Can I reuse last year's compost from my pots?

The reason last year's compost disappoints is not that the nutrients have gone — that part is easily corrected. It is that the physical structure has collapsed. Peat-free and peat-based composts both break down over a season into finer particles with fewer air spaces, and a plant in an airless mix is fighting from the start.

Safety

What you'll need

• Garden sieve or a riddle
• Fresh compost
• Slow-release fertiliser
• Bucket or tarpaulin to mix on (optional)

Step by step

1. Decide what it is for before you decide whether to reuse it.Border mulch, no reservations. Undemanding summer bedding, yes with reconditioning. Seed sowing or a long-term container plant, no — start with fresh.
2. Reject compost from a failed plant.If the plant died of root rot, vine weevil or a soil-borne disease, put that compost on a border well away from the same crop rather than back into a pot. Look for the white grubs of vine weevil as you tip it out.
3. Tip it out and sieve the roots out.Old root mass holds no water and takes up the space compost needs to occupy. A garden riddle over a wheelbarrow does this quickly. Dampen it first so it does not raise dust.
4. Mix it half and half with fresh.Fifty per cent fresh multipurpose restores enough structure and air for bedding, tomatoes in a bag, or a pot of annuals. Straight recycled compost slumps.
5. Add loam if the pot is staying put.A third of loam-based John Innes gives weight, holds nutrients and stops the mix collapsing again by August.
6. Put the nutrition back deliberately.Mix in slow-release fertiliser granules at the rate on the pack, or plan to liquid feed from the moment you plant rather than from six weeks in. The old compost is carrying nothing.
7. Never reuse it for seed sowing.Seedlings need a fine, low-nutrient, disease-free medium. Old compost is coarse with root debris, carries weed seed and damping-off organisms, and has an unknown and uneven nutrient level.
8. Spread the rest on borders.Spent compost still has real value as bulky organic matter. Spread 25 to 50mm on the surface of a border and let worms take it down. This is the highest-value use for most of it.
9. Keep it dry and covered until you use it.A heap of spent compost left in the rain goes anaerobic and sour. Bag it or cover it, somewhere it will not sit in a puddle.

Tips

• Sieving the roots out is the step that makes the difference, and it is the one people skip.
• Growbags are worth emptying onto a border rather than binning — a used bag is two good barrowloads of organic matter.
• If you only reuse it in one place, make it the bottom half of a large pot, with fresh compost in the top third where the roots start.

Common mistakes

• Topping up pots with old compost and adding nothing — The plant starts in an airless, exhausted medium and never gets going. It is a slow failure that looks like bad luck.
• Sowing seed into last year's compost — Weed seed, damping-off fungi, root debris and an unknown nutrient level. Seed compost is cheap and the one place not to economise.
• Reusing compost from a pot that had vine weevil — The grubs overwinter in it and go straight into the next plant. Spread it thinly on a border instead.

Questions people ask