When should I pot a plant on into a bigger pot?

Potting on is judged by roots, not by how the top of the plant looks, and not by the calendar. The whole decision comes from sliding the plant out of its pot, which takes ten seconds and tells you everything.

What you'll need

• The next pot size up
• Fresh potting compost
• Slow-release fertiliser (optional)

Step by step

1. Slide the plant out and look at the rootball.Spread a hand over the surface, turn it over, tap the rim on the edge of the bench and the plant comes out intact. Roots visible at the wall but not circling means it is time.
2. Do not go by roots at the drainage hole alone.A single root through the hole is normal long before the pot is full. Roots coming out of the hole in numbers means you are late rather than on time.
3. Go up one size only.Roughly 30 to 50mm more diameter, and no more than about double the volume. This is the rule people break most often and the one that costs most.
4. Understand why a big jump fails.Roots dry the compost they occupy. Compost with no roots in it never dries, so it stays saturated, cold and airless, and it goes sour. The plant sits in a cold wet jacket and does nothing for weeks.
5. Tease out circling roots if it is already pot-bound.A solid white cage that holds the shape of the pot needs the outer roots pulled free with your fingers, or four shallow vertical cuts down the sides with a knife. Left alone they carry on circling and strangle the plant.
6. Match the compost to how long it is staying.Multipurpose for anything moving on again within a season. Loam-based John Innes for anything staying in the pot for a year or more, because peat-free and peat-based mixes collapse within a season.
7. Keep the planting depth the same.The old compost surface should sit at the new one. Burying the stem of a woody plant rots it. Seedlings and tomatoes are the exception and can go deeper.
8. Leave a watering gap.About 25mm between the compost surface and the rim, so water pools and soaks in rather than running over the side.
9. Water it in and then leave it alone.Water thoroughly once, then wait until it needs it. Freshly potted plants are the most commonly overwatered plants there are, because the new compost stays wet while the roots are still in the old ball.
10. Pot on in growth, not in winter.Spring and early summer for most plants, so the roots move into the new compost quickly. A plant potted on in November sits in cold wet compost for four months.

Tips

• Roots at the wall, not circling: that is the whole test.
• The commonest reason a plant sulks after repotting is not shock — it is the extra volume of wet compost around it.
• Water the plant an hour before you move it. A damp rootball holds together; a dry one falls apart.

Common mistakes

• Moving a small plant into a very large pot to save doing it again — The unrooted compost stays saturated and airless for months, roots rot at the edges, and the plant goes backwards. One size at a time genuinely is faster overall.
• Potting on before the roots have filled the pot — You get the same wet-jacket problem in miniature, and you disturb roots for nothing.
• Planting deeper than it was — Buried stems on woody plants and most perennials rot at the collar. Keep the old soil level.
• Using multipurpose for a long-term container plant — It breaks down and collapses within a season, leaving the roots in an airless mass. Loam-based compost holds structure for years.

If it doesn't work

Questions people ask