Watering plants in containers

Watering plants in containers should be a very straightforward affair.  The fundamental principles of keeping the soil evenly moist whilst aerated should mean that indoor plants in almost every situation will thrive.

What do plants need?

The amount of water required to maintain an indoor plant is decided by a mix of factors including plant species, size, environment, state of health and, in some cases, the time of year.

Species

Water use varies considerably between species. Although differences in leaf area make some contribution, the physiological nature of the plant is more important. Cacti and succulents, for example, are adapted to live under arid conditions where water must be conserved. Their water requirements are minimal except during active growing and flowering periods. Spathiphyllum on the other hand, has its origins in the tropics where water is plentiful and its lush, expansive foliage can indulge in massive transpiration.

Size

Clearly the bigger the plant the more water it is likely to use for photosynthesis, transpiration and other metabolic processes. Larger plants are also likely to be in larger containers with a greater volume of growing medium. This means that more water will be needed to keep the soil evenly moist around the roots. Remember, a container with a diameter of 45 cm (18 inches) has almost twice the volume of a container that has a diameter of 35 cm (14 inches).

Environment

The most important environmental factor affecting water usage is light. Generally speaking, plants in bright light will require more water than those in poor light because they are more physiologically active. Humidity and temperature will also affect the rate of water use, but in most indoor situations, these conditions will be fairly uniform. It is worth remembering that evaporation from moist compost is similar to that from an expanse of open water and increases rapidly with temperature. This means a good top dressing will help conserve water as well as enhance the appearance of your plant display.

Plants are quite adaptable and they will slowly get used to their conditions as they grow. However, once they have adapted, they cannot tolerate wide fluctuations in their conditions. This means that consistency is the key to successful watering.

Watering consistently

The best way of ensuring consistency is to know when the soil is dry enough to be watered. Do not water when the soil is too wet, or when it has dried out so much that the plant is wilting.

Plants in containers obtain all of their water from the compost. Even where an irrigation system is fitted, the water that a plant uses is that which has been delivered to the soil around the roots. Irrigation systems only deliver water into the soil for the plant to use. If the soil around the plant's roots is dry, then the plant cannot get any water.

Conversely, if the soil is too wet, adding extra will only cause problems - lack of oxygen at the roots, decay, fungi and sciarid flies are just some of the problems that can occur in saturated soil.

Wherever possible, specify the use of subterranean irrigation systems.  These are a reliable and very cost-effective method of delivering water. Details of subterranean irrigation systems can be found by clicking here.  Top watering container plants should be avoided as it will cause compaction of the soil, wash away any top dressings and encourage pests such as sciarid flies (fungus gnats).