As noted above, the ability of the growing medium to drain off excess water is critical. More interior plant deaths result from overwatering than from any other reason. All planters must permit the removal of standing water. The layer of coarse gravel already referred to is one method. Other methods include setting containerized plants on top of inverted pots within the larger planter and incorporating drains and spigots into planter bottoms. Where drainage from the planter base is planned, additional planning must ensure that carpets, tiles, and other floor surfacings are not damaged by the runoff water. Sitting the plants on gravel beds into which the water can drain and then evaporate is one method.
Hand-in-hand with planning for drainage goes provision for proper watering. Some interior plantings require continuous moisture; others do better if permitted to dry out between regular waterings. Obviously, the two types of plants would not coexist compatibly in the same planter.
An interior plantscape must be watered according to schedule, not according to the judgement of a custodian or other unqualified individual. The right watering frequency is most easily determined in controlled environments such as enclosed shopping centers. It is most difficult in locations where environmental variables are not stable, such as by open windows or in drafts near doors.
The need to water a planting can be determined by feeling the soil and observing its color. A gray surface color and failure of soil particles to adhere to the fingers indicate dryness. Moisture meters are also available for a more carefully controlled reading of the growing medium’s water content.
Although some automated watering systems exist, there is a surprisingly limited use of them in large installations. Nationwide, there are interior plantscapes utilizing hundreds of plants that are all watered by hand. Most definitely, the technology of watering is still in the developmental stages.
When water is applied, it must be in a quantity adequate to wet the soil deeply, not shallowly. Shallow watering encourages shallow rooting and increases the vulnerability of the plants to damage from drying. Deep watering promotes deep and healthy rooting while providing the soil leaching necessary to prevent soluble salt buildup.
The quality of water used on the interior planting may vary with the location. The most likely source will be municipal water lines. Most public drinking water contains chlorine and often fluoride as germicides and tooth-decay deterrents respectively. Neither additive will harm plants under normal conditions. Although chlorine is potentially harmful, the amounts used in drinking water are dissipated by aeration as the water bubbles from the faucet or hose nozzle. More heavily chlorinated swimming pool or fountain water can damage plants and should never be used as a watering source. NOTE: Plants grown around enclosed pools require good air exchange in the room or the chlorine gas from the pool may damage them.
Water is a source of soluble salts. Water that has been softened by means of cation-exchange softeners may be dangerously high in sodium that can be toxic to plants. In buildings with such water softeners, alternate sources of water should be sought for the plantings. In regions of the country where the need for water conservation causes recycled water to be used on interior plantings, a chemical analysis of the water should be made to determine if any toxic chemicals are present that could damage the planting.