As with most aspects of caring for indoor plant life, watering houseplants is a matter of understanding the plants you are dealing with.
To begin, however, there are a few basic principles.
The first is that very few plants tolerate living in standing water, and those that do typically live in wetlands and not in homes. This is not owing to their appearance, as there are many gorgeous wetland plants, but more to the fact that standing water combined with mud has an odor. This odor signals an anaerobic (non-oxygenated) soil environment in which specialized bacteria and fungi grow, many of which are pathogenic to non-wetland plants. Therefore with standing water, it not the excess of water that immediately kills the plant, but rather it’s the conditions standing water creates for other organisms that can penetrate and rot all but the most specialized of roots.
This is a long-winded way of saying: provide your indoor plants with drainage. Most houseplant roots need oxygen and much as they need soil and water, as this helps keep the right balance of soil bacteria and fungi.
The second houseplant watering basic is always check on or keep a mental note of how often you water your plants. Sure, it is fine to water them once they get sad and wilted, but they will be much more lush if they never get so water stressed. A good way to check the moisture in the soil is by probing the soil with a finger an inch below the surface. If it is still wet, most plants don’t need water. If it is dry, everything but a cactus or succulent probably wouldn’t mind a drink.
More high-tech solutions like moisture meters and monitors are available for the plugged-in indoor gardener, but for most, simple mental notes and checks can suffice.
The third principle is very intuitive: watch your plants for wilt or other signs of water stress. If you don’t catch a too-dry or too-wet plant before, it will certainly let you know there is a problem.
Watering rules for different kinds of plants
Paying attention to the natural environment from which a plant comes and the shape of its parts (like roots and leaves) can go a long ways toward predicting what kind of watering regimen will suit it best.
Firstly, the environment in which it evolved: a plant that comes from the forest floor of a rainforest will typically be water-demanding, whereas a plant that comes from a desert would rot with the same watering regimen. This seems like common sense, but one of the most common ways to kill a houseplant beyond complete and utter neglect is by over-watering.
Secondly, plant morphology can provide some cues for optimal regimens for watering houseplants.
Plants with thick, water-rich leaves (succulents) generally store their own supply of moisture for a much longer period than broadleaf plants. Much of this storage capacity has to do with transpiration, which is the process by which plants move water from the ground via roots, and out through holes in the leaves called stomata. Cacti have evolved tiny, protective, non-photosynthetic leaves (spines) that don’t allow water to escape, whereas broadleaf plants vent water over a much larger surface area.
Similarly, plants with a long taproot or extensive root network have often evolved in drier climates, where they must cast a wider or deeper net for moisture. These need less water than shallow-rooted plants.
Beyond a watering can, there are some low-maintenance methods that can provide more constant soil moisture to non-succulent houseplants that will tolerate it.
Most of these involve an action called “wicking,” or capillary action which is the process by which water moves through narrow spaces in all directions, regardless of gravity.
So-called “self-watering” planters are planters with a water reservoir in the bottom, and a wick or channel to the soil above. These planters can be filled with water on a much more infrequent basis, and the plant uses water as it is needed.
External wick systems can also be created with a reservoir beside the plant and a nit of narrow tubing. These systems work best if the plant is situated below the water reservoir.