You should be as concerned about improving soil quality in the greenhouse as in the garden. I grow a cover crop of cowpeas in the off season to improve the soil. Another possibility is the growing of forage crops—grain grasses, mixed crucifers, peas, etc.—as cut-and-come-again forage for poultry in the winter. The biomass of the root systems of some of these plants, especially rye, is quite large. If you rotate your forage plots over the greenhouse beds, their soil will over time increase in tilth, fertility, and humus as the spent root systems decompose.
Use of compost is of course always a good idea, especially for its boost to the microbial populations in the soil. As in garden growing, mulches help moderate the temperature in the soil, conserve soil moisture, and decompose over time, increasing soil tilth and fertility.
One caution regarding soil care in the greenhouse: It is important to avoid over-fertilizing with nitrogen. Green leafy crops can sometimes accumulate unhealthy levels of nitrates, especially in the low light conditions of the winter greenhouse. I never add nitrogen fertilizers in the greenhouse. Composts are a good source of fertility, though it is better to use plant-based rather than manure-based (higher in nitrogen) composts. I am even concerned about the nitrogen fixed by the summer cowpea cover crop, and plan to follow it with a quick mixed grain cover to “sop up” some of the nitrogen before the fall greenhouse planting season comes in.
The Moderated Winter
A homestead greenhouse can add tremendously to sustainable food production. However, adding huge amounts of artificial heat in order to grow tomatoes in January or cucumbers in March is anything but sustainable. Hence, I strongly recommend relying on the protection from winter’s extremes provided by the structure itself, and on naturally cold-hardy plants, in order to bring in your winter crops. Let’s look at each of those points in turn.
Most new greenhouse owners are surprised to learn that a plastic-skinned greenhouse gets quite cold at night—in fact, the air temperature will often be only a few degrees above ambient temperature outside. Upon reflection, this is not really so surprising: The translucent skin that acts as a portal for the radiant heat of the sun during the day is also no barrier to the radiation of stored heat out of the structure into the cosmos at night. How is it, then, that the greenhouse is so effective at keeping plants alive, even when winter is turning the ground to iron outside?
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 loses that stored energy at a prodigious rate, true, but it has a huge amount of heat it can lose before the soil starts to freeze.
The other way in which the structure protects its sheltered plants is by moderating the extremes of winter. Plants which are adapted to low temperatures will still be badly stressed if the temperature plummets from a high of 40° F in the late afternoon to 18° by dark; or if sharp winds join in on the abuse (“wind chill” isn’t just a problem for us humans); or if they get rained on in low temperatures. The greenhouse acts to slow the abrupt temperature changes, and to keep wind and cold rains at bay.
Remember, however, that the moderating influence of the greenhouse is effective only when we are growing naturally cold hardy plants. I have harvested lettuces, completely exposed in the garden, in mid-December in a fairly benign winter. Spinach will often survive a cold winter in the garden, and rejuvenate when encouraged by the sun come spring (here in Zone 6b). You might say that we are using the greenhouse to imitate for naturally cold hardy plants like these an unusually mild winter—not to teleport them to somewhere in the tropics. (More on plant selection in “Crops for the Winter Greenhouse”.)
Two factors make possible the survival of cold hardy plants through freezing temperatures. I referred above to the way the growing beds store solar heat during the day, and can lose that heat profligately before the ground starts to freeze. But by the point that the ground does start to freeze, it’s morning, and the cycle begins anew. I’ve gone into my greenhouse many a morning after a 10° night to find only a quarter of an inch of frost on the surface of the growing beds. That quarter inch is no problem—as long as there is no freezing deep into the root zone, the plants are not unduly stressed.
The other factor is a neat little trick the plants have learned, but which is apt to be unsettling the first time the gardener sees it. When the air temperature gets below freezing, certain plants such as lettuce have learned how to move water out of their cells into the intercellular spaces, to prevent rupture of the cell walls when the water crystallizes (freezes). In this state, the leaves become limp. When the gardener comes into the greenhouse after a deep-cold night, the sight of drooping lettuces seems to indicate a disastrous end of the winter growing effort. After half an hour of sunlight, however, the plants’ cells rehydrate, and they perk up as bright and pretty as they were before their descent into the chill.
The Mirror Season
Experienced gardeners may have some difficulty adjusting to the paradoxes of winter gardening. Unlike in spring, when the season is gloriously opening out into greater warmth and longer days, in the fall it is shutting down into a time of ever-greater darkness and deeper chill. The implications for growing are tricky, and take some getting used to. This reversal in the general trend of the season is perhaps the biggest adjustment the winter gardener has to make. We have to re-learn many of our assumptions, particularly about scheduling our crops. Do remember that the biggest challenge is apt to be the reduced photoperiod, rather than the low temperatures (assuming we make appropriate choices of cold hardy crops).
The bad news: During the darkest time of winter, there is insufficient solar energy to support vigorous growth. If we have started our plants too late to make most of their growth before the short days, they will indeed survive the cold temperatures, but instead of growing actively will sit and sulk, awaiting sunnier days.
The good news: On the other hand, if we get the timing right to produce, say, a mature head of lettuce by the dark days, the window of opportunity for harvest expands enormously. That perfect head of lettuce that would demand “use it or lose it” within a matter of days in June, will sit contentedly in prime condition, awaiting your pleasure, for two or even three months in the middle of winter.
One implication of the dormancy at the heart of the winter harvest season: When you start your crops in the late summer or early fall, start far more than you think you will need. As you make your earliest harvests, 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 the longer days make possible some late-winter crops.
It is best to water deeply from time to time (in lieu of frequent shallow waterings). Water in the morning, as soon as the frost is off the leaves, to give the plants time to dry before descending into nighttime cold again. Avoid over-watering, which makes plants “sappy,” less able to stand the 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 or so deep, it is better not to water.
It is important to appreciate how hot a closed greenhouse can get on a sunny day, even if the temperature outside is quite cold. Do not stress your plants by leaving the doors to the greenhouse closed when it is sunny. I typically shut up my two large doors (one at either end) at night, then open them wide during the day. If the day is unusually cold, blustery, and cloudy, I will prop the doors partially open. However, I always ensure there is some air movement through the greenhouse during the day.
Incidentally, I found that open doors at either end of the structure provide sufficient ventilation during the winter—exhaust fans were superfluous. I know growers with 20×96-ft greenhouses who report that ventilation is adequate using open doors alone. Remember that as the air in the greenhouse heats in the sunlight, it will rise and exit the structure, and more air will be drawn in from outside, providing constant natural air exchange.
My approach to leaf-eating insects in the greenhouse is, as in the gardens and orchard outside, not so much about control as about balance. Hence I encourage all the flowering plants I can inside the greenhouse. I planted yarrow throughout the beds last fall, and it bloomed late into the fall and quite early in the spring. Perhaps it was the flowering yarrow that encouraged the obvious increase in lady beetle population—in any case, I had far less trouble with aphids this spring. I also allow unharvested chicories, crucifers, and onions (grown from bulbs discarded in the kitchen because of sprouting) to flower and boost insect populations.
My impression is that the insect season gets an early start in the warmth of the spring greenhouse, then the lady beetles and their comrades migrate out into the garden as it starts to bloom, boosting earlier insect diversity there.
And speaking of insects: Be aware that a greenhouse provides good habitat for black widow (and of course other) spiders. It is important not to leave stacks of emptied plant pots and cell-packs lying about—the dark space between such cast-offs and the ground is just where our lady the black widow likes to set up, weaving her scraggly web, making a yellowish cocoon for her eggs, and awaiting prey. The undersides of the worm bin lids are also prime real estate. Though feared for her venomous bite, she is shy and unaggressive. Just respect her need for privacy and watch where you put your fingers. Sooner or later you’ll likely see her. Tell her I said hello.