How do I root prune a plant to keep it in the same pot?

There is a limit to how big a pot you want on a patio, and eventually a good plant reaches it. Root pruning is the standard nursery answer: you renew the compost and reset the root system without increasing the pot size. It is the same principle as bonsai at a domestic scale, and it works on most woody plants.

Safety

What you'll need

• A sharp long knife, old pruning saw or a spade, true
• Fresh loam-based compost, true
• Secateurs and loppers, true
• A cane or thin stick for firming
• A tarpaulin

Step by step

1. Do it in the dormant season.Late autumn to late winter for deciduous plants. Early spring for evergreens, once hard frosts are past. Never in summer, and never on a plant that is flowering or carrying fruit — the reduced root system cannot support active growth.
2. Water well the day before and get the plant out.A moist rootball cuts cleanly and holds together. Cut round the inside of the pot with a long blade first, then lay the pot on its side and draw the plant out.
3. Assess what you have.A pot-bound rootball is a solid cylinder with a dense felted outer surface and roots circling at the base. Almost no compost is visible. That circling layer is what you are removing.
4. Slice a slab off each side.With a knife, saw or the edge of a spade, take a slab of 25 to 50mm off all four sides — think of squaring off a cylinder. Take the total removed to roughly a quarter to a third of the root volume, no more.
5. Take a slice off the base.25 to 50mm off the bottom, flat, which removes the mat of roots that has been circling the base of the pot. Then tease the remaining outer roots loose with a hand fork so they point outward rather than round.
6. Cut back any thick coiled root cleanly.Woody roots that have wound around the ball will never straighten and will eventually girdle the plant. Cut them out at the point where they start to circle, with secateurs, cleanly.
7. Return it to the cleaned pot on fresh compost.A layer of fresh loam-based compost in the base first, so the plant sits at exactly the same depth as before. Then work fresh compost into the new gaps at the sides, firming with a cane so there are no air pockets.
8. Reduce the top growth to match.Take a comparable proportion off the canopy — a third, if you took a third of the roots. A full head of leaves on a reduced root system wilts, scorches and often does not recover. This step is not optional.
9. Water and shelter it.Water thoroughly, then keep the pot out of wind and full sun for two to three weeks and keep the compost just moist. Do not feed until you see new growth — feeding a damaged root system does nothing useful.

Tips

• Root pruning suits bay, box, olive, acer, hydrangea, camellia, fruit trees on dwarfing rootstock, wisteria and most deciduous shrubs. It suits palms, conifers, most grasses and drought-adapted succulents far less well.
• Do it every three to five years and it becomes routine maintenance rather than surgery.
• If you want the plant to keep growing, pot on instead. Root pruning holds a plant at its current size — that is what it is for.

Common mistakes

• Taking more than a third of the roots — Beyond about a third the plant cannot support itself and either dies outright or spends two seasons producing nothing. Half the root is a nursery operation on a nursery plant, not a patio one.
• Leaving the canopy untouched — The leaves keep transpiring at the old rate from a root system that can no longer supply them. The plant scorches from the tips inward within a fortnight.
• Doing it in spring just as growth starts — The plant is mobilising reserves into new shoots and has no capacity to regrow roots at the same time. Dormant, or very early, or not at all.
• Reusing the old compost — The point of the operation is as much about renewing exhausted compost as about the roots. Fresh loam-based compost in the gaps is where the next three years of growth comes from.

Questions people ask