Repotting is one of the most important growing tasks for good development and growth. You can always improve the “natural” growing conditions (light, temperature, watering) by moving the plant, but regularly giving the roots more space and nutrients is one of the keys to success.
Regularity above all
This should be done every year, or even twice a year if the plant grows very quickly or if you grow in a greenhouse or conservatory. Large plants (over 5 years old) only need repotting every two years, alternating with top-dressing. However, growth may have slowed, and repotting may turn out to be unnecessary. When you take your begonia out of its pot, you should see a fine root ball, even all round, and nice and white. If not, it is best to wait.
Spring is the best time to do it, especially indoors. The begonia will then take advantage of the longer days and warmer temperatures to develop its various organs, roots included. If you need to repot a second time (in a greenhouse), October is the latest, before growth slows or flower induction begins.
No XXL
Begonias do not like pots that are too wide. They even like to feel a little cramped. There seems to be a paradox between this and the need for regular repotting, but both must be taken into account. Being herbaceous plants that grow in light soils, on rocks or at the base of tree trunks, under moss, begonias do not have a large root system. So do not drown them in containers that are too big: too much soil would hold an unnecessary amount of water, which is harmful. As a rule, choose a pot just one size up – from 12 cm to 14 cm in diameter, or, if the plant is already very large (a cane-like begonia, for example), 16 cm. In short, “one finger’s width on each side”. A little extra: since their roots do not go very deep, begonias prefer low pots or bowls to pots that are too deep.
Acid soil, but no more than that
Begonias are said to like acid soil, and you will usually choose a potting compost with a pH below 6. That said, lightness is what matters most. Once again, observing nature guides us. The very fine roots grow easily in soil made of humus and decaying leaves, under moss, or clinging almost in the open air to rocks or tree trunks. Soil that is too heavy will stop growth by smothering the rootlets. Lightness, drainage and aeration are easily achieved with fine-grade expanded clay or very porous perlite*, which are much better than sand, which is too heavy. Avoid using too much garden loam: it is often too compact and can turn into a heavy block, like concrete, that is hard for both water and roots to penetrate. If you want to use it, mix it with potting compost or, better still, with compost, provided it is not too rich.
Mould, then water
To avoid damaging the roots, you can repot… an empty pot. In the new pot, simply “repot” your begonia’s old, empty pot. By firming the soil around it, you “mould” the shape of the root ball in advance, without injuring the roots or the plant (a slip of the thumb happens so easily). Once the shape is made, a slight twist lets you remove the empty pot and gently set your root ball in its place. Water generously so that the substrate settles against the roots, and you are done.
Keep the rim
All classic pots, whether clay or plastic, have a small lip at the top, often called the rim. When repotting, stop the surface of the root ball at the bottom of this rim. Old gardeners say that this small space between the top of the root ball and the top of the pot matches the volume of water needed to water the soil in the pot. This little trick will help you give just the right amount at each watering: neither too much nor too little.
