Most pot plants grow best if they are potted into containers which are only slightly larger than the existing root system. Consequently it is only a matter of time before the roots have penetrated the compost and reached the inside of the pot. From then on the roots are forced to grow laterally between the compost and the pot with the result that their ability to take up both plant foods and water becomes restricted. The plant is now said to be pot-bound.
The first indication that a plant is becoming pot-bound is that the growth is slowed down and more frequent watering is required. Roots may also appear through the drainage holes at the base of the pot. This pot-bound diagnosis can be confirmed by removing the root ball from the pot for examination. You do this by spreading the fingers of one hand over the top of the compost and then inverting the pot before tapping its rim on the edge of a table. This should free the root ball and allow the pot to be lifted away. If there is a mass of roots on the surface of the root ball then the plant is clearly in need of being potted on into a larger pot. If, however, no surface roots are showing then the fault probably lies with the root system. This can be checked by carefully removing some of the compost when it is likely that browned and rotting roots will be seen. These should be removed before the plant is re-potted in a clean pot of the same size using fresh growing compost.
How to re-pot
First ensure that the root ball is moist but not waterlogged. Then put a shallow layer of fresh moistened compost in the base of a clean new pot. The root bail is then placed on top of this layer and a check made to see that the base of the plant stem is about 2cm (¾in) below the rim of the pot. If it is not, the depth of the basal layer of compost should be adjusted. Now fill in the gap between the root ball and the pot with more compost, gently firming this either with your thumbs or with a blunt stick.
When to re-pot
The best time to re-pot is in spring when active growth is beginning again after the winter rest period. Some houseplants benefit from annual re-potting whilst others need this treatment only every two or three years. A few types such as aspidistra, bromeliads, palms, Peperomia, Sansevieria and Epiphyllwn do best if left undisturbed for several years. Re-pot only if the plant growth begins to slow down.