We grow rather a lot of pot plants and have to treat them in a variety of ways to manage root growth. First group of plants are those we treat like bonsai eg. the Dessert Roses (Adenium sp.), Cyphostemma and other plants generally considered pachycaul or caudiciform plants; these do not require anything more than to be left to themselves. They do not mind being 'pot bound'.
The second group are plants such as Haworthias which make large succulent roots. These too, don't mind being pot bound. In this they are like agapanthus and cliveas but whereas it wuld be asking for trouble to cut off the roots of Haworthias (unless you are trying to ropagate sp. such as H. truncata) the much larger agggies and cliveas can be severely reduced with a sharp spade or serrated-edge bread knife.
Citrus and camellias we grow in pots too, along with bay trees, oleanders, olives, Paliurus spina-christi, brugmansias and brunsfelsia. These do need root pruning when they are re-potted. At the same time top growth is also cut back by about half to one third tho' some can be cut more without harm. Citrus are treated lightly, oleanders the hardest. As for root pruning we tip the plants out of their pots and slice off the complete root mass at the bottom, usually around an inch or so; then we shave the sides of the rootball by about half an inch, and then we plunge in a sharp knife (we keep an old serrated bread knife for the purpose) and make four downward cuts at regular intervals. The potting mix/ soil should be on the dry side when this is done. We then repack the bottom of the pot with an inch of new potting mix to which we add extra slow-release fertiliser granules. Next the plant goes back in the pot and the space around the perimeter of the root-ball is topped up with fresh potting mix, also with extra fertiliser and the lot gently tamped down - gently mind. Finally we leave the plants for a few days in a light but sheltered spot before drenching them with water and returning them to their positions in the garden. We do this to ensure severed roots have a chance to heal before the potting soil is made wet, which could run the risk of rots developing at the raw cuts. tn