Do any of your houseplants wilt between normal waterings? Do the roots protrude from drainage holes? Has there been little or no new growth? Are there white salts on the soil surface? If you answered "yes" to any of these questions, it's probably time to repot.
Generally speaking, young and fast growing plants will need repotting every six months to a year, older ones every few years. The best time to repot is at the start of the growing season, usually early spring.
You can tell when a plant needs repotting by knocking the soil ball out of the pot and checking the roots. To do this, invert the plant, and gently tap the pot edge on the table to loosen the soil. If the roots are exposed with little soil covering them, it's time to repot.
Before repotting, check the roots to see if any are diseased. Good, healthy roots will be firm and white with many tiny root hairs covering them. If only a few roots appear diseased, the problem may be corrected with fungicide. Discard any heavily diseased plants.
For repotting, you will need good quality potting soil (available at garden stores), peat moss or organic matter, and pots one to two inches larger in diameter than what you are using now. Adding one-third peat moss or composted organic matter to purchased potting soil will ensure good aeration and water holding capacity.
When repotting, keep the old soil at the same level in the new pot. Don't bury the stem base. Firm the new soil around the old soil ball, being careful not to pack it down too tightly. Allow at least one-half inch space from soil to rim to facilitate watering.
Finally, don't forget to fertilize, following instructions on the label. Water well, but don't overwater.
If a plant is too large to repot, topdress it every few years. To topdress,
scoop out the top two or three inches of soil, taking care not to disturb
the roots. Refill the pot to its original soil level using a fresh potting
soil and peat mixture.