Temperature and humidity

One of the most important skills that an interior landscaper possesses is the ability to match a plant to its indoor environment. The knowledge required to achieve this is often the result of several years experience coupled with the results of horticultural research.

In addition to aesthetic considerations, factors such as light and temperature need to be considered. The growth characteristics of the plant, its shape and size may also be very important.

For a plant to thrive indoors, it needs to be in a position where it can receive just the right combination of light, warmth, humidity, water and nutrition. The latter two are almost always provided directly by the people who look after the plants, but the other factors are harder to control.

When deciding which plant to use in an indoor location, it is helpful to understand a little about the conditions that the species experiences in the wild. Most indoor plants are tropical or subtropical in origin. This means that they are already quite well adapted to the temperatures found in most buildings. Another feature of the tropical parts of the world is the lack of seasonal variation compared with more temperate areas. This is important because the indoor environment of most buildings is also fairly constant throughout the year, which again means that tropical plants will be well adapted to conditions indoors.

Many indoor plants come from either the humid tropics or arid areas. The lush rain forests of South America and Asia are the natural homes of many of the most familiar indoor plants such as Ficus benjamina (weeping fig) and Spathiphyllum wallisii (peace lily). This gives a clue to their water requirements which are fairly high. Yucca elephantipes, on the other hand is from a very dry area, which means that the plant can thrive on much lower amounts of water.

Although the amounts of water and light available to a plant are the most important factors governing its survival indoors, a plant’s health and appearance may also be affected by the nature of the air around it. Extremes of temperature and humidity, or the presence of pollutant dust and fumes can be very damaging to the leaves and encourage pests, diseases and disorders


Temperature

In most buildings, the temperature is quite uniform and suitable for a wide range of plants. There are, however, a few situations where the temperature can be more extreme and care must be taken to install only plant species that will tolerate the conditions, or to avoid installing live plants altogether.

Too hot
Aside from cacti and succulents, most indoor plants find it difficult to cope with excessively hot conditions. The rate at which water transpires from the leaves may exceed the rate at which it can be taken in by the roots, even if the growing medium is very moist. This may cause the plant to wilt, lose leaves and suffer permanent cell damage. Such hot, dry conditions also favour the spread of plant pests, such as Two-spotted (red) spider mite, Tetranychus urticae

Particular care must be taken to avoid putting plants close to south-facing windows in summer. The sun is likely to scorch the leaves; the same is likely to occur if the plant is too close to a radiator or hot air intake.

Too cold
Aside from frost, which is lethal to most indoor plants, cold is generally tolerated better by plants than heat. Draughts and sudden changes in temperature can cause leaf drop, but provided the plant is given the opportunity to acclimatize and is not over-watered, it can survive surprisingly well in very chilly environments.

The point about acclimatization is an important one to remember when moving plants around a building. Suddenly moving a plant from one temperature to another e.g. a warm office to a draughty corridor or vice versa, is likely to shock it into losing a lot of leaves.


Humidity

Despite being an essential ingredient of photosynthesis, only 1-2% of the water entering a plant is used for that purpose. The remainder evaporates from the surface of the leaves by transpiration, which increases with light levels and temperature. It also varies with humidity, which can be quite variable in buildings.

Diffusion of water from the moisture-laden air spaces of the leaf to the outside goes on rather slowly when the surrounding air is humid. However, when the surrounding air is dry, diffusion and thus the transpiration rate are increased. In extreme cases the leaves may wilt, because the roots cannot take in enough water to keep pace with the rate of water loss. Other problems associated with low humidity are brown leaf edges and tips, and a higher incidence of pests such as Two-spotted (red) spider mite, Tetranychus urticae.

Regular misting helps minimise the problems associated with low humidity.


Pollutants

Plants may be damaged if their environment contains excessive amounts of dust, fumes or another pollutants. In particular:

• Dust on the upper surface of the leaves reduces the amount of light reaching the plant

• Grease and grime may block the breathing pores (stomata), which allow air and moisture transfer in the leaves

• Chemicals and chemical vapours may cause leaf burn similar to that induced by excessive heat.

All of the above factors should be borne in mind when planning building refurbishment. Like people, plants can be very sensitive to their surroundings and need the same consideration when making changes.

Author: Kenneth Freeman

Additional material: Mike Lothian