PLANTING AND CARE
Large or small, a greenhouse garden, like any other, should be designed carefully and never planted until you have the plan firmly in mind. Each plant should be given room in relation to all the others, and not merely at planting time but with maturity time in mind. Allow plenty of space for spreaders to spread and creepers to creep; and make sure every climber has its support. And, of course, plants in close proximity should have compatible cultural preferences in soil, moisture, light, and the like.
Light and Sunlight
It shouldn’t be difficult, in a complete greenhouse garden, to arrange for areas of full sunlight, partial or dappled sun, and shade, and to do it in a natural manner. Tall plants will get the sun first and for the longest time; but you could also build a slope, a sand dune, or a rock garden that would face the sun and get the most of it. Partial-sun plants could be placed where they are only lightly shaded by other plants or, if necessary, by shadings on the sections of glass above them. Shade-loving plants can grow under small trees and bushes.
Almost any greenhouse, in any area, should have shading applied to the glass in early summer. Several shading compounds are available, in white or green, and are mixed and applied with either a wide brush or a sprayer. These compounds are constituted so that they slowly weather away during the summer and leave the glass clear by autumn. But if you happen to have plants that need all of the winter sun possible, it is wise also to clean the glass.
Temperature
Greenhouses are generally classed as cool (40–60 degrees), moderate (50–70 degrees), or warm (60–80 degrees). This refers to the minimum night temperatures at which the thermostat is set to regulate the furnace or heater. In warm climates where an air-conditioner is used, it follows the same scale. Greenhouse plants are usually classified similarly. But if other cultural conditions are favorable, many plants will tolerate a much wider range.
In order to grow the widest variety of plants, I set my greenhouse thermostat at 60 degrees in winter. This is a little warm for some cool-growing items such as geraniums, and causes outdoor garden plants (annuals, perennials, and vegetables started from seed) to grow slightly leggy and lank. But they recuperate in a hurry when they are moved outside. This temperature is slightly cool for subtropical plants; some gesneriads merely hold their own through the winter, neither growing nor flowering; some extremely tender begonias are forced into dormancy, from which they awaken in warm spring weather.
Humidity and Ventilation
With such a motley assortment of plants, I try to keep relative humidity at 75 per cent or higher. The minute the gauge dips below that mark, we wet down the walks and benches. Of course, this happens only on bright, sunny days. During dark, gloomy weather we water and mist as little as possible.
Constant circulation of fresh air is extremely important all year long. Our doors and vents are wide open in summer. On quiet winter days we may open doors or vents just a crack for a short time. Automatic ventilation is a marvelous convenience in late winter and early spring when the sun suddenly seems to get very hot and threatens to scorch everything under the glass.
Soil
Whether a greenhouse garden is a small flat, part of a bench, or a large ground bed, soil should be prepared carefully to suit the kind of plant that will grow in it. You’ll find “recipes” of all kinds in any authoritative greenhouse garden book. Prepared soil mixtures should be at least a foot deep for beds in which you plan to grow plants of moderate size. For fruit trees and other larger plants, make it even deeper.
Fertilizing
The fertilizing schedule which works so well for my house plants, seems to do equally well for my greenhouse plants. I shall stay with it until I find something better. But it does seem possible that a large greenhouse garden, with large areas of carefully prepared and nutritious soil, would probably need feeding slightly less frequently than plants in pots. I shall have to try this, when my dream greenhouse comes true.
Watering
One of the “musts” in my dream greenhouse will be both hot and cold water, and a provision for mixing them as they flow into the hose. Thus, watering will be like a gentle summer rain. I’ll have an extra-fine nozzle to break the stream into a delicate mist. Warmed water in a fine spray surely should not injure tender foliage in any way. And think what fun it would be to water a garden this way, enjoying the slightly musty fragrance of a warm summer night when soil is moistened by rain—and all of this in January when the snow is piled deep in drifts and banks outside the house.
Grooming
These are the little everyday pleasures of gardening in a greenhouse—snipping back a wandering branch, chiding a creeper and pulling it back from crawling over a neighbor, picking off a faded flower, supporting a branch heavily laden with flowers or fruit. It is real fun, and makes the difference between an overgrown mass and a carefully tended garden in which each plant appears at its best and in harmony with the others. Cleanliness—the removal of all organic matter before it rots, the rinsing away of dust and dirt—is the best protective measure against infiltration of insects and disease.
Insects and Disease
When my retirement time comes, I may have to make a choice. Which do I want most—the convenience of fumigation with greenhouse “bombs,” or the presence of pet frogs, ladybugs, friendly insects, chameleons, and the like among my greenery? I am sure I will choose the latter, because I will have plenty of time to tend my greenhouse garden; and it’s not too much trouble to “spot spray” any plant that shows signs of problems. Anyway, it’s risky to use strong fumigants in a greenhouse that is attached to a dwelling, unless it is tightly closed in and has no cracks or fissures for leakage. And who could be sure of that. Anyhow, if I killed all the insects, what would the frogs find to eat.