To grow well, house plants need conditions similar to those of their native habitat. Generally speaking, all plants require light and moisture and to be kept within a range of maximum and minimum temperatures. The precise requirements will vary with the seasons of the year. If the range of these requirements is not observed, the plant dies. None of these factors acts independently of the other; rather, they are interrelated. For example, less water is required at lower temperatures because less evaporates when it is colder. When it is hot, more water evaporates, so more is required.

Seasonal fluctuations in temperature and rainfall are recorded on climate diagrams of the world. If we know where a plant originally came from, then we can use such climate diagrams to establish its requirements throughout the year. Most house plants stand up well to British summers and some can even be put out in the garden in their containers. However, they must be taken indoors again well before the onset of frost and wintry weather.

Light and plant growth

No plant, however, finds totally constant conditions throughout the year conducive to good health and good growth. The amount of light is a factor vital to green plants. Light is what controls the biochemical process called photosynthesis. During photosynthesis inorganic substances (carbon dioxide and water) are transformed into organic substances (sugars) by the action of sunlight. These organic substances form the body of the plant and are essential to its basic life functions. So light directly influences the growth and development of plants. During the course of the evolution of different types of plants, the various groups adapted to widely varied light conditions. The ways in which the plant groups adapted depended on the location of their native lands and the character of their habitats, as well as the phases of their life cycles – in other words their periods of rest, flowering, fruiting, etc. So well have plants adapted to different conditions that the diversity of habitats in which all sorts of plant species are able to grow is immense – ranging from the scorching heat of the desert to the deep shade of the herbaceous layer of a rain forest.

A plant’s origin, the conditions of the habitat to which it is adapted and the type of situation it occupies in the wild must be kept in mind if it is to be grown successfully in the home. A plant’s general appearance serves as a good guide to its individual light requirements. Plants with delicate, pale green to blue-tinged leaves spaced fairly far apart on the stem generally require shade. Plants with stiff, leathery or fleshy leaves and short internodes, often covered with scales or hairs tinged reddish-violet, silvery or dark green, and plants of compact habit, can be put in full sun.

Plants are usually divided into three large groups: those requiring full sunlight, those requiring light of varying intensity but not direct sunlight, and those requiring permanent shade. These groups merely serve as guidelines and there are many plants that come somewhere in between the three without any distinct demarcations. This is particularly true of plants in the second group. Plants in this category include those that require light but not direct sunlight, or else only the slanting rays of morning and evening, those that thrive in diffused light and those that require some degree of partial shade. (Diffused light, for example, is light coming through a rather thick curtain or a thin leafy screen; while partial shade would be found in the undergrowth or indoors away from a window.)

The particular light conditions of any given home can be judged only by the owner himself, but there are a few general rules that can serve as a guide. The corners and walls on either side of a window are the most shaded parts of any room. The light in the corners opposite a window is also poor. The best light conditions are in the window itself. The amount of light passing through the centre of a window to a distance of approximately 150 cm (59 in) represents about 60 per cent of all the light entering a room. The further from the window, the more rapidly the intensity of the light decreases. Much also depends, of course, on the aspect. Windows facing south, southeast or southwest, and to a lesser extent, east or west, provide the most light. Where windows face northwest, northeast or north, the amount of light penetrating the room is much less and the duration and intensity of sunlight is very limited. The intensity of light is similarly influenced by the proximity of trees or neighbouring buildings.

Except for definitely shade-loving plants, such as Selaginellas, and light- and sun-loving plants, such as cacti and bulbs, all species of decorative house plants tolerate diffused light. An easy check for diffused light is that when an object is placed between the plant and the window, it casts a distinct shadow on the plant.

Humidity and moisture

Moisture, both in the soil and the atmosphere, is vital to the life processes of a plant. Plants absorb water through their roots, epidermis and its derivatives (hairs, scales), and return it to the atmosphere by transpiration through the leaves. The absorption and evaporation of water must balance. If the plant has insufficient water, it will wilt. Excessively wet soil, on the other hand, will damage the plant’s tissues and it will die.

Soil moisture is provided by watering the plant in accordance with its specific requirements. Again it is important to keep in mind its origin, the conditions of its natural habitat, the temperature of the room and the season of the year. During the growing period, plants generally require more water than during the dormant period.

Atmospheric moisture can be adjusted by syringing and misting, by packing damp moss round the plants, by using humidifiers and by growing the plants in closed, glassed-in spaces. The quality of the water is also important. Hardness, acidity or alkalinity (pH) and the degree of pollution will all affect plants. Plants should never be watered with hard water.

The growing medium

Food is another important influence on a plant’s growth. In the wild, green plants obtain food from the soil. Soluble salts, containing what are called biogenic elements, are dissolved in the water absorbed by the plant’s roots. Most plants live with their roots anchored in the soil or substrate. Soil is a complex mixture of organic and inorganic substances and is the result of processes taking place in nature over a period of many years. The indoor gardener who wants to simulate a plant’s natural conditions must try to provide an alternative growing medium that meets the biological needs of his house plants. Soil requirements differ widely. The types of soils required by plants from a primeval forest, savanna or desert, for example, are very different from those required by marshland or aquatic plants. House plants are generally grown in a blend of soils or soilless composts. Commercial potting mixtures of various types are readily available, or the grower can make his own blend from the components listed below. Leaf mould is formed by the decay of composted leaves. It is not particularly nourishing and so is only used for growing young plants or for sowing seeds. The quality of leaf mould depends on the type of leaves from which it is made. Beech and oak leaves are best, while horse chestnut leaves are the least suitable. Looseness is an important characteristic of leaf mould. It is added to almost every soil mix at nurseries. Young leaf mould, which is practically unrotted, is used for growing orchids, bromeliads, begonias and ferns.

Rotted turves are obtained by composting turves to which lime and farmyard manure have been added. Pouring liquid manure over them also works well. This is a very nourishing, heavy growing medium with large concentrations of organic substances. It is used in soil mixes for growing chrysanthemums, pelargoniums, petunias and palms. Frame soil is produced by rotting manure in a frame to which older frame soil has been added. The resulting soil contains up to 25 per cent humus, is a dark colour and is moderately heavy. It is added to a great many mixtures for growing vigorous species noted chiefly for their ornamental foliage.

Composted soil is prepared by composting plant remains to which farmyard manure, lime and loam have been added. After two or three years, this yields a rather light soil, with 10-15 per cent humus and a pH of 7.

Pine-leaf mould is the litter from spruce or pine woods. It is a loose, light soil with a pH of 3.5-5.5 (acid). It dries out rapidly and provides little nourishment. It is suitable in mixtures for many plants that like acid soil, such as azaleas, begonias, sinnin-gias, ferns and heath plants.

Peat is formed primarily by rotting sphagnum moss in an acid environment or by rotting wetland plants on low moors in a rather more neutral environment. It is added to practically all mediums for growing house plants. It improves the soil structure and retains moisture. It is very good for sowing seeds because it is free of micro-organisms. Loam and peat compost is obtained from peat that is saturated with nutrients and to which heavy loam is added. It is used in almost all mixtures for growing house plants. Heath mould is another acidic medium. It is formed by the decay of heath plants and is added to soils for growing heaths, azaleas, camellias and similar plants.

Other substances, both natural and artificial, are often added to the growing mediums. Charcoal is one of the natural ones. It is commonly added to the soil mix used for growing epiphytes. Having a large surface area, it absorbs excess moisture and prevents the soil from becoming waterlogged. It also has disinfectant properties, so it is used when propagating plants from cuttings to dust the cut surfaces. Both fresh and dried sphagnum moss have a wide variety of uses. Fresh sphagnum moss is excellent for growing carnivorous plants such as Drosera and Dionaea. It is also regularly used in mixtures for growing orchids, bromeliads and anthuriums. In the case of anthur-iums, the top of the soil in the flowerpot is often covered with sphagnum moss to keep it moist. Fern roots are often added to epiphytic mixtures to make them looser. The roots of the Royal Fern, Osmunda regalis, and Adder’s Fern, Polypodium vulgare, are used in this way. The advantage of these particular ferns is that their roots decompose very slowly so that they need replacing only after five to seven years. Crushed pine bark and chopped beech leaves are also frequently added to epiphytic mixtures.

Of the artificial additives, one that is widely used is perlite – andesite expanded at high temperatures. Minute and porous, perlite is excellent for making soil looser. Because it readily absorbs water, along with nutrients dissolved in the water, it is used in propagators. Ceramic granules made from brick-clay are used to aerate the soil. These are commonly used in hydroponics or spread on the surface of dishes for growing succulent plants. Sieved crushed brick is added to compacting soils to lighten them. It is often used by cactus growers. Granulated or crushed polystyrene is similarly used to lighten soil mixtures but it is not water absorbent.

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