Solar Greenhouses

Winter gardening strategy:

The greenhouse protects plants from winter extremes not only by slowing temperature changes but also by keeping wind and cold rains at bay. Plants such as lettuce are not bothered much by freezing air temperatures. They have learned a neat little trick to survive, which is unsettling the first time you see it: As air temperature drops, the plants move water out of their cells into the intercellular spaces, so that freezing doesn't disrupt the cell walls. The leaves go limp (and the frantic gardener assumes the crop is lost) - but then they perk back up as the sunlight warms the greenhouse and the cells rehydrate. The more critical factor is to prevent freezing deep into the root zone - this is the key to successful winter gardening. You could think of the soil inside the greenhouse as a rechargeable battery. During the day, it charges from the heat energy of the incoming sunlight. At night, it quickly loses that stored energy, but it has a huge amount of heat to lose before the soil starts to freeze.

Experienced gardeners might have some difficulty adjusting to the paradoxes of winter gardening. We have to relearn many of our assumptions, particularly about scheduling crops. Unlike in spring, when the season is opening out into greater warmth and longer days, in the fall it is shutting down into increasing darkness and deeper cold. The biggest challenge will likely be the shorter day length, rather than the lower temperatures.

The bad news: During the darkest time of winter, there is insufficient solar energy to support vigorous growth. If you start your plants too late to accomplish most of their growth before the short days, they will survive the cold temperatures, but instead of growing actively, they will sit and sulk, awaiting sunnier days. The good news: On the other hand, if you get the timing right, you can produce, say, a mature head of lettuce before the darkest days and it will stay fresh much longer than in the summer. That perfect head of lettuce that would spoil within a matter of days in June will stay in prime condition for two or even three months in the middle of winter. When you start your crops in the late summer or early fall, start far more than you think you will need. As you harvest, you will not be able to start new crops, but if you have plenty "in the bank" at that point, you can continue making generous harvests until longer days make possible some late-winter crops.

Greenhouse crops: (Start greenhouse crops in the outside garden, then move them into the greenhouse when needed.) The greenhouse is simply too hot for direct sowing in late summer and early fall, when most winter crops need to be started.

Salads. Lettuces are quite resistant to frost, though not as cold hardy as some other winter garden plants. In the chill and reduced light of the winter greenhouse chicory's bitterness is tinged with sweet, and the stringy toughness is replaced by a delightful juicy crunch. (An unusually good source for chicory seeds is Seeds from Italy.) Lesser-known salads include mâche and edible chrysanthemum. Some are astoundingly cold hardy, such as claytonia (or miner's lettuce) and minutina (Herba stella). And don't forget scallions as an easily grown addition to winter salads.

Cooking greens. Spinach is extremely cold hardy. Make several sowings during the winter growing season. Plant crucifers, including mustards, raab, Oriental greens such as bakchoi and tatsoi. Chard (or Swiss chard) is a type of beet bred for its large tender leaves and rapid re-growth, rather than its roots. It is cold hardy and productive. Green onion and garlic tops also make great cooking greens. Brassicas that head (such as cabbages and broccoli) are more likely to develop large, tight heads if grown in the late-winter greenhouse rather than in the fall. Loose leafed kale, however, is an excellent crop for the fall-winter greenhouse if you start your transplants early enough. Get an early start.

Root crops such as beets or carrots are not suitable for planting in the fall greenhouse; they will grow, but do not receive sufficient energy in the shortening days to "make root." Excellent results can be had growing carrots, beets, potatoes and daikon (as well as the smaller radishes) in late winter, and harvesting these crops up to two months earlier than their siblings in the garden.

Use the greenhouse to give an early start to tomatoes, peppers and eggplants. Move the fastest-growing plants outside when the season has advanced enough, and harvest ripe fruits a month early. (A few tomato plants grown in the greenhouse itself offer those first vine-ripened tomatoes even earlier.)

More techniques:

Know when to water. It's best to water deeply from time to time in lieu of frequent shallow waterings. Water in the morning to give the plants plenty of time to dry before temperatures fall at night. Avoid overwatering, which makes plants "sappy," less able to withstand cold and other stresses, and less flavorful and nutritious as well. Test the soil with your finger: As long as you feel good moisture half an inch deep or so, it's better not to water.

Encourage natural ventilation. A closed greenhouse gets surprisingly hot on a sunny day, even if the temperature outside is quite cold. Don't stress your plants by leaving the doors to the greenhouse closed when it's sunny. Good ventilation is important for disease prevention as well. Balance your insects. Insect management is not so much about control, as it is about balance. Plant flowering plants to provide pollen and nectar that attract lacewings, ladybugs and other beneficial insects. Beneficial insects seem to migrate out of the greenhouse into the garden as it starts to bloom, boosting insect diversity there.

Of course, plant types need to be rotated to different places in the greenhouse from year to year.

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Here is a list of the largest vegetables that will need the most spacing in your greenhouse:

Bush type Beans: minimum of five feet between rows
Cabbage: a foot between rows
Peppers: about a foot between rows
Cantaloupes: two to three feet between rows
Squash: two to three feet between rows
Tomatoes and Watermelons: minimum of two feet between rows

You also need to get the maximum number of plants into the relatively small space of your greenhouse. Whether your aim is fruit and vegetables for the kitchen or exotic flowers for the house, you need to exploit your greenhouse to the full extent of its potential. Otherwise it just becomes inefficient.

Many people would like to grow plants but do not have much space. You may only have a balcony or a tiny yard. The smaller the space you have the more sense it makes to use a greenhouse because it enables you to make the most of that space all the year round. A tiny area can produce a surprising amount of plants if it is approached in the right way. You may not become self-sufficient in fruit and vegetables but you will be able to grow a supply of fresh salads and herbs or flowers for the home from even a small greenhouse.

The greenhouse is an intensive system of growing and that is what you need if you have only a small space available. Even if you have plenty of land you may want to think about a mini greenhouse next to kitchen door so that you can grab a handful of herbs or salad greens as you prepare supper.

Vegetables will reach maturity earlier if you raise you seeds in a greenhouse. Your crops will be ready before outdoor sown plants are ready for harvesting. With all plants sown in the greenhouse you must be careful to harden them to outdoor conditions slowly. Put them outside when the risk of frost has passed. Do this during the day at first. As they get used to the colder conditions you can leave them out at night. When you are sure they are strong enough plant them in their final position.

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