Plants grow in the most extraordinary places and in the most extraordinary ways. In the tropical jungle regions of Brazil bromeliads perch on trees (and even telegraph poles), collecting water, decaying leaves and insects in a vase of leaves, and flourish. By contrast in the deserts of South West Africa there is a succulent called 'Baby Toes' which grows buried in the sand except for a tiny window peeping above ground to let in the sun. In all parts of the world, except those permanently covered by ice, there are plants which have no difficulty in growing in the most adverse conditions. Yet no plant of its own accord grows in a house - at least not until the roof falls in and sun and rain and decay enter. Then they invade.
Have you noticed that in the story of the flood, when Noah built his ark he only took animals on board - two of everything. Plants do not get a mention. Quite right, too, for even if the animals had left them alone the chances are that they would not have survived the voyage. Outdoors they stood a much better chance. The olive cannot be considered to be a moisture loving tree, but even after ten months under water it was still able to produce a branch for the dove to take back to the ark. Yes, plants are great survivors in the great outdoors. In fact there is no such thing as an indoor plant; only outdoor plants which we try to grow indoors.
Mind you, when we do take outdoor plants indoors, not all of them settle down and grow well. This is usually due to cultural problems; i.e. it is all our fault. It could be we have chosen the wrong plant, or because we have chosen the right plant and treated it in the wrong way. The 'right' plant to choose is one which will find indoors at least some of the conditions which were an essential part of its original habitat. The obvious starting point is temperature.
Different seeds germinate at different temperatures. Above that there is a range of temperatures in which the plants will grow best. At even higher temperatures they will still grow, but not so well, and above that there is a temperature at which they will die. Below the germination temperature there is a fairly narrow range of temperatures in which they rest. Below that there is another temperature at which they die. To a certain extent plants will get by in less than their optimum conditions, but they cannot be pushed too far.
A look back over the past 200 years or so shows how these basic facts determined the kind of houseplants grown at various times. At the start of the last century the craze for cacti started, followed by one for ferns and foliage plants like aspidistras. All these plants have one thing in common - whatever their other needs they had to be kept cool, either all year round or when they were resting. This was something that Victorian houses were well able to provide. Also, they were tough enough to withstand the darkness of foggy winters and asphyxiation from gas fires. Our houses have changed since then (thank goodness); they are warmer, lighter and less polluted. As a result houseplants have changed, and we now grow tropical exotics which need warmth. Plants like Peace Lilies, Orchids and Bromeliads. The problem is now reversed: with more effective heating our houses can be too warm in winter for a host of plants which must or should have a cool resting period. The importance of a rest period cannot be exaggerated, but it is too often ignored. Everyone who has kept a tortoise or has watched a dormouse sleeping all winter in a TV wildlife documentary knows how important hibernation is for some animals. Winter is approaching and food is going to be scarce, so, to conserve energy they put themselves, as it were, into cold store. Even plants which grow outdoors in temperate climates like ours behave in a similar way. The temperature falls to a point at which the plant stops growing. It then needs less energy and therefore less food, which is just as well, because the plant needs light to manufacture its food, and there is not enough light in winter for it to produce enough to keep growing. Bring such plants indoors and the warmth induces them, against their habit, to go on growing, but unhealthily, because they will be suffering from light starvation.
Tropical plants have a different life style in their native haunts from these temperate plants. In their natural environment the temperatures are usually high enough to keep them growing all the year, and as light levels hardly change the plants can manufacture enough food to keep them growing healthily. Bring them into our houses and they will keep on growing in winter if the room is warm enough, but they will suffer from light starvation. For their future health they must be forced to take some rest, even though this is against their nature. It can be done by moving them to a cooler room - if there is one in a centrally-heated house - and then watering less and feeding not at all.
Temperature levels, therefore, are not the only considerations in choosing a plant, since they are inextricably linked with levels of light, but they remain the essential starting point. In the summer months when the heating is turned off, how cool or how warm a house is depends basically on the temperature outdoors, and we have no control over that. Even so, some rooms are warmer or cooler than others. For that part of the year when we light fires or turn up the boiler we have more control over how warm we keep the home, or at least part of it. Those plants which must have cool resting periods in winter may be more suitable for houses with partial or no central heating, since they are likely to have at least one room cool enough for such plants. Those plants which need considerable warmth day and night in winter as well as in summer are most suitable for houses with full central heating, providing they can be given enough light during the dark winter. If not, they will stay healthier if a place can be found for them to rest for a time at lower temperatures.
Of course, plants have other needs besides adequate levels of temperature and light and suitable resting periods. These - such as watering, fertilising, pruning and repotting - are all part of the routine of looking after your plants. Some basic knowledge of how plants work helps in understanding why you are urged to do one thing and not another. But that is another story.
Happy gardening.
written by Bill Hutchings
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page last updated 22 AUGUST 1999