be lower, she says, but warns that insect and disease pressure may be different from field production and more similar to that found in a greenhouse system.

Demchak believes field tunnels can play an important role in Canadian horticulture production. “Our (Pennsylvania’s) maximum temperatures get a little bit warmer, while (Canada) is slower to warm up in the spring. The bottom line is, you are better positioned to work with high tunnels.”

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