Of all the things done to house plants, watering is probably the one cultural practice that causes the most problems. Watering is affected by the type of soil used, light exposure, temperature, and humidity. Many factors determine when a plant needs to be watered. Just as many environmental factors change with the seasons, so do the water needs of plants.
The two main aspects of watering to be considered are frequency of watering and amount of water applied. The watering frequency is simply how much time passes between waterings. The frequency will vary over the course of the year. Avoid watering on a fixed schedule such as every week or every 5 days. A fixed schedule does not necessarily give plants water when they need it. In fact, watering on a fixed schedule may mean plants are overwatered at one time of the year but underwatered at others. With few exceptions, plants should be watered when the soil feels dry to the touch. This means the frequency of watering will vary with the rate at which the soil dries out.
Apply enough water so some comes out the drainhole at the bottom of the pot. This flushes out salts which can lead to root injury. Do not let plants sit in excess water. It will be re-absorbed and the salts dissolved in the water will be re-absorbed. There is no easy way to tell when plants in undrained containers have had enough water.
The actual technique for watering is not difficult. The best way is to use a watering can with a long, narrow, spout. This allows the placement of water on to the soil. Watering plants with dense foliage will be more difficult if this type of watering can is not used. Try not to put the water on the leaves and crown. Occurrence of rot diseases is more likely if water is continually poured on the crown.
The water should not be very warm or very cold. Tepid water, or water near room temperature will probably be best.
Bottom watering is a practice where the plant is set in and absorbs water from a container filled with water. Plants regularly watered from the bottom should occasionally be watered from the top to get rid of excess salts in the soil. The bottom layer of soil becomes saturated when a plant continuously sits in water. Any roots growing in this saturated layer will die. The soil available for plant growth is thus reduced.
Some plants enter rest periods at some time of the year. Resting plants may use less water so are more likely to be overwatered. If a plant slows or stops growing in late fall or early winter, it may be entering a rest period. At the same time the furnace may start running and dry the air out, causing the soil to dry out more quickly. Concern about overwatering resting plants in winter must be tempered by consideration of the effect of lower house humidity.
Terms such as "constantly moist," "allow to dry between waterings," and "moderately moist" are often used. These terms can be confusing. They are not precise and can be interpreted in ways which result in plant injury. Few plants are able to grow in constantly soggy soil. Few plants can last long in soil which has dried out completely. Between these two extremes is plenty of room for watering mistakes. If cultural instructions say, "the soil should be constantly moist," or "the plant likes moisture," watch the soil closely. When the soil surface has barely begun to dry, water the plant. The plant able to grow, sitting in a constant supply of water, is the exception not the rule. If cultural instructions say, "the plant likes to dry between waterings," the soil surface should dry before the plant is watered. At no time should all of the soil in the pot become dry. Use watering terms as guides, not rules, as to how a plant should be watered.
Plants intolerant of a lot of moisture can be grown in well drained soil mixes. Plants requiring more soil moisture can be grown in soil mixes which hold more moisture.
Soil moisture meters can be used as a guide to determine when a plant needs water. These meters can be useful but can't replace knowledge about plant needs. The meters can give false readings in soils with excess salts. The readings may vary depending on how deep the probe is inserted into the soil. Spend time with the meter, using different readings and different probe depths, to determine which readings give best plant growth.
The main symptom of watering problems is wilting. Plant wilts if given too much or too little water. The only way to tell the difference is to check the soil. If it feels wet or soggy, the plant has been overwatered. Plants consistently overwatered often develop stem or root rots which causes the lower leaves to yellow and drop.
Water from a home water softener, used for a long period of time, will kill plants. This type of water softener often uses rock salt. The most common symptom is browning of the leaf margins.