Repotting: when to do it, and when to leave it alone
Repotting feels like care, which is why it gets done to plants that did not need it. It is a disruption the plant has to recover from, so it should be justified.
The signs that mean yes
Roots growing out of the drainage holes, or circling visibly on the surface. Water running straight through in seconds because the pot is more root than soil. A plant that dries out within a day or two of watering in the growing season. Growth that has stalled through spring and summer despite good light.
A plant that has physically outgrown its pot and tips over is also a fair reason, though a heavier pot sometimes solves that without disturbing the roots.
The signs that mean no
You just bought it. A plant needs a few weeks to acclimate before another disruption, unless it is sitting in sodden soil.
It looks unwell. Repotting a struggling plant adds stress to a plant that has none to spare. Diagnose the light and water first.
It is autumn or winter. Outside the growing season the plant cannot regrow damaged roots quickly, so wait for spring unless it is an emergency.
The decorative pot no longer matches the room. Use a cachepot.
One size up, and no more
Roughly two inches of extra diameter. A much larger pot surrounds the root ball with soil that no roots are drinking from, that soil stays wet, and root rot follows. This is the most common repotting mistake and it looks like generosity.
How to actually do it
Water the plant a day before — a hydrated root ball comes out cleanly and is less brittle. Support the base, tip the pot, and ease it out rather than pulling the stem.
Tease apart circling roots gently so they grow outward, and trim anything black, mushy or foul-smelling with clean scissors. Healthy roots are firm and pale.
Use fresh potting mix appropriate to the plant — an airy chunky mix for aroids, gritty and fast-draining for succulents and cacti, standard mix for most others. Do not add a layer of gravel at the bottom; it raises the waterlogged zone rather than improving drainage.
Set the plant at the same depth it was before, firm the mix lightly, water it in, and then leave it alone. No fertiliser for a month — fresh mix already contains nutrients, and the roots need to recover first.
The alternative nobody mentions
If a plant is happy at its current size, you can top-dress instead: scrape off the top inch of tired soil and replace it with fresh mix. It refreshes nutrients with none of the disruption, and for a large plant in a heavy pot it is often the sensible choice for years.