Although it is well known that central heating dries out the atmosphere of a room it is not always appreciated that plants generally prefer more humid conditions. Houseplants whose natural habitat is a steamy tropical jungle are most affected, but even plants from more temperate regions suffer to some extent. Indeed the only plants which thrive in this unnaturally dry air are desert plants such as cacti and succulents.
PLANT REACTIONS TO LOW HUMIDITY
In a dry atmosphere the rate of water loss by transpiration is greatly increased. Consequently the leaves tend to dry out and begin to wilt. This effect is most pronounced in plants with thin papery leaves. With these the first sign of damage is that the leaves become brown and shrivelled. More resistant plants react by temporary wilting of the foliage. Continuous exposure to dry air induces early leaf fall and can eventually cause complete defoliation. Flowering plants generally suffer badly if they are kept in low humidity. The flowers soon fade while the flower buds dry up and fall without opening. Plants with leathery leaves suchas Sansevieria and succulents are least affected and survive quite happily under these conditions.
METHODS OF INCREASING HUMIDITY
One partial solution to this problem is to display sensitive plants in the kitchen or bathroom where the air is usually moister than in the rest of the house. Another approach is to use humidifiers to increase the general level of humidity in the house. This is rather a drastic step to take, so it is better to use one of the following methods of increasing the humidity in the immediate surroundings of the plants.
This involves sinking the plant pot in moist peat in a larger waterproof display pot. All you need do then is to keep this peat continually moistened in order to ensure that the air around your houseplant remains moist. An added bonus is that the moist peat provides a reserve water supply for the plant and also insulates the growing compost from an overnight drop in the temperature.
These are shallow plastic trays which are designed to hold a thin layer of moist gravel on which the pots of houseplants can be placed. Plants grouped on gravel trays thus benefit from the humidiiing effect on the moist gravel. Watering of the plants is also simplified since any surplus water can be allowed to drain into the gravel.
An attractive alternative to the use of gravel trays is to group the plants in deeper decorative containers and pack moist peat around the individual pots. It is of course essential that the container is waterproof. These large containers can also be used for multiple plantings of houseplants in growing compost. There are, however, some disadvantages associated with multiple planting. Firstly, it is not possible to water according to the individual needs of the various plants. Secondly, since drainage holes are not provided, there is a risk of the compost becoming waterlogged and so affecting root growth.
An even more effective way of ensuring constant high humidity around the plants is to grow them in a large bottle or fish tank. This method has the additional advantage that watering requirements are minimized. It is, however, only suitable for the display of rather small, slow-growing plants. More information on this technique is given on page 225.
This treatment is of particular value as a way of increasing the humidity of the air surrounding single plants, but is also suitable for use with groups of plants. All you do is lightly mist the foliage and surrounding air with tepid water using a small hand sprayer with the nozzle adjusted so as to give as fine a spray as possible. Misting is best done early in the day when the plants are not exposed to bright direct sunlight. Late evening applications should be avoided as these provide conditions which favour the development of grey mould. Most plants benefit from being misted, but with flowering plants such as Achimenes, begonia, calceolaria, cineraria, cyclamen and geranium it is important to restrict the misting to the surrounding air and to avoid wetting the foliage and flowers. Cacti and succulents are the only plants which must not be misted since they are liable to develop rot in moist air.
Remember to watch out for your wallpaper and furnishings when misting. In a bathroom the plants can simply be placed in the bath for their daily syringe, but in other rooms it may involve removing them to a more suitable place. Local humidity will be increased if plants are misted together.