As basic as water is to a plant's survival, it can also cause problems when a plant sits in excess water. Plants that sit in water are susceptible to root rot and moldy soil. Providing a plant with drainage underneath the pot can prevent these problems. There are a few ways to provide drainage, depending on the type of pot or planter you use.
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Double Pot Drainage
The double pot method of drainage can ensure that your plants do not sit in water once they have soaked up the amount of moisture they require. The plant should be placed in a pot that is large enough to hold the roots in potting soil. That pot should be placed into a larger pot without drainage, over an inch or 2 of non-porous material such as gravel, marbles or pebbles.
Single Pot Drainage
For single pots that have no drainage holes, non-porous materials can provide a barrier between sitting water at the bottom of a pot and the roots themselves. Marbles, gravel or pebbles can act as the barrier. These materials will not hold water--rather the water will run past them and sit at the bottom of the pot, away from the soil and roots. To use the gravel in a single pot without drainage, pour a 2-inch layer of gravel into the bottom of the pot before adding topsoil.
Built-In Drainage Trays
Some ceramic and clay pots are fused to a drainage tray. They have small molded or drilled holes around the bottom of the sides of the pots. These holes expel the excess water, where it is held in the tray. This can help to keep excess water away from the roots and soil, but the tray needs to be emptied regularly or the soil will soak the water back into the pot. The built-in trays are not removable, so the pot will need to be tilted over a sink or outdoors to get rid of the excess water.
Separate Drainage Trays
Pots that have drainage holes without trays can be placed on separate trays that can be removed to pour off excess water. These trays can be purchased at many garden supply centers or home improvement stores, but you can also use anything flat with a lip around it. Large plates, dinner trays or pie tins can be used for larger plants, where saucers and bowls can be used for small houseplants.