A hole at the bottom of the container is critical. It allows water in the soil to drain freely so adequate air is available for the roots. While various kinds of plants have differing drainage needs, few can tolerate sitting in stagnate water. Healthy roots mean healthier plants. So be sure there are holes for drainage. Shoreline plants love wet soil, so if the pot does not drain, consider using them. See the water gardening section for more information.
Wet soils favor root rot, because they leave little space for air to get to the roots. Plants rarely recover from root rot. More information on root rot:
If the pot does not come with a hole in it, figure out a way to make a hole. One way is to drill one. Some decorative resin or plastic pots have pre-punched holes at the bottom for easy removal. Generally, very little soil falls through the hole. To keep soil from falling through large drainage holes some folks use a coffee filter paper over the hole, though this is not necessary. Small holes in the bottom of the pot allow the water to drain out and very little soil media is lost.
Sometimes a plant is already planted in a pot with no drainage at all. The best solution in this case is to take it to a sink, water it, then after a few minutes turn it on its side for a minute or two to let excess water drain out.
Beware of pots with permanently attached saucers where emptying the overflow is difficult or impossible. I learned this the hard way when my basil leaves turned black because the roots were waterlogged - the result of a permanently attached saucer. Drainage of excess water is vital for the health of plant roots. It is much better to use a pot with a detachable saucer. Be sure to empty the excess water so soils have an opportunity to dry out somewhat.
Slipping a container inside a slightly larger one is called double potting. When double potting is used, the plants grow in a pot liner. Often this is a plain plastic pot. This allows you to slip the pot liner in or out of the decorative container without disturbing the plant roots. Check to be sure that the plants in the pot liner never stand in water (unless you have aquatic plants) If water accumulates in the bottom of the larger container, remove the inside pot and drain the water from the outside pot. Place gravel in the bottom of the outer pot if the decorative pot is deep enough. A little excess water can accumulate in the gravel without the plant roots having to stand in water.
Double potting techniques may be used:
It is a myth that a layer of gravel (inside the bottom of an individual pot) beneath the soil improves container drainage. Instead of extra water draining immediately into the gravel, the water "perches" or gathers in the soil just above the gravel. The water gathers until no air space is left. Once all the available soil air space fills up, then excess water drains into the gravel below. So gravel in the bottom does little to keep soil above it from being saturated by overwatering.
Damp gravel placed in a saucer underneath the pot may help by increasing the humidity in the immediate area of the plants as the water evaporates from the gravel surfaces.
More info about gravel in bottom of a pot can be found at: http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Container%20drainage.pdf
Self-watering pots use various methods to effectively draw water from a bottom reservoir into the soil without causing the soil to become too wet. Water may be drawn up into the soil by capillary action (or wicking) through small soil columns, rope wicks, or the use of moisture sensors. Self-watering containers are especially useful for weekend cottages and people who do not have time to check water needs daily.
Consistently available water is great for vegetables, and tropical houseplants. Imagine a dozen stalks of sweet corn producing ears on your patio in a self -watering container like the Earth Box™. Plants that need to dry out like thick-leaved cactus and succulents do not usually warrant the extra cost of a self-watering container.
For more information, use a web search engine using the keywords "self-watering planter" or "self-watering container" or "earth box."
Decorative pots and wraps without drainage holes. Sometimes you fall in love with a wonderful container that has no drainage hole. This would be the perfect time to find a pot liner to fit inside so both you and the plants are happy. In addition to decorative pots, decorative foil or plastic pot wraps are a form of double potting. The wrap keeps water from leaking out where it is not wanted. To protect the plant from becoming prone to root rot, pierce a hole in the bottom of the wrapper or foil. Then place the container on a saucer. Or, take the container to a sink, remove the wrapper, and then water. Let the water drain freely out the holes in the bottom of the pot. After the pot finishes draining, replace the wrapper.
Changing seasonal displays. Double potting makes changing out seasonal plants a breeze. Fresh plants are easily rotated in and tired ones out of a large landscape container holding multiple plants. Double potting makes it possible to sink individual potted plants into the landscape (or remove them) without disturbing the roots.
Combining plants with differing requirements. Plants that have different soil drainage (aeration) requirements can be combined in a landscape planter if they are in their own separate pots. This allows different watering practices for each plant. On the other hand, if there is not enough light, two sets of plants can be used. As individual plants begin to decline, rotate them back to a higher-light nursery. Replace them with healthy plants. It is less work to choose plants with similar environmental needs or ones well adapted to conditions available. However, this is not always possible.
Reducing fluctuation of soil temperature. Roots are generally more susceptible to cold damage than stems and leaves. When overwintering an otherwise hardy plant, sink the potted plant into the ground. Then mulch over the soil with wood chips, soil or other material to help it survive winter. The soil temperature underground does not fluctuate like container soil which is exposed to wind and extremes of temperature.
Restraining invasive plants. Some plants are attractive, but spread invasively by underground roots. They invade space that does not belong to them. For example, most people love mints for their fragrance and culinary uses, but they can spread aggressively in the garden. Sink a large pot into the ground. Then place the invasive mint in a slightly smaller pot liner. This limits the spread of the roots.