Consider a garden that is about twenty thousand square feet in size (about one-third the size of a football field). Imagine all the food you can grow in that space—lettuces and cabbages in the cool spring and fall, tomatoes and peppers in the hot summer and all sorts of other things in between.
Now, try to imagine growing just as much food in one-tenth of the space just described—two thousand square feet. Impossible? In many locations, it is not impossible at all. Although many people erroneously believe that there are only a few places with a climate suitable for growing produce year-round, the power of “protected growing spaces”—and especially high tunnels and greenhouses—makes it possible. With the additional support of modern technology, year-round growing is becoming easier than ever, even for a hobby farmer.
MAKING IT LOCAL
Reviewing a few facts about the current U.S. agricultural model can help illustrate why one might want to grow fresh food year-round. Around 75 percent or more of the nation’s fresh vegetables come from just two valleys in California.1 Even in the organic vegetable market, massive monocultures provide most of the nation’s meals. What is the end result? Not only does this system typify the proverbial (and foolish) strategy of “putting all of our eggs in one basket,” but it also entails food traveling massive miles for every bite of fresh produce and prioritizes varieties selected primarily for transit and shelf stability rather than nutrient density and biodiversity.2 Consolidated growing, processing and handling contribute to a food system that also suffers from massive food safety issues.
A few weeks ago, I walked through Costco and experienced a direct reminder of just how far and long “fresh food” often travels. Grabbing a package of organic lettuce, I checked the date—it was over two weeks old! While the produce still looked fresh (courtesy the power of a two-thousand-mile-long cold chain along with other technologies to extend shelf life), we know that quality declines rapidly and substantially for many types of produce once they are detached from the ground. Other stores I checked were little better. Growing produce in your own high tunnel or greenhouse can provide a better option.
THE POWER OF THE SUN
If you have ever had to walk a long distance to your car during the dead of winter, you probably looked forward to the warmth you were sure to find inside the car, which the sun provides even in subzero temperatures. The same power of the sun that makes a car hot on the coldest of days can keep vegetables growing through the long, dark days of winter.
When we first moved to our property, I was able to witness first-hand just how powerful protected growing spaces could be. I needed some lumber, so I went to an Amish neighbor’s sawmill. Next to the mill, he had a lovely greenhouse. He asked if I wanted to step inside, as it was a very cold and windy February day in Kentucky, well below freezing. As we stepped inside, my glasses immediately fogged up, obscuring my ability to see anything. I took them off and rubbed them with my shirt—and when I put them back on, I swore I had been transported to Central America!
There were trees laden with lemons and limes, bananas (!) and rows and rows of fruits and vegetables from floor to ceiling. I could make a 100 percent locally-grown Kentucky fruit smoothie for the first time! I knew then and there that while field growing is important, it just doesn’t compare to protected growing spaces. Even better, technology is making such spaces more productive, more efficient and easier to manage than ever before.
FROM WALLS TO HIGH TUNNELS
The original protected growing spaces were walls—yes, walls! Historically, large walls not only helped break up wind but served as heat sinks, capturing the sun’s energy during the day and protecting the plants close to the walls and in enclosed courtyards at night. In old Louisville, many people don’t understand that the backyards with tall, beautiful brick walls that they see are a throwback to a time when such houses still engaged in some level of food production.
Greenhouses started coming into vogue when glass became more affordable, especially in northern countries. Currently, a number of Nordic countries are the world’s leaders in protected-space produce production,3,4 even though they all sit at higher latitudes than Maine! What this means is that, across much of the United States, there really isn’t a reason not to localize and extend vegetable production to most of the calendar year.
Greenhouses tend to be more permanent structures or buildings that include heating and cooling systems; while they usually are made using glass or polycarbonate-type panels; increasingly, many are also using plastic (see Table 1).5 The invention of clear plastic (both sheet and panels) and tubular metal piping allowed the birth of the modern high tunnel. Although plastic poses a perennial environmental challenge in so many other ways, it solved the problem of how to make protected growing spaces both sustainable and affordable.
High tunnels are almost always covered with plastic, though some may use polycarbonate panels for the endwalls. A high tunnel is generally more of a temporary structure—not built to the same structural strength as a greenhouse. Given their ease of construction and relatively low cost (you can DIY build a two-thousand-square-foot structure for around fifteen hundred to twenty-five hundred dollars), thousands of high tunnels are built in the U.S. every year.
In the past, the approach to heating and cooling represented the key difference between greenhouses and high tunnels: growers used active, controlled methods for heating and cooling greenhouses—relying on modern heating, ventilation and air conditioning (HVAC)—whereas the inside environment of high tunnels was controlled using passive strategies such as solar gain, venting, sidewall curtains and the like. This distinction has become less pronounced over the past fifteen or so years, however, with many tunnels now using supplemental heating and cooling systems as well. As a result, people often use the terms “greenhouse” and “high tunnel” interchangeably. (For the remainder of this article, I will refer to high tunnels.)
HOW MODERN TECHNOLOGY CAN HELP
There are significant challenges to managing high tunnels well. For instance, a winter day in Kentucky might start off well below freezing, in which case the high tunnel would need to be closed up tight for the night; some crops might even need additional protection from the cold. That same day, however, the outside temperature might reach forty-five degrees and full sun by lunch time, pushing the tunnel temperature up to seventy-five degrees or warmer in under two hours—if the outside temperature later climbs into the sixties, the tunnel, without venting, could easily hit the ninety-degree mark. If you are home all the time, no problem. You can open doors and vents and sidewalls, or close them as needed. But is everyone always home? Nope, not even farmers!
Fortunately, modern technology makes it easier than ever to meet plants’ needs in a high tunnel environment. Plants’ four basic needs are water, air, an amicable temperature for the crop in question (which generally means additional heat) and nutrients, including the microbial community that allows plants to access the nutrients. Enter modern technology. From irrigation to ventilation, high tunnels are now benefiting from much-needed automation. Low-cost sensors can track everything from soil moisture to air temperature and humidity, and it is possible to partially or fully automate many different processes. For instance, is the tunnel becoming too hot? Ridge vents will automatically open. Is the soil too dry? The irrigation system will cycle on when the soil sensor sounds the alarm.
When it comes to nutrition, soil tests are crucial for high tunnel growing. No matter what you are growing, a soil test will be the most important thing you can do to ensure proper nutrient levels and produce healthy, nutrient-dense crops—although such testing is often neglected. Because you can grow so much more in a high tunnel, and grow it much more quickly, the soils in high tunnels get far more of a workout than outdoor soils. Moreover, tunnels don’t get rain, which means that salt build-up and other nutrient issues can set in more quickly. So soil testing is a must.
Tunnels offer unique ways to provide water to plants and soil. Because rain doesn’t fall in a tunnel, farmers usually provide water via irrigation systems. Hand watering is possible but isn’t efficient or recommended. Disadvantages of hand watering include the potential for causing sun scald and disease problems on the plants; in addition, hand watering uses far more water than necessary and a great deal is lost to evaporation, which increases humidity in the tunnel and again can contribute to disease. Fortunately, all sorts of irrigation systems now exist, from drip tape to drip spikes, which not only conserve water and reduce disease but also reduce weeds. This is because these irrigation systems can deliver water directly to the plants and in amounts close to what is needed, giving weeds less access to the water they need to proliferate.
Because you are delivering water anyway, it makes sense to double up and address both water and nutrient needs at the same time. Delivery of nutrients to plants via irrigation water is called “fertigation.” This type of system allows a custom-tailored approach to plant and crop needs, making it possible to apply small amounts of fertilizers and amendments as needed and avoid the problematic application of large amounts at once. Such small applications can help eliminate nutrient loss, leaching and groundwater contamination, among other benefits.
Plants also need air. Air plays many roles, such as removing excess heat and moisture, while providing nutrients that plants and soil need. Tunnels that don’t have enough air flow can actually suffer from insufficient carbon dioxide (CO2), along with excessive heat and humidity. Many methods exist that allow you to control and customize the air flow in a tunnel to balance the need to retain heat (especially in the winter) with the need to remove excess humidity and the need for fresh air. First and foremost are drop-down side curtains, along with ridge and end vents. While many are mechanical and require a person to open and close them, some companies now make vents that will automatically open and close based on the temperature in the tunnel, with no electricity required! Other companies make electronically controlled systems, which, coupled with modern Internet technology, let you check on—and open and close—the tunnel with your phone.
If there is one area of a tunnel to automate as much as possible, especially if you are away from home a lot, it would be the venting. Storms come and go in the summer, and it is generally best to close a tunnel completely when a storm strikes. But you can’t do that from the office if your tunnel is all manual. Instead, imagine pulling up your high tunnel on your phone and closing the sidewalls and ridge vents while at work ten miles away. Or imagine that instead of having to rush out of bed in the middle of the night because of a thunderstorm the Weather Channel failed to warn you about, you just tap a few buttons and go back to sleep. That is technology that truly serves us!
THE OLD IS NEW AGAIN
When it comes to regulating colder temperatures, greenhouses and tunnels generally still depend on conventional approaches (such as electric and wood-powered methods) to provide supplemental heat. There are two interesting innovations in this area that use the power of thermal mass, although truth be told, both are old approaches.
The walipini (meaning “place of warmth”) is an underground greenhouse that uses the earth, venting and good solar orientation to control the inside environment. The earth may also be lined with stones or similar material to create a heat sink. Depending on location and soil, plants are grown in the ground or in raised beds with soil brought in for growing.
A Chinese-style high tunnel, which can refer to either tunnels or greenhouses, involves a structure that generally has a southern orientation and a northern wall made of materials that serve as a thermal sink. This northern wall is crucial to maintaining the structure’s temperature stability—during the day, it helps keep the structure cooler by absorbing and storing incoming solar energy, while at night it keeps the structure warmer by slowly releasing that stored heat. The structure may be either partially or fully above-ground. Chinese-style structures will sometimes still use supplemental heat sources as well, but in many climates, they may not need them.
UNIQUE GROWING ENVIRONMENT
Greenhouses and high tunnels are a unique growing environment. Because the square footage in a tunnel is limited compared to outdoor space, as well as more expensive, more valuable and subject to specific disease and pest pressures, growers have developed particular plant varieties that perform best in this environment. Many are specifically adapted for greenhouse growing conditions, including lettuces that grow more upright (to use less space per large head), pelleted seeds that allow tight and accurate spacing of carrots and other small seeded crops.
All of these developments are a good thing. Three-week-old lettuce that requires a three-thousand-mile cold chain journey to reach our plates (and now may be grown in plastic pots on glyphosate-sterilized soil and yet still carry an organic label!) isn’t a good use of technology or of the resources that make such technology possible—but year-round growing that gives people more local access to fresh foods is. High tunnels and greenhouses are helping make this happen.
1. California Department of Food and Agriculture. “Agricultural statistical review.” In California Agricultural Resource Directory 2010-2011, pp. 17-39. Available at: https://www.motherjones.com/files/2agovstat10_web-1.pdf.
2. Local vs. organic: which is best? https://responsibletechnology.org/local-vs-organic-best/.
5. Greenhouse building materials: should I choose glass, polycarbonate, or poly film? https://dengarden.com/landscaping/greenhouse-glass-guide.
This article appeared in Wise Traditions in Food, Farming and the Healing Arts, the quarterly journal of the Weston A. Price Foundation, Summer 2019