Unfortunately there is no hard and fast rule about how often to water a houseplant. The amount of water each plant requires for optimal growth depends on several factors, including plant species and structure; environmental conditions such as light, temperature, humidity and air movement; composition of the potting mix; and the type of container used.
There are several ways to determine when a plant needs water – the weight of the container, the feel of the soil, and soil color can indicate how much moisture is available for the plant’s use. For plants that require even moisture and do not tolerate dry roots, watering should occur when the top layer of soil begins to feel dry. For plants that prefer a slight drying of the root system between waterings, it’s best to feel the soil below the top layer on a regular basis and water before the soil becomes completely dry or any wilting occurs.
The most convenient and efficient way to irrigate houseplants is to place a saucer under the plant to catch any excess then pour water on the soil surface. Completely soak the surface of the soil and watch for excess to drain into the saucer. After watering, check the soil below the surface to be sure it is evenly moist. If the soil is overly dry and has pulled away from the side of the pot, water may run down along the side of the pot and not soak into the soil.
Do not allow plants to stand in water as most plants do not tolerate waterlogged soil conditions. When the soil is thoroughly moistened, discard any excess water that has collected in the tray.
Generally speaking, it is better to water plants from the top than it is to subirrigate, or water from the bottom. Subirrigation is the process of watering plants by placing pots in a shallow amount of water in a tray or saucer, and allowing the soil to draw water up from the bottom via a wicking action. Subirrigation can lead to an accumulation of excess soluble salts in and on the surface of the soil. High salt levels inhibit plant growth by reducing water uptake, restricting root growth, burning foliage, limiting seed germination, and inhibiting flowering. If you choose to water from the bottom it is recommended to periodically water from the top down to leach soluble salts from the potting soil.
For more information, see the following Planttalk Colorado™ video(s).