Planning for the Greenhouse
Don’t fight the forces; use them.
— Buckminster Fuller, Shelter, 1932
Chapter 2 laid out the very big picture. Now, let’s zoom in to evaluate the range of possibilities for extending the growing season. This book focuses on solar greenhouses, which are structures that feature an insulated north wall, double layers of glazing, underground frost protection, and various methods of thermal storage. Solar greenhouses are designed to stay much warmer than outdoor temperatures, creating lush, abundant, year-round growing environments — enabling growers to grow and experiment with many crops normally ungrowable in their climates. However, they are not the only way to extend the season. Simpler methods of crop protection also have a place in the range of solutions, summarized in Fig. 3.1. Understanding these options provides context for where solar greenhouses fit in, and whether they are the best fit for you.
Hoop houses, row covers and cold frames all provide a single layer of crop protection and usually used as season-extenders. They provide some frost protection for crops, creating an indoor environment a few degrees warmer than outside. Studies show inside temperatures under a single layer of protection are on average 2°F–4°F warmer than the minimum outdoor temperature.
FIGURE 3.1. Season Extension Options.
The next step is to add a second covering using row covers in addition to the outer structure, as shown in Fig. 3.2. Eliot Coleman is well known for using this technique to grow cold-hardy greens year-round on his farm in Maine. The added layer protects crops enough to sustain hardy greens even when the outdoor temperatures drop to −8°F (−22°C). One of the keys to Coleman’s successful winter farm is growing crops that tolerate frost and low-light levels. This supports Coleman’s basic philosophy of simplicity and limited intervention. For other growers, the goal is to produce more of the food we consume — which goes far beyond cold-hardy greens.
To do this, we turn to greenhouses that can withstand much greater outdoor temperature extremes and maintain a stable year-round growing environment. This is where solar greenhouse design comes in. By trapping heat and retaining it overnight (using insulation and thermal storage), a greenhouse can maintain an environment above freezing for all or most of the year, expanding both the growing season and the variety of crops we can grow.
Within the category of solar greenhouses, there is also range of options based on the design and growing goals. One way to frame the options is by their minimum indoor temperature, just as outdoor growing zones are categorized. Some solar greenhouses are three-season structures that freeze over in the winter. This is a common strategy among growers in climates with very harsh and low-light winters (for examples, see the case studies from Canada and the northeastern US). Then there are greenhouses that get close to frost in the winter. These are often called Mediterranean greenhouses, reflective of the milder winters of that region. These greenhouses enable many more things to grow year-round: cold-tolerant vegetables and perennials, and trees like figs, olives and varieties of citrus.
FIGURE 3.2. Row Covers Inside a Hoop House. Credit: Mehaffey Farm
Finally, there are the hothouses, also called tropical greenhouses. These have minimum temperatures of 40°F–50°F (4°C–10°C), which permits growth of heat-loving crops like tomatoes, peppers and eggplant as well as perennials like bananas, guava and citrus. This type of greenhouse requires more insulation, multiple layers of glazing, and significant thermal storage to create an indoor environment that is several growing zones above the outdoor growing zone.
Light is usually the limiting factor for growth, making good growth of many full-sun crops difficult in northern areas. Which season extension options work for you depends on your climate, your goals for the growing environment, and your resources.
One of the reasons traditional greenhouses perform poorly in most climates is that they use a “one-size-fits-all” approach. The same plastic box will operate very differently in Maine than it will in Texas. Solar greenhouse design applies a different mindset: by tailoring the structure to the local environment, one can work with the elements, rather than against them.
To design a structure that works cohesively with the outdoor environment, you must know a bit about your local climate, both its challenges and resources. From a greenhouse design perspective, the two most important variables that make up the local climate are light and temperature.
There are a variety of ways to describe and measure each. Regional temperatures can be characterized by their averages or their minimums, or by other metrics like Heating Degree Days (which reflects the energy requirement to heat a building). For the purposes of this book, we find that the USDA Growing Zone map, shown in Fig. 3.3, is most helpful as a description of temperature zones. It categorizes climates by their minimum temperature. You can find versions online that show much more detail by state.
Like temperature, there are many ways to measure light and many factors that contribute to the light levels at a given location: percentage of possible sunshine or cloudiness; the intensity of sunlight based on latitude and elevation; day length, etc. We delve into this subject much more in Chapter 5. In short, the simplest metric greenhouse growers use is the daily light integral (DLI), which measures light intensity over a period of time. The DLI integrates all these factors into a simple number indicating the total light levels available to plants over a 24-hour period. You can see DLI numbers for the US in the first map in the color section.
FIGURE 3.3. USDA Grow Zone Map. Credit: US Department of Agriculture
The outdoor climate is only one element to consider. On top of this, you also need to decide what kind of indoor environment you want to create. Some basic questions can help elucidate. You don’t have to have all the answers right away; the important thing is to have these questions in the back of your mind as you move through the design process so you can develop clear goals to inform your greenhouse plan.
• What do you want to grow and when? Your goals for your indoor environment will inform all manner of decisions in the design process. Do you want to grow tomatoes year-round? Then you need a greenhouse that stays above 50°F (10°C). Do you just want to grow greens in the winter? If so, a three-season greenhouse that avoids freezing can suffice. There is a huge difference between those two growing environments, and thus the structures/systems required to create them.
Often, growers look for groups of crops that have similar temperature requirements, such as warm-season annuals, cool-season annuals, and/or different perennials that have similar needs, and then they design the greenhouse to meet the minimum temperatures required by that group. A list of common greenhouse crops and their temperature ranges is given in Appendix 1.
• Why do you want a greenhouse? Are you growing as a hobby or commercially? Commercial greenhouses typically require narrower temperature ranges, which necessitates more advanced climate control systems. In a residential setting, a freeze or loss of crop is not catastrophic, and more variation is tolerable.
• Have your considered other uses for your greenhouse? Clearly, you want to grow plants, but could it also be an area for sitting/relaxing, education or storage? Might you integrate animals into your greenhouse? We recommend you start by sketching out a floorplan early on in the process, and allow this to evolve as you refine your design.
• What is your time commitment? Do you want to be able to leave the greenhouse and go on vacation, or can you check on it daily? This will determine the need for automated systems, such as vent openers or fans. On a broader level, it influences how big the greenhouse should be. Think of how much additional garden space you can manage based on your schedule throughout the year. To decide on a greenhouse size, we recommend residential growers consider their outdoor gardens as a guide. Think of your past garden plots, how much time they required and how much food they provided. Do you want something larger or smaller? Commercial growers naturally have many more factors to consider — production numbers, labor requirements and expenses — all of which should be detailed on a business plan.
• What is your budget? The primary factors determining greenhouse cost are usually the labor and level of technology. If doing the work yourself, collecting recycled materials, and using simple systems, greenhouses can be built on a shoestring budget. If hiring out the work and using more advanced systems, costs naturally rise. As the greenhouse gets larger, the cost per square foot decreases.
All of these factors come together and influence each other in a web of variables that makes every solar greenhouse unique, outlined in Fig 3.4.
Coleman, Eliot. The Winter Harvest Handbook. Chelsea Green Publishing, 2009.
Jabbour, Niki. The Year-Round Vegetable Gardener. Storey Publishing, 2011.
USDA online interactive map for Plant Hardiness Zones, planthardiness.ars.usda.gov
