Temperature tolerances are directly affected by the humidity in the air. Many plants will accept a temperature that is higher than average if the air is moist. Some will accept a temperature that is below average if the air is not too moist. In air that’s too hot and dry, leaf edges will often turn brown and crisp; when it is too cool, a plant may stop growing temporarily; when it is both too cool and too moist, there may be danger of rot. Don’t trust the thermostat that governs your house heating system to tell you whether the temperature is right for a plant; measure it on the very spot where the plant is growing.
Humidity
This is often a more crucial factor than temperature, and one that is more troublesome to change. Relative humidity is a measure of the amount of moisture in the air as compared with the maximum amount of moisture the air can hold at a given temperature. It is expressed in percentage figures.
Most plants find it difficult to breathe in dry air; they need some moisture around, on, and in their leaves. (People are better off when they have it, too.) Alone, or teamed up with unsuitable temperature, low humidity can cause leaves to wither, buds to drop before they open, and even an invasion of mites or some other pest.
A humidity gauge is not nearly so familiar a household item as a thermometer, but it is equally useful, usually better looking, and not much more costly. And it can come up with some eye-opening information. The humidity gauge will tell you very quickly, for example, when an air-conditioning system is missing its recommended mark of 50 per cent relative humidity. It will stop you from overwatering in muggy weather. It will show you that humidity can be dangerously low in the window where your plants are growing, even though it is perfectly acceptable three feet away. (Perhaps there is a radiator beneath the window.)
What you do to increase humidity depends on how low it is, and on what type of plant you are growing. At our house we have three systems for the humidity problem—the lazy, the halfway, and the necessary nuisance. The lazy way works fine for dry-air-type plants such as most cacti and other succulents. We simply grow them where the hot blasts from the radiator are strongest. They don’t wither, and when on sunny days we water them, we also mist them with a fine water spray. (For an easy misting device, we have an old Windex bottle with a spraying attachment in the cap. It does a fine job. Of course, there are other commercial products with built-in sprayers. Perhaps you have one in your kitchen.)
To go “halfway,” you set up some simple humidifying device. The containers which you fill with water and hang behind the radiators are effective to some extent. But don’t forget to keep them filled. Pans of water on top of the radiator also help. A steam kettle, such as you use when the kiddies have a chest cold, can be used for a few hours. But remember, that steam is hot, so be careful not to get it too close to your plants. Just keep it in the general vicinity; please, not too close! Here is something else we often use—put some gravel, sand, perlite, or vermiculite in a plate or platter, pour in some water, and put your flower pot in it. The water will evaporate and moisten the air around your plant. If you have a number of plants, get a metal or plastic tray. If you have a decorative garden, you might want to have a tray made to order, to fit your space. Paint it a pleasing color. I have seen such trays decorated with decals, but personally, I’d rather rely on my plants for the necessary decoration. In having a tray made—we’ve had several made by our tinsmith—the larger the tray, the better.
The “nuisance” system is usually necessary only for very delicate tropical plants that flourish in highly humid jungles and rain forests. They will need the platter, or tray of moist sand, plus some enclosure to hold in the moist air. For a single small plant, this can be an inverted bell jar, or a tent made from a polyethylene bag (the sort vegetables are often packed in) and propped over the jar and plant. A larger bag can be used to protect several plants. You can even use kitchen-type plastics such as Saran Wrap. A collection of tropicals almost calls for an indoor greenhouse, like the oldtime Wardian case. In our window greenhouse we can close the inside sash and, with the aid of pans of water, keep the humidity just about where we want it. On cold nights the electric-coil heater isn’t always adequate, so we open the sash to get the benefit of the heat of the room.
The plants that need this extra humidification are definitely in the minority; most are content with simple measures, or none at all.
Soil