But repotting isn't a tonic. It's a disruption. A good one when it's needed, and a genuine setback when it isn't. Some of your plants do need it right now. Most of them don't. And a few will go backwards if you do it this month.

Here's how to tell the difference.

First, two jobs that get confused

“Repotting” gets used for two things that have almost nothing to do with each other. Potting up is moving a plant into a bigger pot because the roots have run out of room. Refreshing the mix is same pot, new mix, because the old one has broken down.

Different signals, different urgency, completely different outcome. And plenty of plants that get potted up in spring only ever needed the second one. Going bigger when the actual problem was structure hands you a larger volume of mix that stays wet for longer, with roots that can't reach most of it. That's the mechanism behind a repot that makes a plant worse instead of better — and it's more common than anything else we hear about at this time of year.

What tells you to go and look

Two things put a plant on the inspection list. Neither of them is a verdict on its own.

Time. Bark, coir and composted bark are organic materials, and organic materials break down. As they do they lose their shape, the mix compacts, and the air goes out of it. How long that takes depends entirely on what's in the mix and how hard the plant is growing — a chunky bark-heavy mix can hold its structure for years, a fine one won't. So a few years in the same mix isn't a reason to repot. It's a reason to go and check.

A recent purchase. Every plant you buy arrives in a grower's mix chosen for commercial nursery conditions — irrigation on someone else's schedule, light levels your lounge room can't match, feeding managed by people who do it for a living. Some of those media suit a house fine. Some stay saturated for days. You won't know which one you've got until you've watered it a few times.

How to actually check

Three things. None of them take long.

Slide it out. Hand across the top, fingers either side of the base, tip the pot and ease it off. What you're reading is how much of the pot the roots have actually taken over. Densely circling the outside, packed through the bottom, barely any mix left between them — that's ready to go up. Plenty of usable mix still in there and room for roots to keep exploring — put it back. What you're not reading is whether the root ball holds together: a chunky mix will crumble in your hands while full of roots, and a collapsed one can come out as a solid brick with hardly any roots in it at all.

Finger Probe Test. Push a finger down into the mix. Loose and crumbling apart rather than packed into a dense mass means the structure is still working. This is a read on the mix itself, not on how wet it is — moisture is the Pot Weight Test's job.

Watch what the water does. Water racing straight through in seconds is usually channelling down the sides of a shrunken root ball rather than wetting it. Water pooling on top and taking an age to move is compaction. A pot that stays heavy long after a watering is a pot that isn't drying back — the mix has stopped holding air, and the roots are sitting in it. Persistent fungus gnats are often the same story told a different way: mix that's constantly damp at the surface. (If that last one is your main symptom, we go deep on it in why your potting mix stays wet days after watering.)

Repot now

Everything here is something you found, not something you assumed.

Roots have genuinely run out of room. A dense circling mat, roots coming out the drainage holes in volume, a plastic pot starting to bulge. Go up a size.

It fails the finger probe. Packed, dense, no crumble. The structure has collapsed. Fresh mix in the same pot is usually all that's needed — this is the one people mistake for needing a bigger pot.

The water is behaving badly. Channelling, pooling, or a pot that won't dry back no matter how long you leave it. Same diagnosis as above, arrived at from the other direction.

The grower's mix isn't coping in your house. If a plant you've brought home stays saturated for days, compacts down hard, or keeps making dry-back impossible to judge, change it. Not because it's new — because you've watched it and it isn't working.

You already know it needs to go up. If a plant has been on your mental list since autumn, do it now, at the front of the season, so it has all of spring and summer to establish. That's the real argument for spring: not that plants need repotting in September, but that a plant repotted in September has the longest possible runway.

What can wait

Plants that are just a bit snug. Plenty of indoor plants are perfectly happy slightly tight. Roots touching the side of the pot aren't an emergency. A pot that's been completely replaced by roots is a different conversation.

Anything you repotted last spring. Not never — a fast grower in good light can fill a pot in a season, so slide it out if you're unsure. But the default answer is no.

Plants that haven't started moving yet. A plant that's actively growing is far better placed to establish into fresh mix. If yours is sitting exactly where it was in July, there's no prize for doing it on 1 September. Wait for a new shoot, a new leaf, something telling you the growing season has actually started. Late September, October, even November is fine. Nothing is lost.

Plants that are struggling — until you've diagnosed them. Repotting isn't a treatment on its own, and this is the hardest one to sit on, because when a plant looks unhappy, doing something feels better than doing nothing. Work out what's going wrong first. If it's light, watering frequency or a pest, disturbing the roots just stacks another stress on top. But if the diagnosis leads you underground — rotting roots, collapsed mix, a pot that simply won't dry — then repotting is part of the fix, and you do it now.

Plants you want to look tidier. That's a decorative pot's job. Drop the whole thing into a cover pot and leave the roots out of it.

The middle option nobody uses

Top-dressing. Scrape away the loose, tired mix at the surface, discard it, replace it with fresh. No root disturbance, no size change, no setback.

It's especially useful for large specimens — plants you're not going to haul out of their pots every spring. A two-metre plant that's growing well doesn't need a repot, but it does need someone to maintain the mix it's got. It's also useful anywhere the surface has slumped and you want the level back without pulling the plant apart.

What it won't do is fix compaction further down. If the root zone has collapsed all the way through, top-dressing puts a fresh layer over a problem rather than solving it — that one needs a proper refresh.

If you're repotting, do it properly

One size up. That's all. Two to three centimetres more in diameter. Over-potting is one of the easiest ways a spring repot goes wrong — a big pot holds a big volume of mix, only a small part of it has roots in it, and the rest sits wet for weeks. Slow decline, no obvious cause.

Same depth. Sit the plant at the height it was already growing at. Burying the crown to make it look better in the new pot causes more problems than it solves.

Water it in, then recalibrate. Water thoroughly until it runs out the bottom — that settles the new mix into contact with the root ball and removes any large gaps around it. Let it drain properly, then lift it. That weight is your new reference point for the Pot Weight Test. The pot has changed, so your sense of what “watered” feels like has to change with it. The surface will dry first regardless; the weight is what tells you what's happening down where the roots are.