Every dedicated grower needs a greenhouse – especially in winter. Greenhouses or their close cousin, the hoop house, are wonderful ways to extend your growing possibilities. They also reduce crop damage from snow, pest, and wind and give you a quiet place of escape from the stresses of ordinary life.

My father at the age of 95, gardens throughout the endless Irish drizzle in a 40’ x 15’ hoop house. Gardening keeps him alive and healthy. At Christmas, he is still harvesting tiny succulent white alpine strawberries and providing vegetables for his large extended family. My own Gothic greenhouses (see last months article) are a warm haven as the fall frosts arrive. They extend my tomatoes well into October and hold dreams of early fava beans planted in November. Throughout the cold months, the collards, chard, and carrots keep the family fed with healthy food, as snow lies deep on the ground.

For the winter grower, a greenhouse is an essential tool. Like every good tool, it should best fit the job at hand. Make your choice of design and materials based on your specific needs, your environment, and your budget. There are dozens of different designs to choose from (see resources). My schedule allows only a few hours each week to tend my garden, so I prefer things to be simple, natural and easy to maintain. I do not heat my greenhouses; it seems wasteful to use extra energy when the sun and soil are so generous.

A greenhouse is essentially a means to trap and hold solar energy. An ideal location should receive six plus hours of direct sunlight during the winter months. Because my land slopes sharply to the East, I make the most of the morning light by having two of my greenhouses run North to South. While these greenhouses get plenty of winter sun, the main problem is how to store this free energy and reduce its loss through the thin plastic walls.

Commercial greenhouses decrease heat loss with double or triple walled solid panels, or double-skinned poly films with air blown between. These are excellent solutions but can be expensive and difficult to construct yourself. My own answer is to make the greenhouses as airtight as possible, and ensure the North and West walls are well insulated. In the winter, only the East and South sides of a greenhouse needs to be open to solar energy. On one of my greenhouses, I stack bales of straw against the North side covered with a tarpaulin to help the snow slide off. I attach bubble or solid foam insulation on the inside of the others.

Once you have reduced the loss of energy, the next question is how to store as much heat as possible. Heat energy is best held and released using thermal mass – any material that will heat up slowly when it is warm and discharge that heat slowly when it is cooler. The most available thermal mass is soil. Well-tilled soil has minerals and moisture to provide mass, with air and humus to act as insulators. When the soil is kept protected with plants, mulches and row covers, it can retain amazing amounts of energy and release it slowly, warming the plant roots where it is most needed.

As fall arrives, I add extra thermal mass to my greenhouses. The East sides of the greenhouses are lined with 50-gallon plastic drums filled with water. Food or soap manufacturers (Pangea Organics in Boulder, for example) often have barrels available free or at a low cost. My larger greenhouse can contain as many as 14 barrels — 700 gallons of water. Some of this water freezes and thaws as temperature and sunlight shifts, but the soil and plants around the barrels remains unfrozen. In early spring, I use the water from the barrels to water the thirsty plants.

My solar solutions work well to grow a range of cool loving vegetables, but what if you want more? For the enterprising grower, the answer is found in the pit greenhouse. Mikl Brawner of Harlequin’s Garden in Boulder, Colorado has built unheated greenhouses that grow tomatoes through the winter at temperatures down to 0ºF (see resources).

These cunningly designed growing sites, do not look like your typical greenhouse. They sit low to the ground with only the transparent South window revealing their function. You enter through a West door that takes you down steps, four feet below ground level. The temperature is moderate — the feeling quiet, earthy and cocooned.

In this underground room, you are surrounded by an insulated concrete foundation. A five feet wide bin of soil sits against the North wall growing every kind of plant imaginable. The solid North wall rises vertically six feet high; it is insulated and painted black to capture heat. Opposite the door is a large triangular East window. On the South side, plain glass slopes at a 60º angle from the ground up to the apex; a poly film attached under the rafters gives added insulation. In the summer, hop vines trail up the glass South face providing natural shade.

The pit greenhouse is the most efficient use of sun and soil energy. Mikl tells me he only had to add a little extra heat on six nights last winter when the temperatures dropped to 30ºF below. As a creative work of love and function, it demonstrates just what is possible with ingenuity and effort. Whatever your growing needs, use the generosity of sun and soil wisely. Your solar powered greenhouse will offer you superb harvests throughout the cold winter months.

Resources
Choosing a greenhouse: http://www.greenhousebuyersguide.com/types/
Pit Greenhouses: Mikl Brawner. http://www.harlequinsgardens.com/mikls-articles/
Gardening in the Fall and Winter: Roland Evans. Maximum Yield, Sept/Oct 2007
Build your own Gothic Greenhouse: Roland Evans. Maximum Yield, Sept/Oct 2008