GREENHOUSES AND SEASON EXTENSION METHODS

As discussed in Pt. 1 [0], not all crops that we grow are ideally suited to our climate. While gardening can be done exclusively outside during the warmer parts of the year, greenhouses and other season-extension methods greatly aid our ability to produce many of our crops. Even in summer, certain crops thrive better with the protection of a greenhouse than if grown outside.

Greenhouses and cloches (which are essentially mini-greenhouses) moderate the growing environment in several ways. Most significantly, they can trap and hold heat, and they allow for regulation of soil moisture. Especially on sunny days, but even on cloudy days (when 70 percent of rays are said to reach the earth), solar radiation is converted to infrared radiation (heat) when it hits dark surfaces (soil or plants). Whereas plastic or glass glazing allows the passage of light rays, it partially blocks the passage of heat, creating a “greenhouse effect.” The greenhouse also provides insulation against the nighttime loss of heat that has accumulated within the greenhouse during the day. It not only encloses warmed air, but protects against the loss of heat from the soil, whose temperature drops less readily with the protection of a greenhouse and whose thermal mass can act as a storage bank or heat sink to maintain mild temperatures even when outside temperatures are low.

Ventilation is essential for any greenhouse, not only to prevent excess buildup of heat on sunny days, but to guard against mold and disease problems, allow beneficial insects to keep pests in check, and prevent “locker room syndrome” on busy days. A well-ventilated greenhouse is a happy greenhouse.

Extra thermal mass besides that provided by the soil can be useful in moderating temperatures, especially in greenhouses used in seed-starting. Seed-starting tables can be constructed on top of water-filled black metal drums. The drums will not only hold heat that accumulates during the day, but will also help prevent temperatures from rising dangerously high (the same way a full tea kettle takes longer to boil than an almost-empty one, and is in much less danger of burning dry and melting onto your stovetop). Ventilation is still important in such greenhouses, but temperatures are easier to control.

Various designs for building greenhouses are widely available, and the purpose of this section is not to provide instructions on how to construct one. Materials can range from the inexpensive or free (scrounged PVC pipes or rebar covered with reused plastic) to expensive (custom-designed metal-tubing or wood construction with durable forms of double glazing). About the Beetless’ GLASS GREENHOUSE, whose lyrics have unfortunately been lost (see APPENDIX [0]), we know that it “discussed the importance of proper orientation, adequate ventilation, thermal mass, and appropriate glazing and roof angles.” Apparently “the engineers in the crowd loved it, but no one else could get into it.”

Cloches are often constructed from sections of PVC pipe (or, less commonly, rebar) made into hoops to cover individual beds, onto which plastic can be clipped and lifted when more ventilation is needed. (These are the type favored by PVC PAM, from the Beetless’ APPENDIX [0].) Other designs can involve strawbales or haybales, wood, metal, old or new windows, or any other materials that in combination can achieve the same ventilation-capable heat-trapping effect.

At Lost Valley, we use greenhouses for starting virtually all of our crops in the spring, with the exception of those larger-seeded crops and root crops that can be planted directly in beds. Greenhouses allow us to keep soil temperatures within the germination ranges of the crops we are starting, and to keep growing temperatures favorable as well. Crops like tomatoes and peppers, which need a long growing season to produce fruit, depend on greenhouse sowing (ideally in March) to size up adequately by the time the danger of frost has passed, when they can be transplanted into garden beds. Greenhouses also allow us to control irrigation of seedflats—many crops could be flooded out if left outside during spring rains.

Our starts greenhouse is outfitted with tables and shelves on which we place seed flats and pots until plants are ready to be hardened off by moving to the outside tables which surround the greenhouse. (In some cases, expecially if they are going into beds in another greenhouse, vegetable starts may be transplanted directly without this hardening off stage.) In the hardening-off process, plants are acclimated to more rugged outside conditions. Sometimes this involves several trips out of and back into the greenhouse before full outdoor acclimation has taken place, a process described in the Beetless’ I'VE GOT A SEEDLING [0]. Outside tables are also used for sowing and raising starts once temperatures inside the greenhouse have become too hot for various crops. Cool-weather crops will move to the outside tables first; finally, warm-weather crops may find a place there as well. Shade cloths can be used to moderate greenhouse temperatures, but by the time they seem necessary most of our plants seem to be ready to harden off anyway.

Our other greenhouses cover garden beds, and provide a more moderate growing environment for plants during the winter and a warmer growing environment during the summer. Most salad greens survive well during most winters in our “bed” greenhouses, whereas they tend to rot, be torn up by rain, ice, and/or snow, or be eaten by slugs if grown in outside garden beds. Tomatoes and peppers have usually fared better in greenhouses here than if grown outside during the summer—they ripen fruit earlier, and stay alive longer in the fall before succumbing to death by frost or attrition. Those bed greenhouses can be sealed up fairly well in the winter, with the ends opened to allow ventilation during sunny spells. Rolling up the sides partially provides the best ventilation during the summer.

Other season-extension techniques involve the use of Reemay (a spun polyester covering), water-filled bottles or tubes next to or around plants, partial milk jugs placed over tender heat-loving plants, or other materials that help raise plant and soil temperatures and guard against frost damage. Tomatoes, peppers, and other heat-loving plants can also be grown in large pots, which are moved around to catch sunlight and brought inside when cold temperatures threaten, extending the harvest season.

 
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