Dr. Leonard Perry, Horticulture Professor
University of Vermont

Of all the culture given to houseplants, watering is probably the one cultural practice that causes the most problems—more often overwatering— and where inexperienced gardeners can go wrong.  It really isn’t that difficult or rocket science once you consider environmental factors, and the individual plant needs.
The type of soil used as a potting medium affects watering, as does light exposure, temperature, and humidity.  Just as many environmental factors change with the seasons, so do the water needs of plants.  Plants in a warm room, particularly near a wood stove or with forced-air heat, will dry out sooner than those without these conditions.  Hanging plants often dry out more quickly than those in pots on a table. If the air is dry, placing plants on a tray of pebbles kept moist will help them retain moisture longer.  Using a room humidifier nearby helps too. 
Watch the weather too, and try not to water plants by windows when it is very cloudy outside, or forecast to be cloudy and rainy.  They won’t get sun to help dry them out, and so may stay too wet for too long. 
A good potting soil will have good aeration and water drainage, yet will hold onto some water as well.  These often contain a large amount of peat moss.  Some of the newer “organic” mixes drain very well, but hold little water.  Plants in these mixes will need lots more watering, or repotting into a peat moss-based medium.  At least with soil media that drain exceptionally well, there is less chance to overwater!
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 five 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 under-watered at other times. It is a good idea to get on a fixed schedule to check them for water, once you know your plants and how fast they dry out. 
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.  Especially if you’re new to growing houseplants, poke your finger an inch into the soil to make sure it is dry below the surface too.  There are inexpensive watering gauges you can find at many hardware and home stores that help you learn when a plant is dry.  Some of the more elaborate ones even include a computer chip and database of common plants, integrating a specific plant water needs with the current soil moisture.
If you determine a potted plant is dry, lift it to feel the weight.  Doing this with various pots you’ll soon develop a feel for when to water, just by the weight of the pot.
Apply enough water so some comes out the drain hole at the bottom of the pot. This flushes out salts that can lead to root injury, and ensures you are not merely watering the surface of the soil. Do not let plants sit in excess water, though. It will be reabsorbed and, thus, the salts dissolved in the water will be reabsorbed. Plus, the plants will stay too wet, leading to root rots.     

If you have a saucer under the pot, make sure after a short time (half hour or so) to empty the water out.  This allows water to be absorbed from the bottom—especially important if the soil is really dry.  If you have a pot within a more decorative pot—the latter without a drainage hole—make sure the outer pot doesn’t fill with water. 
The best way to water is to use a watering can with a long, narrow, spout. This allows the placement of water directly on 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, as rot diseases are more likely to occur if water is continually poured on the crown.
For watering, tepid water, or water near room temperature is best. This is especially important during the cool season and winter when water may come out of faucets icy cold.  This can shock roots, particularly of tropical plants, and lead to root disease and death.
Bottom watering is a practice where the plant is set in and absorbs water from a saucer or 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.  As already described, make sure plants don’t continually sit in water—just for a short time until some is absorbed by the soil.
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.
Some plants naturally require less water.  These include cacti and succulents—those plants with thick and fleshy leaves, ponytail palm, Chinese evergreen, and snake plant.  I have some “indicator” plants that I watch, such as the peace lily, which tell me it’s time to water when they start to wilt. 
The best advice is that if in doubt about whether to water or not, don’t.  It is better for plants to be a bit dry, than too wet.

Links for watering succulents

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