Repotting Calculator

How to tell a plant needs repotting

A plant tells you it needs a bigger pot in several ways, and none of them is urgent enough to panic over. Roots growing out of the drainage holes or circling the surface of the soil are the clearest sign. Water running straight through the pot the moment you pour it in usually means the pot is mostly roots and very little soil, so there is nothing left to hold moisture. Growth that has stalled in spring and summer despite regular watering and feeding is another sign, as is a plant that topples over because the pot is too small for its height. The surest check is to slide the plant out of its pot and look: if you see more root than soil, with roots circling the bottom, it is time. If the root ball still holds plenty of soil, put it back and check again next year — repotting a plant that does not need it does more harm than waiting. Some plants, notably snake plants and many orchids, actually prefer to be slightly tight in the pot.

Why only one or two inches bigger?

It is tempting to jump a small plant into a big pot “so it has room to grow”, but that is one of the most common repotting mistakes. A large pot holds a large volume of soil that the small root system cannot drink from, so the soil stays wet for days or weeks after watering, oxygen is pushed out, and the roots rot. The one-to-two-inch step keeps the soil volume close to what the roots can use, while still giving them room to grow into. Fast growers can take the full two-inch step; slow growers such as snake plants, ZZ plants and cacti do better with a one-inch step. The pot’s shape matters too: a pot that is much deeper than it is wide holds wet soil at the bottom where the roots may never reach, so a standard proportion — roughly as deep as it is wide — suits most plants. Always choose a pot with drainage holes.

How to repot, step by step

Water the plant a day before repotting so the root ball holds together and the plant is not stressed. Prepare the new pot with a little fresh mix in the bottom, so the top of the root ball will sit about an inch below the rim when the plant is in place. Slide the plant out — squeeze a plastic pot gently, or run a knife around the inside of a terracotta one — and tease the circling roots at the bottom loose with your fingers. If the roots are a dense mat, you can trim the bottom inch off with clean scissors; the plant will regrow them quickly. Set the plant in the new pot, fill around it with fresh mix, and firm it gently with your fingers — do not stamp it down hard, or you will squeeze the air out. Water thoroughly until it runs from the drainage holes, then put the plant back in its usual spot and leave it alone for a week or two while it settles. Do not fertilise for the first month: fresh mix already contains nutrients, and feeding a plant that is recovering from repotting can burn new roots.

Choosing the mix and the timing

Use fresh potting mix, not garden soil and not the old mix from the pot — old mix is exhausted and compacted, and it can carry pests or salts. The potting-mix calculator works out the ingredients for the new pot size, and the general 2:1:1 mix suits most houseplants, with extra grit for succulents and cacti. Spring is the best time to repot, because the plant is about to grow and will fill the new pot quickly; repotting in the middle of winter, when growth has stopped, leaves wet soil sitting around inactive roots. If a plant is in flower, wait until the flowers finish. After repotting, the watering routine changes slightly — fresh mix holds water differently from old mix — so check the soil with your finger rather than watering on a fixed schedule, and use the watering calculator and the watering schedule generator to settle into the new routine. The houseplant care guides give the repotting interval for each species.