Drainage is the ability of soil to retain or lose moisture, which is essential to the well-being of potted house plants and outdoor Garden plants alike. The wide range of plants from Cactus to Water Lily demand an equally wide range of drainage.
Most plants depend on moisture to carry dissolved nutrients to their roots. But too much moisture may plug root pores and cause roots to "drown". Roots need to breathe air much as humans do, making adequate drainage a must.
Some of the elements affecting drainage are soil mass (amount of soil), soil composition (sand or sawdust), Soil retainers (porous or non-porous), Sun, temperature, wind, mulch, compaction, and insect populations.
Soil Mass may be limited to the soil found in a small planter or all the soil composing Planet Earth. The smaller the mass, the quicker the moisture will be absorbed and evaporateed. So small pots need to be watered more often than larger pots.
Given occasional rains, gardens may not require any additional watering from the Gardener. But elevated beds will reqire more than those plantings that are level with the surrounding soil. And gardens that are "sunken", or at a lower level than the surrounding soil may require much less watering. If you have such a Sunken Garden, provide an escape path for standing water, or you may find yourself with a Frog Pond.
Soil Composition can vary from sand which retains very little moisture and requires frequent watering, to loam and peat which is said to absorb as much as thirty times its weight in water. Choose yours based on your plant's preferences.
Retainers can be Pots or Borders in Flower Gardens. Plastic Pots are usually impervious to water. Terra-cotta Pots are porous when new, but tend to clog as they collect salts from the soil and change to a blotchy whitish color. Their color also changes as they absorb moisture. This color change may tell the trained eye when a particular plant needs water. Allow your Plant to choose the Pot you use.
Some writers claim that drainage in pots is aided by the addition of stones in the bottom. But you may reason that any water which has not been restricted by the soil above, will neither be restricted by similar soil below. With this thought in mind; a paper towel placed in the bottom of the pot to cover all drainage holes may be a better choice, since it will act to keep soil in the pot while allowing excess water to drain out.
The border of a