by Karl H. Kazaks

High tunnels provide a practical way for growers of vegetables and cut flowers to extend their growing season and increase the profitability of their operation. Just as with open field production, however, soil health in high tunnels plays an important part in the cultivation of crops. Recently, NRCS hosted a webinar on crop rotation and management strategies within organic high tunnels with the aim to help growers maximize their long-term success.

“By developing a crop rotation in your high tunnels where you suddenly have windows of time to pop in cover crops – whether early spring, summer, fall – that’s the real value of rotation,” said Cary Rivard, associate professor at Kansas State University and director of the K-State Research and Extension Center-Olathe.

The suitability of cover crops and the ideal timing of their use depends on the location of your operation. In southern latitudes, it could be too hot in mid-summer to grow cover crops, said Krista Jacobsen, associate professor in the Department of Horticulture at the University of Kentucky.

There are different challenges in the north. “Overwintered legumes struggle in colder climates when planted intermixed with grasses,” said Associate Professor of Horticultural Science at the University of Minnesota Julie Grossman.

Thus, Jacobsen said, in order to determine your ideal crop rotation you need to understand the temperature, light and market conditions for your particular location.

“Many factors influence the microclimates of high tunnels,” Jacobsen said, but in general they are determined by the climatic conditions of your area. Typically in the north, you can extend the cycle of warm season crops by a month on each end of the growing season with high tunnels. “Having just a few degrees of warmth as compared to open fields makes a huge difference,” she said.

But light limitation is also a component to high tunnel management. Because every crop has a minimum light level requirement for growth, crop production is limited by low light during cool seasons in northern latitudes and by cloudy conditions everywhere. Jacobsen advised growers to think “about light and temperature when designing crop rotations.”

“In Ag 101, rotation is taught as basic to any crop system for its importance in pest control and fertility management,” said Rivard. The same principles which apply to open field agronomy apply to high tunnel growing.

When rotating crops, Rivard stressed, it’s important to rotate across crop families, as plants within the same family have similar soil nutritional needs and tend to suffer from the same pest and disease problems. Switching from tomatoes to peppers to eggplants – three crops within the Solanaceae family – isn’t the type of rotation which will greatly benefit your operation. Instead, switching from vegetables to cut flowers – which tend to be in different families and which do well in the low-wind and lower-light conditions of high tunnels – could be a useful management strategy for your operation. Hemp may also be an option.

“The nice thing about cannabis is it is in a completely different family and it is high value,” Rivard said.

You need to make sure what you’re doing is profitable, Rivard said. This means evaluating what type of structure you’re using and understanding your yearly maintenance costs. For beginning growers, a caterpillar tunnel – lower cost and easy to disassemble and move – could help you practice crop rotation if you’re able to move the structure regularly (every two years).

As for maintenance costs, Rivard has done studies, both at NC State where he did his doctoral studies and in Kansas, which showed that the cost to maintain a high tunnel is about 50 cents per square foot of covered ground per year. This does not include the labor cost of growing your crops.

Tomatoes are a high value high tunnel crop, but when you grow them back to back, year after year, you run the risk of stressing your soil fertility and becoming a breeding ground for problems like botrytis.

The good news is there are a number of alternative crops which can be used in a rotation with tomatoes in high tunnels. (Grafted tomatoes do not count – “Your yields will go up, though,” Rivard said. The extra value of using grafted tomatoes should make it easier for you to manage rotating in a medium-value crop after tomatoes.)

One such crop Rivard has studied is grafted strawberries, growing them in place of tomatoes. Their yield per square foot was about $1.80, as compared to about $2 more than that for tomatoes. Rivard expects the revenue of strawberries could be higher if grown in higher density – something he expects to study in the future.

Growing strawberries in the warmer months is best suited for more northern locations, Rivard said, but you could develop a rotation of winter greens, tomatoes, winter cover crop, strawberries, winter greens, tomatoes.

Rivard has also studied growing melons as an inter-year replacement for tomatoes. Their value per square foot was about 90 cents, but because their growing season ends before tomatoes, you can put in a cover crop after melons and before winter greens.

Besides melons, Rivard is looking to test other cucurbits, including watermelons and cucumbers. English cucumbers also have a long shelf life. A grower Rivard has worked with has found success with multi-colored brassicas (such as yellow and purple cauliflower) as an inter-year replacement for tomatoes. They need to be planted earlier than tomatoes but their season ends much earlier, giving you the opportunity to plant a summer cover crop and also plant winter greens earlier.

Sweet potato slips also have great potential and in Rivard’s research provide a higher gross than tomatoes – over $4 per square foot. Given the earlier end of their life cycle, you can (as with brassicas) plant a summer cover and then plant winter greens earlier.

Altogether, you could come up with a six-year rotation: tomatoes to cucurbits to strawberries to peppers to brassicas to sweet potato slips, still growing winter greens and using cover crops in the windows opened up by the crop rotations.

This sort of rotation is particularly suited to operations with multiple tunnels. Not only will it benefit your soil health, it will also help managing your labor. As Rivard said, “Integrating crop rotation gives you the ability to stagger out your labor needs. It will also break up the pest cycles.”

When it comes to deciding what cover crop to use, consult with your local Extension agent. Generally speaking, cover crops which do well in open fields will do well under tunnel. But keep in mind the termination process and the equipment needed to operate in a tunnel could, due to space constraints, be different than what you would use in the field.

In far northern latitudes, cover crops will overwinter. When overwintering a mixture of legumes and grass, you will get more biomass than legumes alone, but the legumes will struggle to perform. Also, herbivory can be a problem, particularly with hairy vetch.

For warm season cover crops, a “really high biomass crop,” like Sudex, Rivard advised, should be terminated early to allow you to manage the growth and the residue.

As you prepare for spring’s growing season, take stock of your operation and see if you can incorporate more crop rotation and cover crop usage in your high tunnels.