Over-watering and under-watering account for a large percentage of indooor plant losses. One common question gardeners ask is, “How often should I water my plants?” There is no simple answer. Differences in species, water-use rates, potting media, and environment influence water needs. Two important rules of proper watering technique are: Never permit the soil to dry out completely between watering; and never allow plants to stand in water for an extended time. Roots may die in both situations. As a general rule, a plant needs water when the top 1 inch (2.5 cm) of soil is dry in pots less than 6 inches (15 cm) in diameter and the top 2 inches (5 cm) are dry in larger pots. Insert your index finger to the 1- or 2-inch (2.5- or 5-cm) depth and feel the soil to check for moisture. If the soil feels damp, do not water.
Always water until a little water runs out of the bottom of the pot. This technique serves two purposes. First, it washes all the excess salts (fertilizer residue) from the soil. Second, it guarantees that the bottom two-thirds of the pot, which contains most of the roots, receives sufficient water. Do not let the pot stand more than several minutes in the water that has run out. Empty the saucer.
Water quality is not usually a problem when ordinary tap water is used on indoor plants. However, water that is artificially softened should not be used to water house plants.
When testing, pay attention to the soil condition. If your index finger cannot penetrate 1 to 2 inches (2.5 to 5 cm) deep, you need a more porous soil mix, or the plant is becoming root-bound.
Light and water guidelines for selected foliage and flowering plants, Table 11.1 (pdf) in California Master Gardener Handbook. UC. Buy Publication
Plants Indoors: Selections for various environmental conditions. UC. Buy leaflet