by Sally Colby

Growers who are just starting with high tunnel production are satisfied when the structure is bursting with a profitable crop. There’s a significant investment in the tunnel, and growers want to offset costs by continually growing high-value crops. In many cases, that crop is tomatoes.

“The first year, you get a great tomato crop [and] then do it again,” said Dr. Cary Rivard, Kansas State University. “The next year, you think ‘Let’s do it again – we’re still paying off the high tunnel.’ What can happen is growers start to eliminate crop rotation intervals or eliminate rotations altogether. This can be a potentially problematic solution.”

Rivard said many Kansas growers produce for farmers markets, CSAs and small restaurants. Some are scaling up high tunnel production, and instead of having one tunnel with diverse crops, they maintain four or five tunnels for crop rotation.

But Rivard said it’s important to bear in mind that one of the purposes of crop rotation is as part of an integrated pest management system. Using the same growing space for the same crop (or crop family) leads to issues with soil-borne and root-infecting pathogens like pythiums, fusariums and rhizoctonias. Rivard said disease can spread quickly if inoculum is dropped in the soil and a host of the same plant is grown the following year.

Soil fertility is another issue with continuous production of one crop family. Rivard explained each crop removes a specific nutrient profile from the soil, affecting both major and minor nutrient levels. Another benefit of crop rotation in high tunnels is more efficient use of labor – keeping crews busy by staggering crops.

Rivard’s research showed it costs between $0.44 and $0.49 per square foot, per year, to maintain a high tunnel. When developing a crop rotation system, growers have to be able to offset that figure. It’s no surprise that tomatoes bring in the highest gross revenue at $3.66 per square foot, with bell peppers second at $2.30 per square foot.

“Think about rotating across entire plant families, not specific species,” Rivard said. “Within the solanaceous family are peppers, tomatoes, Irish potatoes and eggplant. If you rotate from tomatoes to peppers, you aren’t going to gain much. The pathogens that attack tomatoes also attack their cousin the pepper.”

One option for tomatoes, although it isn’t a true rotation, is to diversify with rootstock. Rivard has conducted research using interspecific hybrid rootstocks – a wild tomato species crossed with a domestic species. “One way you can introduce a small but not insignificant amount of diversity in tomato production is by using grafted plants,” he said. “It allows you to introduce and rotate some rootstocks without having to change the scion variety.”

In order to limit plant pathogen population reproduction, look for tomato rootstock with major gene resistance. “It’s only going to help with diseases that have major gene resistance available,” said Rivard. “Rootstock can function as rotation, but it isn’t the same thing. We don’t want a monoculture with grafted and non-grafted plants. It isn’t going to be sustainable long term.”

Cover crops in high tunnels may be an option for growers who can take a tunnel out of production for a period of time. Rivard said growing a cover crop helps feed soil microorganisms and adds organic matter. In three cover crop trials, Rivard defined three cover crop options: overwintering, summer and winter kill (late autumn into December). Crop options for overwintering include rye, triticale and wheat with hairy vetch as the legume. Summer cover crop choices include millet, sorghum, sudangrass and buckwheat with cowpea as the legume. Winter kill crop options include oats, barley and millet with cowpea as the legume.

A profitable option for some growers is spring-planted strawberries. Rivard said in

a typical annual strawberry production system, varieties like Chandler and Camarosa are planted in autumn. “Here in Kansas, we put row cover over them for the entire winter and harvest in spring,” said Rivard. “In the high tunnel, we changed it up. If you’re planting in fall and not harvesting until spring, you have a huge opportunity cost in the production space in fall and winter when you could be growing greens, cabbages or cover crops.”

Rivard said high tunnel strawberries can be fairly productive. Open-field strawberry growers are satisfied with one pound per plant from open field strawberries, and high tunnels yield even more. “In the high tunnel, we got about a pound and a quarter per plant,” he said. “Strawberries are a high value crop and can be a potentially good way to increase per square foot revenue in the tunnel in summer.”

Although vining crops typically don’t have a high gross revenue on a square foot basis, Rivard conducted cantaloupe and watermelon variety trials in high tunnels.

Melons were planted May 15 as transplants with 24-inch in-row spacing and five feet between rows. Fertilization was provided both pre-planting and through fertigation.

Rivard said although melons were not trellised during the trial, trellising is necessary to prevent unmanageable vine tangling. “We harvested 3,300 pounds of cantaloupe out of the 24’ x 100’ section of high tunnel,” he said. “Our first harvest was June 30, which is quite early.” Yield data showed $0.90 per square foot revenue based on a selling price of $0.54/pound. Revenue isn’t as high compared to tomatoes at $3.66, but it’s double the revenue toward structural costs. Watermelon yields were similar, with revenue at $0.89/square foot. Rivard reminded growers to plant pollinator varieties among seedless varieties, or select parthenocarpic varieties.

Another feasible rotation for the Midwest is high tunnel slip production for sweet potatoes. Rivard said sweet potatoes are typically propagated in the southeastern U.S. via long stem cuttings and then shipped for planting in June and July. Instead of buying slips, propagate them in the high tunnel and either sell or use for field-grown sweet potatoes.

“We bed the tubers as early as we can and cover with plastic,” said Rivard. “The tubers will send up shoots that grow about ten to twelve inches, then they’re harvested as slips.” Rivard found that planting density impacts both quality and the number of slips produced. Quality factors include the number of internodes and leaf mass, both of which can contribute to final yield. With good yield, sweet potato slip production gross revenue is $4.30/square foot.

Rivard encouraged growers to consider a six-year crop rotation as the goal, with three years between tomatoes. “The way to think about each potential cropping system is to think about the revenue per square foot,” he said. “Over the course of six years, we had an average annual gross revenue of $4.09 per square foot. Overhead structural costs for the tunnel of $0.49 per square foot per year is about 11 percent of that gross revenue. That’s a good starting point – if you can pay off your high tunnel with 10 percent of your crop, you should be in pretty good shape.”