Ailments of House Plants

Freeman A. Weiss.

House plants, being creatures of a sheltered environment, may be spared some of the tribulations of pests, diseases, and occasional hardships of nature to which plants growing in the open are often exposed. They are not immune, however, to those afflictions.

Indeed, their indoor environment, which one might presume to be altogether beneficent, is really the cause of most of their ills. The statement seems a little contradictory when one considers how well most plants flourish in a greenhouse. Let us examine for a moment the actual conditions of the environment in which house plants are expected to thrive.

The distribution of light in an average living room is very uneven greatly different from the even lighting under the open sky. We are impressed with this when we try to make photographs indoors without using supplementary illumination and get pictures with extremely bright highlights and deep shadows. To be sure, we put our house plants, except the very shade-tolerant sorts, in our best lighted windows. But even a single thickness of glass may reduce the intensity of outdoor light by one-half. With storm windows, screens, and curtains besides, the light intensity inside a window may be a tenth or less of that outside. Furthermore, the light near a window is strongly one-sided, and it often undergoes extreme and rapid variation as the sun is obscured by clouds, trees, or buildings. Thus the light may be too strong at times, too weak at others, and always from the side, never overhead as under a bright sky. The artificial light we provide for our own convenience usually benefits house plants but little, as it is turned on only during hours of darkness outdoors and is very spotty. The indoor light we customarily read by is but a fortieth or less as strong as outdoor light on an average bright day.

All those adverse light conditions are accentuated during the short and often dark days of winter, when we cherish our house plants most and want them to flourish and bloom.

Humidity atmospheric moisture is so variable that hardly any generalization holds very well, except that as a rule the air inside our houses is much drier than that outdoors, especially in winter when we are using artificial heat. The humidity outdoors ranges generally from about 30 percent to 100 percent of saturation, which means that the air contains from about one-third to the full quantity of water vapor it can hold at any given temperature. Indoors, it is likely to be 30 percent or less most of the time very much lower in dry regions under winter conditions. The humidifying devices sometimes used on heating equipment seldom raise the humidity more than 10 to 20 percent. Thus the upper range of humidity we can maintain in our living rooms corresponds to only the lower part of the outdoor range. Furthermore, the humidity indoors is constantly low because we artificially keep the temperature fairly uniform in our living rooms, whereas outdoors the humidity increases at night as the temperature characteristically falls. It often reaches saturation, in fact, as shown by the deposition of dew or, in cold weather, of frost.

Some horticultural writers (doubtless middle-aged males!) have urged a return to olden times when the teakettle simmered all day on the cook stove and the family washing was done in open washtubs or boilers and the damp clothes were hung in an anteroom to dry all of which tended to keep the humidity up to the level preferred by plants. But that, of course, is impracticable.

The daily rise of temperature and its fall at night, together with the concomitant decline and ascent of the humidity, I have already mentioned as characteristic of the outdoor environment. Further, the temperature indoors varies but little, at least in houses and apartments with automatic heat control. It is undoubtedly beneficial to plants to experience a drop in temperature at night it is their resting period after working all day in the light. Greenhouse managers regularly let the temperature fall at night as much as 10 to 20 degrees. They do so not merely to produce a corresponding increase of humidity, although it does have that effect. The lowering of temperature at night may profoundly affect the growth habits of a plant and perhaps its flowering or fruiting. Those responses are relics of its behavior when it grew outdoors. Some house-grown plants also are benefited by this daily temperature change.

Plants growing in the open ground normally "forage" for water and minerals by sending their roots as deep or far as possible to reach those vital supplies. The soil is a more or less constant reservoir of moisture; the loss from the surface layer by evaporation and plant growth is compensated by a gradual rise from deeper levels. When heavy rains fall, the excess water runs off to lower levels or soaks into the subsoil beyond the immediate range of plant roots. Rain water and water from melting snow are practically free of dissolved minerals. In its passage through the soil, the water dissolves and washes away excess salts and harmful products of fermentations that occur in the organic components of the soil. The alternate fall and rise of soil water causes a corresponding Movement of air into and from the Soil, and that aeration is important to the health of plant roots.

Pot-grown plants, especially those in impervious plastic containers now much in favor, are at a disadvantage in most of these respects, in comparison with those growing in the open. The quantity of soil is strictly limited. Root growth is limited correspondingly. The soil rapidly becomes water-soaked and just as quickly dries out. There is little chance for natural purifying processes through drainage and aeration. The high mineral content of many city water supplies and the necessity for frequent watering lead to an accumulation of lime and other chemicals in the soil of potted plants. The accumulations are often injurious and difficult to remove.

THIS RECITAL of the adverse factors of house-plant culture might make it seem something of a miracle that such plants ever flourish indoors. Many fine specimens, excellent in foliage and flower, testify to the contrary. I exaggerated unfavorable features purposely just to emphasize the fact of their existence. To the extent that the deficiencies of the indoor plant environment, in such respects as light, humidity, temperature, and water, are corrected so as to approach the favorable conditions of the open ground and air, so will the health of house plants be promoted.

That means good light, as uniform as such a variable factor can be made, supplemented if necessary by artificial illumination. It means providing the nearest possible approach to favorable humidity by placing the pots in shallow saucers filled with water or resting them on trays containing moist pebbles, vermiculite, or other clean, water-absorbent material. Plants kept in very dry rooms should be enclosed, if practicable, in a glass or cellophane-covered case, provided with a moist floor. Extremes of temperature must be avoided, especially a constant excess: House plants will generally flourish best at day temperatures not over 70 F., dropping 5 to 10 at night. Watering should preferably be periodic and regular, with moderate wetness alternating with moderate dryness. Except for occasional drenches, which should be followed by prompt drying, to cleanse the foliage of dust and grime, heavy watering and likewise extreme drying are to be avoided. The water should always be near the temperature of the air or, if applied to the foliage, slightly warmer. It goes without saying that house plants must be protected against illuminating gas, especially that produced from coal, and from exposure to other atmospheric contaminants, like the fumes from coal or oil stoves.

Various biological agents also may injure house plants. We are here concerned only with the parasitic microorganisms chiefly fungi and bacteria, but also nematodes and the submicroscopic viruses. The grosser parasites and enemies insects and mites as pests of house plants, have been treated in the 1952 Yearbook, Insects, which should be consulted for information on their recognition and control. Attention is directed particularly to the chapter, "Insect Pests of Flowers and Shrubs," pages 640-651 in Insects.

One could easily make an impressive list of the parasitic diseases that have been recorded at some time or place on indoor plants, but it would be discouraging rather than helpful to the plant grower. Fortunately the diseases that occur at all commonly are few, and principally of three types:

A. Diseases that primarily attack underground parts and are due to soil-borne organisms: Damping-off and root rot, caused by fungi (Pythium, Rhizoctonia); crown rot, caused by fungi (Pythium, Rhizoctonia, Botrytis, Fusarium, and Phytophthora); bulb rot, caused by bacteria and various fungi; crown gall, caused by bacteria; wilt, caused by fungi (Fusarium, Verticillium); root knot, caused by nematodes (Meloidogyne, formerly called Heterodera).

B. Diseases that primarily attack above-ground parts and are caused by airborne organisms: Anthracnose (leaf and stem spots and rots), caused by fungi (Colletotrichum, Gloeosporium); leaf spots and blotches, caused by various fungi and sometimes bacteria or leaf-infesting nematodes; stem cankers and dieback, caused by fungi (Diplodia, Fusarium, Phomopsis, and others); stern galls, caused by bacteria and sometimes fungi; gray mold shoot blight and bud rot, caused by a fungus (Botrytis); mildew, caused by fungi (Erysiphe, sooty mold, caused by fungi (commonly Capnodium, following aphid and scale infestation); false smut, caused by a fungus (Graphiola).

C. Systemic diseases, which affect the whole plant: Mosaic, ring spot, spotted wilt, and yellows, caused by viruses; chlorosis, caused by nutritional deficiencies.

THE IMPORTANCE of the division into soil-borne diseases and airborne diseases lies in the fact that the former are controllable mainly or only by soil disinfection, either by heat or by chemicals. As a rule those diseases are less influenced by environmental factors, such as temperature and humidity, than are the diseases of foliage. If the soil is initially contaminated with the causal organisms or becomes contaminated from introducing infected plants, there is no lasting remedy other than a complete change of soil or soil disinfection.

The diseases of Group B, on the other hand, are influenced greatly by the environment. For the most part they attack house plants only when some environmental or cultural factor is at fault. For example, the shoot blight and bud rot caused by the gray mold, Botrytis, typically occurs only when plants are chilled or are wet frequently and not properly ventilated. Lack of light and excessive humidity, even if only transitory, also favor the disease, which may attack very suddenly. Exposure to moderately warm temperature, with better light and ventilation, and with less frequent or copious watering, will usually check it at once. Avoidance of wetting the leaves when watering, or making sure that the foliage dries promptly thereafter, by setting the plants in a well-lighted and ventilated place (but not in bright sunlight or in a cold draft) will avoid most leaf spots, whether caused by fungi, bacteria, or nematodes. In fact, from the standpoint of house plants, the one good feature of the dry atmosphere in our living rooms is that it gives these leaf diseases almost no opportunity to start.