There are two kinds of plant that we bring into our houses. The more spectacular are the flowering plants, cyclamen, azaleas or African violets. Unfortunately their season of attractiveness is limited. All too soon the flowers will fade and the plants then have little attraction. If you have a greenhouse, you can keep the plant going and prepare it for another season, but we usually do not feel inclined to keep it in the house; certainly not in a conspicuous position. The other kind of plant is less spectacular: its beauty is centerd in its foliage rather than in its flowers, but it has the advantage that, provided you treat it properly, it will grow permanently in your rooms and increase in size and effect from year to year. These plants, grown for permanent effect, are commonly known as house plants.
Most of us do not live in glass houses; therefore the plants we can grow in our rooms are limited in number. Even a room that appears well lit to us, will seem shady to a plant and, as a result, the majority of houseplants are those that can tolerate shade. If a plant is to be permanently attractive, it should be evergreen. We can visualize exceptions, such as the bonsai dwarf trees, where the outline of the tree is attractive even when no leaves are visible, but there are not many of these exceptions to the demand for evergreen plants. Again most of us live in rooms of only moderate size and we require plants of moderate dimensions. We also do not want them to grow too quickly. Re-potting is a tedious operation for those who live in flats or in houses without gardens and we do not want to have to undertake it more than once a year at the most. Although plain green leaves are attractive enough, particularly if they have interesting shapes, leaves that contain some color are usually regarded as more attractive. color in leaves occurs in two forms. Some leaves are naturally colored; for instance those of rex begonias and Cordyline terminalis, but there are other plants which produce colored forms of their normally green leaves. These are described as variegated and the variegation may be due to a number of varied causes, from a virus infection to a periclinal chimaera.
Whatever the cause, the result is that some part of the leaf lacks chlorophyll, the substance that makes leaves green. If the chlorophyll is completely lacking, the area appears white, while, if there is very little present, the area appears golden or yellow. Although some plants appear naturally variegated, nearly any plant can occasionally produce a variegated form. Variegation is an exceptional occurrence and the plant can be perpetuated only by vegetative propagation, normally by rooting cuttings or layers. Seeds are very unlikely to transmit the variegation. By no means all people find variegated leaves attractive, but very many do and, as a result, many plants are popular because of their variegated leaves that would otherwise be little regarded. The popular variegated forms of Tradescantia fluviatilis and Chlorophytum capense may be cited as examples. With half or more of the chlorophyll lacking, the leaves of variegated plants can only do half the work of normal green leaves and so variegated plants tend to grow more slowly than the unvariegated forms. This is not unexpected; a more surprising result of variegation, though it is not always the case, is that the plants are often more tender than the normal forms. There seems to be no very obvious explanation for this.
We can now summarize the qualities that we require for a house plant. It must be compact in habit of growth, tolerant of shade and evergreen. The leaves should be attractive, either by reason of their shape or their color: if we can have attractive flowers as well, so much the better, but with the emphasis to be laid on the permanence of the attraction, agreeable flowers are obviously a bonus. In fact the combination of handsome flowers and handsome leaves is somewhat rare in any branch of gardening. Among the house plants many of the bromeliads provide an exception to this rule, but even with these plants the showiest part of the inflorescence is due to the colored bracts that surround the flowers, and these are really modified leaves.
Temperate climates produce few plants with the characteristics that we require. The various ivies form an important exception to this statement, but, even so, the great majority of house plants come from the tropics. Plants are infinitely adaptable, as a general rule (there are, of course, exceptions and these are generally regarded as ‘difficult’ plants) and most tropical plants will adapt themselves to temperate conditions and even to the fluctuating lengths of daylight, which, even more than the alteration in temperature, mark the chief difference between tropical and temperate climates.
There are certain temperatures, varying from plant to plant, below which plant growth ceases. The plant may survive perfectly well, but it will neither produce fresh roots nor fresh leaves, until the temperature is raised. It is, obviously, more difficult to produce high temperatures when the outside temperature is very low and so it is most convenient to make our winter the equivalent of the tropical plant’s dry season. The dry season in the tropics is generally very hot, but, owing to the lack of water, the plant makes no growth and stays in a dormant condition. This is one of the reasons why all house plant growers are recommended to keep their plants as dry as possible during the winter. How dry you can keep them, will depend on the type of plant you are growing and on how warm you keep your rooms.
The type of heating that you use and the temperatures you maintain in your various rooms during the winter will affect the types of plant you can grow. Some plants, notably begonias, are very intolerant of gas fumes, so that if your rooms are heated by gas, you will not be able to grow begonias satisfactorily. If you have really warm rooms, maintaining, perhaps, an average temperature of 70°F (21°C), they will be far too warm for such plants as ivy or x Fatshedera lizei. With high temperatures such as these, the plants will continue growing during the winter and more water will be required. The winter growth may not be very ornamental, as the lack of light will prevent the formation of good sized leaves.
Many house plants are ‘stopped’ in the spring: that is to say that the tips of the various shoots are nipped out, so as to induce the formation of secondary shoots that will give the plant a nice bushy appearance, and where this is done the weak winter growth can be removed. However, there are plants, such as most of the ficus, that are not stopped and, where these are concerned, it might be better to move them to cooler positions in the winter. However most of us, alas, cannot afford these high temperatures and it is more a question of keeping the room warm enough for our plants and ourselves. In any natural climate the highest temperatures are around midday, but many sitting rooms are kept cold during the day, when people are out at work, warm in the evening, when everyone is at home, but cool off during the late evening and the early morning after people have gone to bed.
Such a contradiction of natural rhythm is sufficient to disturb any plant and it is easy to see that keeping plants in good condition in the winter is less simple in the house than in a greenhouse. If you have some system of regular central heating, the problem is comparatively simple, but for rooms with only sporadic warmth, the matter is less straightforward. However, there are house plants to suit all conditions. It is as well to know what the average temperature of your room is during the winter, otherwise the problem can be resolved only by a system of trial and error, during which you might well lose the plants that you most prize. It is fairly safe to say that no plant will tolerate the conditions that are to be found on a mantelpiece above a coal fire. The atmosphere is far too dry and the alternations of cold and roasting heat are too much for all plants, except the toughest succulents. Even if the temperature is equable, plants that are put too near the window risk being chilled, or even frosted, when the weather is very cold and they should be moved further into the room during these periods.
Even when the temperature is satisfactory, the dry atmosphere that we like in our rooms is not beneficial to plants. This, however, can easily be overcome, by placing the pots in a larger container and filling this container with some moisture-retentive material. Peat is most frequently used, but moss or mica powder does equally well. Some people get perfectly satisfactory results with damp newspaper, which is topped with moss to look more elegant. By these means we can maintain a moist atmosphere in the immediate surroundings of the plant without either affecting the atmosphere of our rooms or the correct state of moisture of the soil ball. With this we come to the most crucial matter in the successful cultivation of house plants.
More house plants are killed by over watering than by other cause. Like human beings, plants cannot live without water, but, again like human beings, they can be drowned. However, this analogy cannot be pressed too far. Human beings need water at regular intervals, but plants need water most when they are making growth. This is usually during the spring and summer. There is a correlation between the growth of the aerial portion of the plant, the portion we can see, and the growth of the roots which we can’t see. If the plant is making new leaves and stems, we can be fairly sure that it is also making new roots. Unfortunately, the root growth is liable to precede the production of new leaves and so these latter may be prevented from developing if the soil is too dry at the appropriate period. On the other hand, if the soil is too wet, the roots cannot breathe and, far from developing, are liable to rot and, unless this process can be stopped immediately, the plant itself will succumb. We can guard against this to some extent by purely physical means. If we have an open soil mixture that drains rapidly and well, the risk of the soil becoming sodden and sour is reduced, although not, of course, obviated altogether. When to give water is only satisfactory learned by experience, but the following hard and fast rules are generally acceptable.
When water is applied, it should be in sufficient quantity to moisten the whole of the soil ball. The water should be at room temperature. Rain water is preferable but not essential.
The soil should be allowed to dry out between waterings. This is not too easy to interpret. We do not want the soil to become dust dry, but on the other hand, we want to avoid saturation. A useful rule of thumb with clay pots and soil mixtures, is to strike the side of the pot with your knuckle. If the resultant sound is dull, watering is not needed, but if it is a ringing sound, water should be applied. With peat mixtures the weight of the pot is a more reliable indication. If it feels light, water is wanted, but not if it feels heavy. The same applies, but to a lesser extent, with soil mixtures in plastic pots. These are much the most difficult to gauge.
During cold weather plants make little or no growth and so require little water. Growth is also slowed down when there is little light. It is safe, therefore, during the winter to keep all watering down to a minimum, even though the room may be kept at quite a high temperature. Naturally plants in warm rooms will require more than those in cool surroundings.
From about mid-April it is probable that growth will start and so more water may be required. Be cautious, nevertheless, until you see new leaves appearing. It is possible to knock the plant out of its pot to see if new roots (characterized by their white tips) are forming and to replace the soil ball without disturbance. When growth is vigorous water will be needed more frequently. ‘Stopping’ checks growth temporarily and watering should be on a reduced scale until a resumption of growth is seen.
By the end of August it is advisable to discourage much further growth and encourage the plant to ripen its new growth. This is done by keeping the plant as dry as possible.
The type of leaf will give some indication of the plant’s requirements. Plants with thick leaves or with succulent leaves (such as the large-leaved ficus and sansevieria) can tolerate longer periods without water than thin-leaved plants. These latter will probably wilt when they become too dry and they should be watered at once. The thicker-leaved plants will not wilt and so should be inspected frequently. Drought, unless acute, will not kill them but may cause subsequent leaf drop.
If leaves turn yellow and fall off, it usually indicates over-watering. However some plants, such as Ficus benjamina, will naturally shed their year-old leaves in the autumn and most plants shed a few leaves in the course of the year. Excessive defoliation is almost certainly due to incorrect watering; although it can be caused by under-watering as well as by over-watering. If the plant becomes unsteady in the pot, this is generally due to root-rot caused by excessive water and is very difficult to arrest. Some leaves will wilt in the summer if they are in direct hot sunlight. If the soil appears to be satisfactorily moist, a syringeing of the leaves with water will generally restore them to their normal turgidity, and in any case, they will resume their normal appearance as soon as the sunlight goes.
Once the question of watering has been mastered, there are few other problems. Rooms are very dusty which spoils the appearance of the leaves of house plants and also prevents them from functioning properly. It is advisable, therefore, to clean the leaves every two or four weeks. This is best done with cotton wool and tepid water and the leaves should be sponged on both sides. New leaves are soft and easily damaged and should be left until they are older. Some people use milk, or oil, or flat beer to give the leaves a more glossy appearance, but these mixtures do not do the leaves any good.
During the summer, when growth is most vigorous, the plants may be fed. A liquid feed is most easily applied and should be given according to the instructions on the bottle. Little and often is invariably better than doses in excess of those recommended. Unless the plant is really well-rooted, feeding should not be applied and is not necessary for plants that have been repotted. Repotting is done in the early summer. For the majority of house plants the John Innes potting compost No 2 is the best. Plants are usually potted on into a pot one size larger. Plants from 13cm (5in) pots are put into 16cm (6in) pots and so on. The only exception is that the 10cm (4in) pot is very rarely used and plants are moved from 7cm (3in) to 13cm (5in) pots. Plants with very thin roots such as begonias and peperomias do better in a mixture of 2 parts of leafmould to 1 part of sharp sand, while epiphytes, such as the bromeliads, are usually given a mixture of peat, leafmould and sharp sand. However, it is only rarely that epiphytes require any potting on, as they use the soil as an anchorage only. After being potted on the plants should be kept on the dry side until the roots have penetrated the new soil. It is best to move plants from 5cm (2in) pots to 13cm (5in) pots after a year, as the 7cm (3in) pots dry out so quickly, but after that most house plants will need repotting only every other year. The second year the plant will need feeding.
The epiphytic bromeliads (aechmea, neoregelia, nidularium, guzmania, tillandsia and vriesia) need rather different treatment from most house plants. They have a rosette of strap-shaped leaves which form the so-called ‘vase’. This must be kept full of water, preferably rain water. The mixture in which they are potted may be kept moist, but this is of minor importance, as the roots serve little purpose except anchorage and it is from the leaves that nutriment is absorbed. During the summer the merest trace of liquid feed may be added to the water in the vase, but this must be done with great discretion.
Most house plants are easily propagated if you have a greenhouse, although some, such as the large-leaved ficus, cordylines and dracaenas, need a good deal of heat to get them to root. There are a few that can easily be propagated in the home. The various tradescantias and zebrinas will root easily in water and so will Cissus antarctica and Rhoicissus rhomboidea. Shoots of succulents, such as Sedum sieboldii and aichryson will root easily, either in ordinary soil or in a mixture of equal parts of peat and sharp sand, which is an ideal mixture for most cuttings. Shoots of the various ivies, taken when they are half ripe, that is neither too young nor too woody, will root easily although rather slowly. The peperomias, with single leaves rising from the base, can be rooted from leaf stem cuttings. The leaf with its stem is pulled off and inserted in the peat and sand mixture when a new plant will form at the base of the leaf stalk. Sansevierias will produce new leaves on rhizomes, but they do not root until a year has elapsed and should not, therefore, be severed from the main plant before this time. If, however, you can find the new rhizome without disturbing the plant and cut half-way through it, it will hasten the formation of roots at the base of the new leaf. Many of the climbing aroids produce aerial roots and these can be induced to develop in soil. These climbing aroids (philodendron, syngonium and scindapsus), will grow more luxuriantly if they are given a cylinder of wire stuffed with moss up which they can grow. However, the moss must be kept damp and this is not easy in the home. They can also be trained on blocks of cork bark.