Don showed a pelargonium that had outgrown the available space and nourishment in its pot. He explained why it is necessary to repot plants every few years, and demonstrated potting techniques using a large yucca (Yucca elephantipes).
Repot any plants that have been in the same pot for two years or more. This is necessary even if the plant has not outgrown its pot, because the potting mix degenerates (it shrinks, becomes water repellent and runs out of fertiliser) and needs renewing. Never buy a cheap potting mix. Instead, buy the best quality mix you can afford. (Tip: quality potting mixes display the Standards Australia ‘ticks’ on the pack.) Potting mixes can contain dangerous micro-organisms, so read the safety directions on the label before opening, and take care not to inhale the air that comes out of the bag. Either repot using the same container, as Don did with the Yucca, or put the plant in a pot about 50-100mm (2-4″) larger in diameter, with extra potting mix. If the plant is very root-bound, cut off about 25-50mm (1-2″) of solid roots from the bottom and perhaps make two or three vertical slits 10mm (1/2″) deep in the root ball with a knife. Then place the plant in a larger pot. Don did not do this with the yucca, even though it was root-bound. He felt that this tough plant would survive very well in the same pot for a few more years. Partially fill the pot with potting mix so the plant sits at the correct level. Centre the plant in the pot. Fill around the root ball with more mix, taking care not to build the soil up around the trunk. It is not necessary to pack the potting mix firmly around the roots, in fact this could impede drainage. Instead, simply lift the pot a couple of centimetres off the ground and give it a sharp tap. This will settle the potting mix and remove air pockets. Water well after repotting. Good quality potting mixes contain fertiliser, but at this stage it is okay to add a slow-release fertiliser, such as Osmocote for pots. After repotting, particularly if you have trimmed the roots, the plant should be placed in a semi-shaded area for a few weeks to re-establish. After that it can be put back in full sun or wherever it is happiest.