Transitioning your high tunnel for fall and winter

During our 100 farms project, we learned that a lot of Minnesota high tunnels go unused throughout the fall and winter. Research with high tunnel farmers shows that those who use the "shoulder seasons" are the most profitable, and keeping living roots in the soil over the winter can improve soil health and moisture dynamics in tunnels. As your high tunnel crops wind down for the season, consider a few options for continuing to use your high tunnel throughout the year.

Option 1: Plant a winter crop

More and more growers are planting winter spinach or other winter hardy greens in unheated high tunnels. This can be a fairly low-labor crop, and a way to provide your customers with local greens at a time when fresh local food options are few and far between. If you're thinking of planting winter greens, you'll need to do so very soon.

Johnnys has a great chart for helping growers decide when to plant various winter-hardy vegetables in a high tunnel to allow for winter harvest. All of these dates are in relation to the “Persephone period” where we have less than 10 hours of sunlight per day, at which point plants slow down significantly. For example:  Spinach should be planted 7-8 weeks before the Persephone period. Persephone periods in Minnesota include:

• Rushford: November 7 - February 4
• St. Cloud: November 4 - February 6
• Duluth: November 2 - February 8
• Thief River Falls: October 31 - February 10

So, now is the time to plant your winter spinach!

Spinach (baby leaf) and baby leaf Brassicas can be planted just 5-6 weeks before the Persephone period, and full leaf spinach, baby kale, and claytonia can be planted 7-8 weeks before.

What if you have plants in the greenhouse until mid to late October? You can start your winter crops early and transplant later. Often this doesn’t work out well with tomatoes, but it can work well to follow a shorter season crop like cucumbers with a winter spinach crop. 

Our colleagues at Iowa State have developed an excellent guide to winter spinach production in the Upper Midwest with information about variety selection, fertilization, and pest management. Check it out here.

Late fall lettuce crop

Option 2: Plant a cover crop

There are two big reasons to plant a cover crop in your high tunnel. 

1. We often see tunnels with bone-dry soil in the spring. This can pose challenges for irrigation, as soil that becomes too dry can become hydrophobic, repelling water. Keeping living plants in your tunnel helps to retain soil moisture, while adding valuable organic matter to the soil and sustaining microbial communities. 

2. Adding sufficient nitrogen to high tunnel crops is a challenge. Using too much compost or manure often results in sky-high phosphorus concentrations, and products that contain only nitrogen may also add salts to the soil. Legume cover crops (e.g. peas, beans, clovers, vetch) can add nitrogen from the atmosphere to your soil, and are an excellent way to balance your high tunnel soil fertility. 

Extension is currently doing a participatory research project where farmers can receive free cover crop seed to try in a high tunnel. We'll send you free seed, and simply ask that you fill out a 5-minute survey after planting, and another one next fall to let us know how it went. This will help us to develop recommendations for the best species and methods for high tunnel cover crops in our region. Sign up and learn more here. This trial is being done in collaboration with the Grossman lab (UMN) and the Sideman lab (UNH).

Winter cover crop trials in St. Paul. Photo: Adria Fernandez

Option 3: Use your high tunnel for curing

Fall weather in Minnesota can be unpredictable. Crops like onions, garlic, winter squash, potatoes, and sweet potatoes all need to "cure" if they are not going to be eaten fresh. Curing is a process of healing wounds on the surface of fruit / bulbs / tubers, and drying skins and leaves before long-term storage. While most of these crops can be cured in the field under the right conditions (warm (~ 80-90 °F) dry weather for 7-10 days), indoor curing is recommended if the weather doesn't cooperate. If you don't have a barn or another covered structure, your high tunnel can be a great place to cure fall produce. Keep the doors open and use plenty of fans to promote good air flow. Either hang produce from the rafters (onions and garlic), or lay it out on benches with good airflow underneath. If you don't have a surface with good air flow, rotate produce periodically so that all surfaces are exposed to air. Check out crop-specific curing instructions on our postharvest handling page.

Author: Natalie Hoidal