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Chapter Nine - Gardening Under Cover

Most serious gardeners in Central Oregon have some kind of protection for their plants. Not only is the season short, but those unexpected frosts rolling down from the mountains on a summer night has devastated many a garden. A greenhouse is ideal, but there are many other options to choose from. Just having a system for covering your plants on cold nights can extend the season.

The polyester, nonwoven row covers are an innovative and helpful addition to season extension. They transmit 75 to 80% of the light, let moisture through, and will give several degrees of frost protection. In general, row covers enhance the growing environment; help keep the soil warm, and raise the air temperature 10 degrees or more. They help to keep the soil from drying, give wind protection and keep the air-born bugs off. Row covers are used particularly to begin the season early and extend it in the fall.

I have seen some great systems of garden beds with PVC pipes curved over in an arc, forming the frame of a tunnel. Then when frost threatens, a cover is pulled over the PVC pipe to create a mini-greenhouse.

A simple cold frame is a great season extender also. There are a variety of forms, but basically, a cold frame is a box with transparent material on a hinged top that can be opened and closed. The top can be old window frames, plastic or fiberglass. This can be used for starting plants, hardening-off greenhouse starts, and propagation. It can grow plants directly, shelter summer vacationing house plants or winter over potted plants.

A portable cold frame is versatile and handy. You can place it over beds for earlier plants, force early bloom in beds of bulbs or pansies, and force rhubarb for early eating. It can also be used to cover plants when frost threatens on summer nights.

Hot Beds

Hot beds are a refinement of the cold frame in that they have some form of bottom heat. This can be with a heat cable buried in the ground, or the old fashioned way with the composting heat of fresh, strawy manure. This can be accomplished quite simply by digging the area two feet deep and filling with 18 inches of fresh manure, then cover with sawdust and you will have excellent bottom heat for your flats of plants. If you don't want to do the digging, just put the manure on the ground, cover with sawdust and move a portable frame over the top.

You could even set up the frame with a light bulb to increase the heat. This will keep the air warm and will help to prevent the plant tops from freezing. Studies have shown, however, that bottom heat is more effective overall for seedling growth.


What you grow in a greenhouse will depend upon what you like. Your success, however, will be greater if you accept the natural ryhthem of seasonal change and plant growth responses to the changes. Growth will be slower in spring and fall because of shorter days and almost nonexistent in winter. For plants grown to harvest, the early spring crop is the most productive. The turning point of growth is February 21, when there is at least 12 hours of light.

Fall and winter crops should be started early enough, like at least by August, so that plants are well established and fairly large before October 21. This is that date that the days become shorter than the nights, and plant growth becomes very slow.

A lean-to structure, south facing against your house or an outbuilding, is a space efficient, easy to heat type of greenhouse. I have one built against an above ground water cistern. This serves a double purpose; the water acts as a giant heat sink, and I get excellent thermal gain in the structure. A lean-to structure can be any size, but are easiest to use if you can walk in them.

The free-standing greenhouse is the ideal, because there is double the usable growing space than in a lean-to. They come in a variety of shapes: the quonset hut, geodesic dome, A-frame or a standard little house. You can build them yourself or from a kit, or have them delivered on a truck completely assembled. In the Central Oregon climate, I have found that a foundation is important to give the plants some protection from the ground cold.

There are a variety of cover options. Glass is the traditional glazing. It has the advantage of good visibility, attractiveness and a long life. The disadvantages are that it breaks relatively easily, and is expensive. Fiberglass is the least expensive option, but be sure to get the type that is designed for greenhouse glazing. Fiberglass allows all of the UV rays to pass through, which glass doesn't. It becomes opaque with time, however, and will need to be replaced in about 15 years or so.

Rigid plastic or acrylic coverings are another option. They are usually the most expensive, but are not as breakable as glass. They can scratch and do become opaque in time, but have a longer life than fiberglass.

Plastic film is good for double glazing, which is recommended for all greenhouses; the inner air space that it creates makes a major difference in the heating. Plastic film is occasionally used as an outside glazing, but this is temporary, as the plastic only lasts three or four months when exposed to the weather and sun.

A greenhouse that faces directly south is the most efficient use of the sun, particularly in the winter. East is marginal, but possible. West will be too hot in the summer and will receive poor sun in the winter. North does not receive enough light for any but a few foliage plants.

Deciding whether to heat the greenhouse is an important consideration. This is the most expensive ongoing process of all. It is possible to grow plants in a greenhouse year around with no heat at all, but not easy. Plants do not grow well in winter, the greenhouse acts as a holding place for plants, rather than a growing house. You may want to just close up the greenhouse in the winter. Letting it freeze can also be good system of insect control.

A "warm" greenhouse is run with nighttime temperatures of 60 to 65 degrees, with at least a 10 degree increase in daytime temperature. This will allow tomatoes, peppers, orchids and other tropical plants to grow. A "cool" system keeps a nighttime temperature of 45 to 55 degrees. Plants that do well in these temperatures are lettuce, other greens, potted citrus, herbs, geraniums, cactus and other house plants. Plants in the cool house have slower growth and take longer to bloom, but there are less insects. A "cool," 50 degree house requires 1/2 of the fuel of the "warm" 60 degree house, so deciding which you prefer is a major financial decision.

There are a number of options for meeting your heating requirements. Taking advantage of solar gain is the most logical. Even with that though you will need some kind of backup system. Electrical systems are common, and are most efficient in a small, well-built greenhouse. You should have a non-electrical alarm system installed that lets you know if the heating system fails.

Gas and oil are also options. You must be very careful that no fumes escape, as they can be toxic to plants. I have also seen a system with a wood stove as main heat.

It may seem to be overdoing it, but a backup system to the backup is important as well. Electricity generation can be interrupted and oil can run out. A kerosene or catalytic heater is a good fall back option to have on hand.

At the other end of the spectrum is the problem of over-heating in summer. Ventilation is critical, the ideal being a low vent in the windward wall, and a vent as high as possible on the opposite. It is recommended that for every 100 square feet of south glazing, you have 10 square feet of openings and vents, including the door. If they are screened, increase your vent area by 25%. Fans are also useful. Vents can be opened and closed by thermostatically controlled units, this is a real advantage if you are not there all of the time.

Another means of controlling temperature consists of blocking the sun. It is common to whitewash glass houses. You can plant tall growing or vining plants on the south side. Shade coverings, such as roll down bamboo curtains work well.

Humidity is another considerations for the healthy growth of plants, 50 to 75% humidity rate is recommended. Misting, either automatic or by hand will increase humidity and cool the house also. When it becomes very hot, I hose down my greenhouse with cool water, this brings the temperature down by many degrees.

Solar Greenhouse

All greenhouses are by definition solar, but what I mean is one that is equipped to collect and store extra heat for heating at night and on cloudy days. It must also be partially insulated so as to retain as much heat as possible for as long as possible.

The orientation should be due south, with no trees or future buildings that will block it's southern exposure. If necessary it could angle up to 20 degrees east of south or 30 degrees west of south. In Central Oregon true south is 22 degrees east of magnetic south.

The north wall should be solid, and the east and west walls are either solid or partially solid. Only 1/3 of the roof should be glazed. All of the solid part of the roof and the walls should be heavily insulated, up to R-19. It should have a foundation that has insulated outside walls. If you have a concrete slab, insulate the outside of that as well.

The south face glazing should be at an angle to optimize the winter sun radiation. It also provides shade from the summer sun. The ideal is the latitude angle plus 20 degrees, in Central Oregon that works out to a 65 degree angle. Dimensions to maximize the sun are to have the length 1.5 to 2 times the width.

All glass needs to be double glazed. There should be a system of night insulation for the glazing as well.

Heat is commonly stored in either water or stone. This heat storage medium is called thermal mass. For a water system you will need four gallons of water per one square foot of south glazing. In a masonry system you will need 150 pounds of masonry for every one square foot of south glazing. This should be spread out, not all in one corner. Keep in mind that a dark color absorbs and holds more heat than a light color.

Pit House

A pit house is a green house that is underground. It requires a lot of digging, but is well worth the effort. Eight feet deep is recommended. In Central Oregon, with all the rock, however, as deep you can get is better than nothing. I have a pit house that is only about three feet deep, but it has made all the difference in the world of what I am able to grow.

You are able to take advantage of the solar gain of the earth in a pit house. Anything below the frost line is a major heat sink, and maintains a fairly even temperature year around because the earth is good insulation.

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