Passive solar greenhouses for year-round production and risk management
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Editor’s note: This article was originally published in the tri-state New Ag Network (www.new-ag.msu.edu) for organic growers. Some of our Vegetable CAT Alert readers will find the information of interest, too.
Using protected cultivation for extended season production is not a new idea. But given the current environment of reduced availability and increased cost of fuel together with increased emphasis on farm profitability and local economy, it is one that has the potential for new impact. Farmers and researchers alike have recognized the benefits these simple structures provide.
There are a number of reasons why we would be interested in growing year-round, but some of the main points often cited include increasing farm viability by creating income throughout the year; increasing availability of fresh, healthy, local produce; decreasing farmer workload in the summer months; decreasing energy dependency for food transportation; and keeping dollars in the local economy to name a few.
While there are many names to describe season extension structures including hoophouse, hightunnel, coldframe and unheated greenhouse, we tend to use the title Passive Solar Greenhouse (PSGH). While all are typically metal framed, poly-film covered structures, a PSGH is covered by two layers of inflated 6-mil greenhouse plastic. Most importantly, PSGHs are unheated, passively ventilated, have no supplemental lighting, and are designed with the intent to grow throughout the year, even in northern climates. PSGH may conjure the image of a custom built “bioshelter” with an insulated north wall and water for heat storage to prevent freezing. That is not what we mean! We want off-the-shelf, low cost, simple construction (view images of the PSGH at the MSU Student Organic Farm). Freezing conditions are not a problem for winter crops. The structure and moisture on the plastic film work like a cloud to trap the heat of the soil in the greenhouse so the soil does not freeze.
Reduce risk in a variety of areas
Risk management reduction is also a major reason for employing these structures. While risk management can mean many things, we think of 1) soil health/organic matter, 2) crop diversity, 3) direct marketing and 4) season extension.
Promoting soil health is a basis of organic production systems and is integral to successful agriculture regardless of production system. Increases in soil organic matter, often through compost applications in the PSGHs and by cover cropping in the field, can improve soil health and subsequently benefit plant health. Increases in soil organic matter improve water absorption and water holding capacity, thereby mitigating runoff common during heavy rains and buffering the effects of drought. In the PSGH, soil organic matter contributions to moisture availability are important for the rapid growth under the protected conditions. The soil organic matter is also the primary source of nutrients. A steady and somewhat limited supply from biological availability can maintain production without leading to lush, damage susceptible growth.
From experience, it is safe to say that nearly all crops grow better in the properly managed PSGH than in the field. That being said, it is important to determine which crops are worthwhile to grow in such valuable space. While we could grow only tomatoes in the spring, summer and fall, by doing so we risk losing the entire crop if there is a crop failure. By increasing crop diversity in the PSGH we are able to reduce risk. Peppers, summer squash, eggplant, melons and basil are crops that do well in the summer season PSGH. Cutflowers and herbs can also be profitable. What we are growing is dependant upon the season. We are not talking about tomatoes in December. For the winter, cold tolerant crops like spinach, kale, chard and dozens more are good choices. There is still time for early tomatoes—our first this year were harvested on June 14.
By selling directly through CSAs, farmers’ markets, restaurants, and on-farm markets, more of the dollar comes back to the farmer. While this is not exclusive to PSGH producers, the structures allow farmers to offer customers something unique and to keep customers buying all year. Production in the PSGH is still susceptible to weather variations and can be used as a tool to teach customers about the impacts of weather on production.
Another advantage of the PSGH is the potential to decrease disease and insect pressure. Many of the fungal diseases common in the field have lower incidence in the properly managed PSGH as the plastic covering excludes rain and therefore minimizes the wet foliage environment under which fungal diseases prosper. There is also a marked decrease in insect pressure within the PSGH with good techniques such as sanitation and crop rotation.