High tunnels are a great tool for growing warm weather crops in Minnesota. The plastic captures heat from the sun so the growing season can start earlier, end later, and get hotter. These conditions help growers produce high quality early and late season tomatoes and peppers as well as specialty tropical crops like ginger and turmeric. High tunnels also improve crop quality by shielding fruit from rain and hail damage and preventing soil splashing which can cause disease.
However, these benefits do not come without costs. Intensive planting, management, and irrigation in high tunnels leads to many soil health problems such as salinity, compaction, and loss of organic matter. Currently, we are in collaboration with faculty from the U of MN Department of Horticultural Science on a research project to investigate whether adding winter annual cover crops in high tunnel rotations can help fix these soil problems. To do this, we’ll measure soil nutrients, soil microbes, and cover crop nitrogen credit.
Winter annual cover crops have many proven benefits in the field; they increase organic matter and soil nitrogen, decrease soil erosion, prevent nitrate leaching, and stimulate diverse microbial communities. These covers are planted in late fall, go dormant in winter, start growing again in early spring, and are tilled into the soil before a cash crop is planted. This is a difficult goal in a high tunnel because the cash crop season starts so early and ends so late that there is little time for the cover crops to grow large enough for growers to receive all potential soil benefits. We are experimenting with different planting dates and seeding the cover crops while the crop is still growing to figure out how to give the cover crops enough time to do their thing.
Another issue is getting the cover crops to survive the winter in high tunnels. In a normal field winter, the cover crops would be insulated by snow and spend the winter fully dormant. In a high tunnel, the daytime temperature climbs above 60F on sunny days and falls down to the outside temperature at night, which can get as low as -35F. It is not known whether cover crops can go dormant in these conditions, and active plants in these conditions are vulnerable to freezing injury or desiccation. This winter we are insulating the cover crops with reemay row cover and making sure the plants have enough moisture.
To recap, high tunnels are a great tool for growers, but there are some challenges in using them wisely and efficiently. One approach to dealing with these challenges could be integrating cover crops, though there are still some kinks to work out. This research project aims to smooth out the kinks by:
- Identifying cover crop species and management practices best suited for over wintering in high tunnels.
- Evaluating cover crop effect on soil health and quality.
- Evaluating cover crop impact on bell pepper growth and production (nitrogen credit, soil temperature, soil moisture).
- Sharing findings with Minnesota and other upper Midwest growers.
For more information about this project, visit Dr. Julie Grossman's lab website.