The second most important aspect of plant care (after light) is watering, though it's by far the question we get asked the most. "How often should I water my plant?" Though of course it's essential to water your plants regularly, there are other factors to consider in addition to your "watering schedule."
How often should I water my plant?
A better question to ask is "How often should I check my plant for watering?" For most plants, it's best to check on them at least once or twice a week. Using a soil probe (if you don't have an actual soil probe, a chopstick or a finger are decent substitutes), determine how far down the soil has dried out. With plants that are still in the plastic nursery pot, it's easiest to test the moisture level of the soil by checking the weight.
Most houseplants should become moderately dry between waterings, 25 or 50 percent of the way down from the surface. The amount of time this takes will change throughout the year, as the hours of daylight fluctuate and plants go through their growth cycles, so watering all of your plants every Saturday morning all year long, for example, might not be the best strategy.
There are exceptions, though: carnivorous plants, for example, should always be completely wet. On the opposite end of the specturm, most cacti and succulent varieties should dry out almost completely before getting watered.
Most plants will begin to droop or wilt if they dry out too thoroughly. They can usually bounce back after a good watering, generally within 2 to 24 hours, but it's best to not let this happen too often.
How much water should I use?
Instead of thinking about a specific volume of water, it's better to fill up your watering can and pour slowly all over the soil surface until the water begins to come out through the drain holes into the saucer. This way you can be sure that even the lowest roots have gotten a drink.
If your plant is potted directly into a container that doesn't have a drain hole (not recommended), it's best to wait until you're absolutely sure that the soil has dried out sufficiently, and then measure out water equal to about 1/4 the total volume of the container.
What kind of water should I use?
Though it's not entirely practical for us city dwellers, collecting rainwater for your plants is the best solution, due to its natural nitrate content. If you're not able to collect rainwater, but do have some outdoor space, putting your plants outside during the rain if they need water is also a great way to clean any dust off of their leaves. (Just make sure your containers have drain holes!)
Second best is filtered (to get rid of the magnesium, calcium, fluoride, and sodium) tap water or bottled spring water, which still has some beneficial minerals (though this can get to be expensive). You can also dissipate the minerals in tap water by filling up a watering can and allowing it to sit out for 24-48 hours.
Distilled water can also be used, but doesn't have any beneficial minerals, so you might notice your plants growing a little slowly.
Whatever kind of water you use, it's best for it be around room temperature.
Watering from below
An alternative method to using a watering can from above is to water your plants from below, as long as the container has a drain hole. This is especially useful if the soil has pulled away from the sides of the pot.
Fill up a deep saucer or bowl with water and place your planter inside it for 15-30 minutes. You should notice the level of the water has gone down, and ideally the surface of the soil will feel moist.
It's best to only use the bottom watering method every once in awhile, as regular top watering pushes excess salt and minerals through the soil that may build up from regular tap water.
It's easier for most plants to recover from a little drought now and then than it is for them to bounce back from over-watering, so always check the plant before blindly watering it.
It's also a good idea to occasionally check the drain holes in your pots to make sure they have not become obstructed.
Occasionally the soil may pull away from the sides of the pot, causing the water to quickly flow down the sides and directly into the saucer. This is usually due to the soil drying out too frequently, or if the plant has become rootbound. If you're not ready to repot, you can fill in the gaps around the sides with new soil. Also, see "Watering from below" above.