Actively growing houseplants need repotting from time to time. The frequency of repotting depends on the growth rate of the particular plant. Fast-growing plants may need repotting annually, while slow-growing plants may require repotting every two to three years. Some plants (e.g., amaryllis) thrive when potbound.
When new growth starts in the spring, turn the potted plant upside down and remove the plant, if possible. A plant can be removed easily from its pot if the lip of the upside down container is knocked against any solid object. Hold your hand over the soil, straddling the plant between the fore- and middle-fingers, holding back the soil, and gently knock the rootball out of its present container. If roots are in a solid mass around the rootball, the plant needs to be repotted. A plant also needs to be repotted if roots are growing out of the drainage hole, or if the plant requires frequent watering or wilts shortly after watering.
Repot only as needed during spring and summer while the plant is actively growing. Do not repot ailing or dormant plants or those beginning to flower.
Remove the plant from its pot and gently disturb the root system so that roots are not in a tight rootball. If the roots are too tight to loosen, score the rootball with a knife to loosen them.
Select a pot that is 1 or 2 inches larger in diameter than the current pot. Place a small piece of broken clay pot over the drainage hole to prevent soil from washing out. Do not add a layer of clay pieces or rocks because this actually slows water movement through the pot. Place enough dry potting mix in the bottom of the pot so that the top of the rootball is within 1 inch of the container top. Do not add soil above the original level on the rootball. Fill around the rootball with mix. Do not pack the soil to firm or settle it; gently press the soil with your fingers. Water well so excess water drains out of the pot. Repeat many times, especially if the soil mix contains peat. Never pack wet media.
For plants in large containers that are impossible to repot, remove the top 2 or 3 inches of soil and replace this with fresh mix every other year or so. This is called topdressing.