How often begonias require water depends on many factors, including the season, the potting medium used, the plant's location (indoors or outdoors) and the color of the soil, according to the American Begonia Society
Each season has varying levels of light, humidity, wind and temperature. Generally, more water is needed when the temperature is high, humidity is low and the plant is exposed to sunlight for lengthy periods. Begonias in porous pots such as clay require more water. Begonias grouped together require less water because they produce more humidity. Dark soil is moister than light soil and requires less water. Glossy or soft and rigid leaves indicate the need for more water.