It is impossible to properly water a houseplant on an arbitrary, predetermined schedule, once a week, for example. Watering frequency varies depending on the size and growth rate of the plant, the nature of the potting soil, the type of container, and environmental conditions such as temperature and humidity.
Potting soil in plastic containers won’t dry out as fast as soil in unglazed clay containers (if you tend to over-water, clay would be a good choice). If the appearance of a plastic container is objectionable, it can be placed into a decorative container. This is known as double potting.
Use room temperature water for houseplants. Before watering a houseplant, use your finger to check the moisture status of the potting soil. For most houseplants — tropicals and flowering potted plants — water when the soil is dry one inch below the surface in a 6-inch pot, or 2 inches below the surface in a 10-inch pot. Then apply enough water to saturate the potting soil so some runs out of the drain holes in the bottom. Make sure it isn’t just running down the side where dry potting soil may have pulled away from the side of the pot.
Note: Flat bottom pots sitting on a flat surface may develop an "airlock" (vacuum) that seals off the bottom, preventing drainage. Choose pots with feet, or elevate to provide air space underneath.
After excess water has drained through, discard any that accumulates in the saucer under the pot. It is important to water to total saturation, with some excess draining through the pot, for two reasons. First, this ensures that the entire soil mass is moistened. Second, water draining from the bottom of the pot carries away dissolved salts from fertilizer application, a process called leaching. Accumulated soluble salts can contribute to root damage due to an osmotic effect.
— Scott Eckert is a Kansas State Research and Extension agent for Harvey County. Horticulture is his specialty. He can be reached at 316-284-6930.