Growing crops in a high tunnel differs from in-field

Presenters at the High Tunnel Production Conference in West Lebanon, NH, emphasized that high tunnels have different requirements and management needs than does in-field farming, so growers must make accommodations.

The specialists who shared their expertise included Bruce Hoskins, assistant scientist in the Analytical Lab and Maine Soil Testing Service; Ann Hazelrigg, Extension professor, University of Vermont; Will Hastings, University of New Hampshire field specialist in Coös County; and Cheryl Frank Sullivan and Becky Maden, both UVM research assistant professors.

“High-yielding vertical crops in high tunnels need a LOT of fertility,” a UVM slide pointed out. Bullet points amplified the message: “Nutrients are needed to produce biomass. Successful growers fertigate weekly (that means to both fertilize and irrigate at the same time by adding fertilizers to the water supply). Tissue samples can help assess soil nutrient status during the growing season. When well-tended, plants can produce for a long season.”

The soil is, after all, under a tunnel’s poly, and rain and snow never reach it. Growers, in essence, create an artificial desert when they build high tunnels, and, over time, salts can build up to the detriment of maintaining healthy soil. Salt is water soluble, so growers can let rain fall directly onto the soil to flush it every four to six years (when they should be putting on new poly covers).

To prevent this problem, high-salt fertilizers should be avoided. This includes fertilizer made from food waste that most often is made from the very salty scraps from cafeteria kitchens, as well as dairy manure and seafood waste.

Adding large additions of compost and peat increases organic matter levels over time. This leads to a highly “buffered” soil that’s somewhere between a potting soil and field soil.

UVM speakers also warned against “pH creep” in which soil pH gradually increases over time, greatly limiting micronutrient availability. This creep can be caused by using high alkalinity well water to irrigate. Any water used for irrigation should be measured for alkalinity. There are two recommended options for those whose high tunnel soil has developed this problem: acidify with sulfur 90 pellets, since very high soil sulfur levels are not a problem, or make acid injections into the irrigation water (with OMRI citric acid). Citric acid is produced using the fungus Aspergillus niger, but both genetically engineered (GE) and non-GE strains of the fungus exist, so growers should check out they’re buying.

“Soil fertility is important in high tunnel yield, but it is just ‘one stave in the barrel’ that also includes light, heat, ventilation, pruning, pest management, variety selection and grafting,” the presenters noted.

Efficiently and sustainably operating a high tunnel requires growers to make numerous decisions, great and small, ranging from how best to “winterize” roll-up tunnel sides to ensuring that plants receive the optimal amount of water, as measured in gallons per minute (GPM). Over 50% of today’s growers use drip irrigation, but overhead irrigation is also used frequently, and hand watering continues.

Growers who plan to use horizontal air fans (HAFs) can use simple math to figure out how many they need by calculating how many cubic feet per minute of air they must move.

Sandy and Paul Arnold of Pleasant Valley Farm in Argyle, NY, attend conferences like this one regularly. “Since 2007, we’ve raised a wide variety of herbs and vegetables in unheated high tunnels,” their website reads. “Our winter crops include spinach, lettuce, salad mix, arugula, kale, chard, Asian greens, mustards and herbs.”

Remarkably, attendee Skip Paul of Wishing Stone Farm in Little Compton, RI, and his wife Liz pioneered organic farming there in 1983. “All totaled, we farm about 40 acres around town, with over 15 greenhouse structures, a packing barn, a commercial kitchen and a retail market space,” their website noted.

Chelsea Schmidt, who works under educator, ethnobotanist and herbalist Betsey Golan at the nonprofit Joppa Hill Educational Farm in Bedford, NH, came to the conference to learn more about how to successfully grow vegetables and herbs in a high tunnel. “It’s been a steep learning curve for me, and this is helpful,” she said.

by Edith Tucker