How do I top-dress a large pot instead of repotting it?

Compost in a container exhausts from the top down: nutrients wash out, the structure collapses, moss and liverwort colonise the surface, and a crust of salts builds up. Top-dressing renews the layer where most of the feeding roots are and where most of the water enters, without disturbing the plant at all.

What you'll need

• Hand fork or old kitchen fork, true
• Fresh loam-based compost such as John Innes No. 3, true
• Slow-release fertiliser
• Gravel, slate or bark for mulch

Step by step

1. Do it in early spring, as growth begins.Late February to April in most of Britain. Early enough that the fresh feed is available as the plant starts growing, late enough that a hard frost is not going to hit soft new roots at the surface.
2. Remove the old top layer.Take off the top 50mm, or up to 100mm on a very large planter. Use a hand fork and your fingers, not a trowel — you are lifting the exhausted layer out, not digging. Stop the moment you meet substantial roots.
3. Take off any surface crust and moss.The white or yellow crust is fertiliser salts and hard-water deposits, and it is water-repellent. The moss and liverwort layer is impermeable too. Both need to go with the old compost.
4. Loosen the surface you have exposed.Prick the exposed compost over lightly with the fork to break any compacted or hydrophobic layer, so the new compost knits into it rather than sitting on top as a separate slab.
5. Mix the feed into the fresh compost before it goes in.A controlled-release fertiliser at the rate on the pack, mixed through the compost in a bucket. That distributes it evenly rather than leaving a concentrated band.
6. Replace to the original level.Fresh loam-based compost, firmed gently, back to the mark on the stem where the compost was before. Burying the collar of a woody plant deeper than it was rots the bark. Leave the watering gap of 25 to 40mm below the rim.
7. Mulch and water thoroughly.A 25mm mulch of gravel, slate chippings or composted bark reduces evaporation, suppresses weed and moss and stops rain splash. Then water until it runs out of the base to settle everything in.

Tips

• For ericaceous plants — camellias, rhododendrons, blueberries, pieris — top-dress with ericaceous compost and use a feed formulated for them, or you will slowly push the pH the wrong way.
• Top-dressing does nothing for a plant whose roots have filled the pot completely. If water runs straight through and the plant wilts daily, it needs repotting or root pruning, not a new top layer.
• This is also the moment to check the drainage hole, put the pot on feet, and give the outside a wash.

Common mistakes

• Adding compost on top without removing anything — The compost level climbs, the watering gap disappears, and eventually the stem collar is buried. Take out before you put in.
• Digging deep and cutting the surface roots — The top 50mm of a container is full of fine feeding roots. Fingers and a fork, gently, and stop when you hit them.
• Using multipurpose compost as a top-dressing on a permanent plant — It slumps within a season and holds too much water at the surface. Loam-based compost keeps its structure and stays where you put it.

Keeping on top of it

• Top-dress with fresh compost and slow-release feed (Every spring)
• Repot or root prune properly (Every three to five years)
• Clear the drainage hole and check the pot feet (Annually, in autumn)

Questions people ask