By Braden Wallenkamp
Outside in Wisconsin, the soil is frozen and most plants have gone dormant for the season. But on a patch of soil, facing the vegetable fields at Gwenyn Hill Farm, are two 30 by 96 foot gothic style hoop houses that cover unfrozen soil and provide space to grow at temperatures up to 30 degrees warmer than outside. Just before Christmas these unheated structures housed an array of leafy greens and fresh vegetables ripe for our CSA members, farmstand customers, and local wholesale buyers. It is thanks to these hoop houses that the growing season is extended.
Hoop houses (also known as high tunnels) are unheated growing enclosures made of greenhouse grade plastic held into place by tubular braces. The houses at Gwenyn Hill Farm, assembled in 2019, have two layers of plastic with a pocket of air in between that insulates the structure. Inside these structures the interior air is warmed by the winter sun penetrating the plastic, imitating temperatures one to two growing zones to the south.
For the fall and winter harvest, planting in the hoop houses begins in early September. This year, because of the continued fall heat, the salad mix was harvested early and sold at the final weeks of the Brookfield Farmers Market. Our hoop house season continued as we harvested arugula, cilantro, baby bok choy, white turnips, radishes, kohlrabi, and a few other hearty vegetables. Many of these vegetables went to our CSA members in the Thanksgiving and Christmas pick ups. In late December we harvested over 100 pounds of carrots that will soon be washed and sold at our Farmstand.
As temperatures stay around freezing and the hoop houses depend on the daytime sun to heat up, we also use interior fabric row covers to insulate our vegetables and help them survive at freezing night time temperatures.
For success in an unheated hoop house, it is important to plant crops that will survive at these low temperatures. Spinach and kale are two such crops that are still thriving in the Gwenyn Hill hoop houses. These plants acclimate to the freezing temperatures as the seasons change. During prolonged periods of low temperatures, the sugars in spinach specifically, increase by over 50%, making the winter varieties much sweeter. This is because sugars act as protectors for the plant’s protein structure and chloroplast membranes therefore helping the plant to hold its shape while normal plant processes slow. Hoop houses allow the plants to function this way because they heat up in the sun and cool down drastically at night. The structure of the hoop house allows humans to harvest these cold resistant plants in not- so- cold settings.
Hoop houses create a desirable growing environment and benefit the local food economy. The vegetables we grow with this season extension method sell in our farmstand and help extend our vegetable business into the off-season. This not only benefits Gwenyn Hill but also the consumer who wants to buy local produce year-round if and when available. Moreover, season extension allows farmers to continue building consumer relationships and encourages year round connection with buyers. This trend is also visible on a broader scale through the expansion of winter farmers markets. The Milwaukee Winter Farmers Market hosts over 50 vendors, several of which offer fresh vegetables grown locally, likely with resources like hoop houses.
As with the trend of supporting local agriculture, hoop houses have grown in popularity as a viable way for farms to extend their growing seasons. Through the U.S. Department of Agriculture’s cost share program for hoop houses, over 10,000 structures have been contracted nationally since 2010, helping farmers across the country bring local food to their communities for more months of the year. As more farms employ the humble hoop house to keep the growing going, more consumers can rely on fresh local produce into the winter.
Once the spinach is harvested and we bunch the remaining kale, the winter harvest will be over. After the soil rests for a few months we will lay down a few inches of compost and plant again for the spring. In addition to growing lots of our usual leafy greens, we’re looking forward to experimenting with Broccoli Raab this spring!
Hoop houses are a viable option for extending the growing season, supporting a local customer base year round, and diversifying farm efforts into the off-season. To support the growth in Gwenyn Hill Farm’s hoop houses, find fresh kale, carrots, and spinach in our farmstand this winter.
Braden Wallenkamp is a recent graduate of Lake Forest College with a degree in Environmental Sciences. She has been a star on the Gwenyn Hill Farm harvest team since spring of 2021.
Want to delve deeper into this topic? Here are some of Braden’s sources for this blog post:
- MOSES – Season Extension Fact Sheet
- Rodale Institute – Hoop Houses 101
- Southern Wisconsin Food Hub Feasibility Study
- New England Winter Spinach Study