Repotting is done too often and too enthusiastically. A new plant does not automatically need a new pot, and a bigger pot is not a kindness.
When it actually needs repotting
- Roots coming out of the drainage holes in quantity
- Roots circling visibly on the surface of the soil
- Water running straight through without soaking in
- The plant drying out within a day or two of watering
- Growth stopped through spring and summer despite good light and feeding
- The plant pushing itself up out of its pot
When it does not
A plant that has just come home from a shop is usually fine for a few months. Let it settle into your light and your watering first. Repotting a plant that is already adjusting to a new environment stacks two stresses together.
Some plants also flower better when pot-bound. Hoya, Spider Plants and Peace Lilies all fall into this group, so repotting them "to help" delays flowering.
Go up one size only
One pot size means 2-3cm (1 inch) more diameter for a small plant, 5cm (2 inches) for a large one.
This is the single most common repotting mistake. A plant in a much larger pot sits surrounded by wet soil its roots cannot reach, and that soil stays wet for weeks. Root rot follows.
If a plant is already struggling, the answer is almost never a bigger pot.
When to do it
Spring, as growth resumes. The plant has a whole season to recover and fill the new pot before winter.
Repotting in autumn or winter means disturbing a plant that has no capacity to regrow damaged roots, which is why plants repotted in November so often decline by February.
How to do it
- Water the plant a day before. Moist roots come out cleanly and damaged roots are easier to spot.
- Support the base and tip the pot rather than pulling the stem.
- Loosen the root ball gently with your fingers. If roots are circling tightly, tease them apart or make a few shallow vertical cuts down the sides.
- Check the roots. Healthy ones are firm and pale. Cut away anything brown, soft or smelling sour.
- Put a layer of fresh mix in the new pot, sit the plant so the soil surface ends up at the same depth as before, and fill around it.
- Firm gently. Do not compact it, since you are trying to keep air in the mix.
- Water thoroughly to settle the soil, then leave it alone.
Do not fertilise straight away
Fresh mix contains nutrients, and damaged roots burn easily. Wait four to six weeks.
The sulk afterwards
Most plants pause after repotting. Some drop a leaf or two. Alocasia, Calathea, Ficus and palms are particularly dramatic about it.
This is normal. The plant is rebuilding fine root hairs that were torn during the move, and it cannot do much else until that is done. Keep conditions stable, water carefully, and wait.
Resist the urge to fix a sulking plant by changing something else at the same time. Two changes at once make it impossible to tell what worked.
Potting up is not the only option
Refreshing the mix
If the plant is a good size and you want it to stay there, take it out, replace the spent mix, and put it back in the same pot. It gets the benefit of fresh soil without the room to double in size.
Root pruning
For a mature plant you want to keep at its current size, trim the outer 2-3cm (1 inch) of the root ball and repot into the same container with fresh mix. Bonsai growers have done this for centuries and most houseplants tolerate it well.
Dividing
Clumping plants like Spider Plants, Peace Lilies, Calathea and ferns can be pulled apart into several plants instead of moved into a larger pot. Repotting day is the natural time to do it.
Choosing a pot
Terracotta is porous, so soil dries faster. Good for succulents, Hoya, ZZ plants and anyone who overwaters.
Plastic and glazed ceramic hold moisture longer. Good for ferns, Calathea and anything that wants consistently damp soil.
Whichever you use, it needs a drainage hole.