From Greenhouses to Garden Cloaks
Extend your growing season!
—by James Loomis
Fall is upon us, gracing us with her limitless bounty and increasingly frequent nippy nights. As the season shifts, the sun takes on a more loving, and less brutal, attitude. The needs of your garden, and hopefully your schedule, become less demanding. Fall is the perfect time to enjoy the rewards for the labors of summer, with time for preserving your surplus harvests.
Remember, the freeze is right around the corner, and if we act now we can guarantee the bounty keeps flowing, unimpeded by frost. With well-timed plantings of a proper selection of plants, one can enjoy harvests well into November with little fuss. With a few simple building projects, one can guarantee harvests right through winter and into spring!
By building a simple structure to utilize the power of solar gain, we can take advantage of the greenhouse effect and increase the temperature inside a simple protective structure built over our garden beds. We can store some of this heat in the thermal mass of our soil, and indeed other well-placed objects, effectively buffering the temperature in our structure once the sun goes down. A well-designed structure can easily and quickly gain 50 degrees F over outside temperatures in direct sun, so the challenge of heating them rapidly gives way to the challenge of cooling them! Any of the following structures will require ventilation on sunny days to keep them from overheating.
Let’s take a quick look at the different styles of structures used to keep the produce flowing well into the winter.
Greenhouses: In the true sense, a greenhouse is a structure large enough for the grower to walk inside of. It features glass, polycarbonate, or other rigid glazing, and an artificial heat source. Glazing refers to the transparent material that covers the structure, allowing the sun’s rays to penetrate and generate the greenhouse effect. Like a parked car in direct sun, a closed space with transparent glazing will quickly collect and amplify the sun’s heat. This is the most expensive and involved of our options.
High tunnel: This is the modern equivalent of the greenhouse, technically different in that the structure is covered by a flexible plastic greenhouse film, rather than a rigid glazing. Economical and easy to erect, the modern high tunnel has all but replaced the expensive and cumbersome greenhouses of the past. Large enough to walk inside, high tunnels rely on either artificial heat or thermal mass to maintain warmer temperatures overnight.
Low tunnel: Identical to the high tunnel, only smaller. The low tunnel is designed to cover only the plants being grown, and the plastic cover is rolled back for access to the plants, as well as to ventilate the structure. Much simpler and less expensive than a high tunnel or greenhouse, these are a great first step for those pursuing season extension.
Coldframe: If the low tunnel is the sidekick of the high tunnel, the coldframe is the same to the greenhouse. Usually constructed from old windows and lumber, these are ideal protection for those of you with raised beds, as half of the structure is there already. The window, or other glazing, is angled at least 45 degrees to catch the low winter sun, and is hinged to allow ventilation and access to the crops inside. These are simple to build if you are handy with tools; just remember to keep it tight, accessible, and easy to ventilate.
The ghetto garden cloak: Before we get carried away with building projects, this one is the sure winner when it comes to protecting tomatoes, peppers and other frost-sensitive plants. Like clockwork, our first freeze of the season in mid- to late October usually rushes in for only a night or two, then temperatures quickly return to warm and sunny. A quick nighttime cover with a tarp or blanket keeps the frost off of the plants directly, and can easily get you a few more weeks for your fruits to ripen. Just remember to pull off the cover during the day!
How to build a low tunnel
The simplest and most effective use of time and materials for season extension is the low tunnel. This is a series of hoops that keeps the plastic glazing elevated, forming a protective pocket of air above the garden bed. When purchasing this plastic, it is imperative that you purchase a greenhouse film. Do not waste your money or time on plastic sheeting from the big box stores. Greenhouse film is engineered to be UV-resistant and maximize light transmission, discourage condensation, and have a life expectancy of four to six years in direct sunlight (double that if stored away in the warmer months). After that point, it makes a great tarp or slip ’n’ slide. Plastic sheeting, on the other hand, will degrade quickly in sunlight, becoming opaque and brittle within a year, which means you’ll toss it into the landfill. Of course, from there it will find its way into the ocean and soon be choking baby sea turtles. Bad move, don’t be that person.
Mark out the location of your hoops. I find that a hoop every three feet along the length of your bed provides ample support.
Make the hoops. The simplest approach is to cut lengths of #9 wire, bend into a hoop, and press into the soil on either side of your bed. A six-foot length of wire works great to cover a 30- to 32-inch-wide bed; increase your length accordingly if your beds are wider.
A second option is to build your hoops from 3/4-inch PVC anchored with rebar. This option allows for greater durability, and your low tunnel will hold up to the weight of snow. For this option, hammer a two-foot piece of rebar halfway into the ground on either side of the bed. Next, cut a five-foot piece of PVC. Slide one end over one of your rebar anchors, and bend the PVC into your hoop, sliding the opposite end over the second rebar anchor. Repeat for each hoop, every three feet.
Attach the film. Lay out your greenhouse film over the hoops, leaving an additional three feet on each end. You’ll want at least an 8- to 10-foot width of plastic to give yourself plenty of room to seal the bottom edges. At this point, while the plastic is clean and new, I like to tape one end to a length of PVC (there are clips sold for this purpose as well). This allows the plastic to be easily rolled or lifted off for access to the plants inside. On the opposite side, you can either bury the plastic, pin with landscape staples, or weight it down with boards or bricks. The key is to keep the plastic as tight to the ground as possible, to minimize drafts. For the wall ends, roll the excess plastic and weight with a cinder block or rock. It should look like a long, clear, half-buried tootsie roll.
Let’s add some thermal mass. Thermal mass is an object’s ability to absorb and store heat. When the sun is out, our low tunnel heats quickly, and in fact will require ventilation when the daytime temperature is above 40 degrees F. However, once that sun goes down, or dips behind some clouds, the solar gain is lost and the interior will begin losing heat. Thermal mass helps to buffer that heat loss, and even hold a considerably higher-than-ambient temperature overnight. This same mass also helps to buffer the interior from warming too quickly.
The denser an object, the higher its thermal mass. Masonry, stone and containers of water work great. The darker the object, the more heat it will collect and store.
I like to place bricks and pavers in between my rows. This helps to radiate heat to the soil, which is another great sink for thermal mass. I also line the north wall of my structures with 7.5 gallon water jugs that I’ve painted black. The addition of thermal mass can make the difference in preventing freezing in your low tunnel.
Plant the right crops
The key to success with season extension is to plant crops that thrive in cool weather. Lettuce, arugula, spinach and all brassicas not only tolerate light frosts but actually taste much sweeter. The plants have the ability to transfer water from their leaf tissue to their roots, avoiding catastrophic damage with a frost. As long as we prevent the ground from freezing by banking the solar gain in the thermal mass of the soil, our plants will be just fine. With season extension we are trying not so much to maintain a constant high temperature, as we are trying to maintain an overall higher average temperature, especially with regards to soil temperature. Keep their feet warm, and they won’t complain so much about their heads. This results in accelerated growth and higher crop survivability as winter sets in.
There you have it, a simple, inexpensive way to keep your garden active right into, and possibly even through, the winter months. Your structure will also allow you to get a jump on spring next season, and be the first on your block harvesting. With a new technique under your belt, you’re on your way to gardening like a boss.