The benefits of growing produce and flowers in hoophouses are well-documented: higher yields, better quality, a longer season, even better flavor in some crops. But hoophouses come with their own set of problems. Among the prominent management challenges are soil quality, fertility, insects, diseases, and weeds.

So far, research on these topics is limited to just a few projects around the country, and this article will summarize the most relevant. New growers should note that  the words “hoophouse” and “high tunnel” are synonymous, and refer to unheated greenhouse structures with in-ground crop production. Most growers call them “hoophouses” but most academics use “high tunnels.” The two terms are used interchangeably in Growing for Market.

Soil quality
Many growers worry about the soil in their high tunnels. Outside, soil texture changes with the seasons in response to rainfall, freezing and thawing. Inside the hoophouse, though, the soil rarely freezes and is not exposed to rainfall at all. It seems intuitive that soil biology and structure must be affected by being continuously covered by a hoophouse.

Beginning in 2005, researchers at Kansas State University surveyed 81 growers with a total of 185 high tunnels in Missouri, Kansas, Nebraska and Iowa. Fifty-four percent had no concerns about their soil quality, but 14 percent felt there were problems and the rest were uncertain. Hardpans were reported by 32 percent and mineral surface deposits by 30 percent.

Sharon Knewtson, who was at that time a Ph.D. student at Kansas State, visited 79 of those farms and took soil samples inside the high tunnels and in adjacent fields outside. She tested the soil for multiple soil quality indicators including pH, salinity, particulate organic matter carbon and water-stable aggregates.

Her conclusion: Soil quality as measured was not negatively affected by high tunnel structures over time. Management practices such as tillage depth or frequency, application of fertilizers, use of cover crops and use of fallow periods had no effect on soil quality. Nor was there a correlation between soil quality and the length of time a tunnel had been in use, with all the tunnels at least three years old and some as old as 10 years.
While the overall message from that study is to relax about soil quality, two recommendations did emerge. The first is that most hardpan problems were caused by construction of the hoophouse and by restricted traffic patterns, e.g. paths. Growers should be aware of the possibility of a hardpan and if one develops, correct it with a deep-tined ripper. The second issue concerns mineral deposits (a white crust) on the soil surface, which may indicate a salt accumulation. Knewtson measured soil salinity in 93 tunnels at 63 farms and found that it was “slight and mostly superficial.”

“The surface mineral deposit reported by 30 % of the survey respondents was not a cause for alarm,” she wrote. “Electrical conductivity was not different in high tunnels with and without reported visible surface minerals. The mineral deposition at the surface could be carbonates, or salts, but as both pH and salinity were within acceptable limits at nearly all locations the presence of a surface mineral deposit may not be of concern.” However, because even slightly elevated salt levels can cause a 10 percent yield decrease in tomatoes and lettuce, she noted that “most growers can avoid yield reduction by leaching salts deeper in the soil profile with heavy irrigation before planting.”

Fertility
Because most plants grow so rapidly and exuberantly in the protected environment of a hoophouse, growers need to pay particular attention to soil fertility. A soil test at the start of the growing season will provide recommendations about adjusting N-P-K and soil pH. But most experienced high tunnel growers also provide regular infusions of organic matter and fertilizer. John Biernbaum, a high tunnel expert at Michigan State University, says that too much nitrogen causes problems in high tunnels, so he avoids manure and instead uses compost made of plant materials. At the MSU student farm, compost is applied before every crop — two or more times a year — at a rate of 25 gallons per 100 square feet (a 5-gallon bucket every 20 square feet). Eliot Coleman, author of Winter Harvest Handbook, makes a plant and seaweed compost that he applies at a rate of 41 gallons per 100 square feet. And Steve Moore, another veteran high tunnel grower at North Carolina State University at Goldsboro, uses 12-27 gallons per 100 square feet.

Other organic fertilizers can be used instead of compost. For four years, the MSU student farm used a Bradfield alfalfa-based 3-1-5 fertilizer instead of compost. The recommended rate of that product is a half gallon per 100 square feet. Plants grew just as well as in the composted beds, Biernbaum said.

He advises that high tunnel beds should be broadforked (see the Johnny’s Selected Seeds catalog for broadforks) to loosen and aerate soil without inverting it. Then they should be raked smooth and, finally, the compost should be spread. Irrigation, plant roots, and earthworms carry nutrients deeper into the soil over time, and having the compost on the surface at the beginning of the crop improves water absorption and seed germination.

Weeds
Weeds grow as vigorously as crops in the favorable conditions of a high tunnel, so weed management is even more important inside than outside. The oldest research recommendations were to cover the entire surface of the high tunnel soil with black plastic mulch, and many growers still do that. Plastic mulch has management problems, too, such as how to get the mulch tight on the soil surface and plant through it when the size of the tunnel doesn’t allow for mechanical mulch laying. Another issue is repairing drip tape underneath plastic mulch if it gets damaged by rodents. And in some locations, black plastic mulch inside a hoophouse heats the soil too much.

For some growers, those problems plus the resource and disposal issues make plastic mulch a non-option. They prefer to grow in bare soil and make weed management a priority. Here’s what Biernbaum teaches at MSU about high tunnels and weeds:

“When starting with a clean greenhouse, the strategies are similar to working in the garden. At the top of the list is not letting current weeds go to seed. It is also important to observe what weed seeds may be blowing in from out side. At each opportunity, fallow ground should be watered and maintained to allow weeds to germinate, and then quickly and shallowly cultivated to clean up the space. Since weeds will typically only germinate from the top two inches of soil, cultivation methods that avoid exposing the seed in deeper soil will usually reduce the amount of seed germinating.”

Carol Miles at Washington State University received USDA funding in October for a project studying biodegradable mulches in high tunnels in Washington, Texas, and Tennessee. Miles has previously researched biodegradable mulches in the field. Reports from that research are available at http://vegetables.wsu.edu/plasticulture.html, and future results of the high tunnel work will be posted there as well.