The modern advice for planting a tree is almost the opposite of what most people do: dig a hole two or three times the width of the root ball and no deeper than the root ball itself. The old practice of digging a deep hole and improving the bottom is now known to be worse than useless, because the roots circle in the improved pocket and never leave it, and because a deep hole in heavy ground fills with water and drowns them. That changes the volume substantially, and this works it out for the whole planting — including how much of the backfill should be compost and how much is the soil you took out.
Hole, trees and backfill
The hole should be wide and shallow. Most are dug narrow and deep.
Two to three times the root ball. For a standard whip, 0.6 m; for a container tree, 1.2 m.
No deeper than the root ball is tall. Deep holes drown roots.
Ten to twenty percent is plenty. More creates a pocket the roots will not leave.
Result
Wide and shallow, not narrow and deep
Tree roots do not grow down. They grow sideways, and almost all of the absorbing root system of a mature tree is in the top half-metre of soil, spreading well beyond the canopy. A hole dug deep and narrow, with improved soil at the bottom, puts the young roots into a pocket of nice soil with a wall of unimproved ground around it, and the roots circle in the pocket rather than crossing the boundary.
The second problem with deep holes is water. In anything but free-draining sand, a hole deeper than the surrounding soil fills with water that has nowhere to go, and the roots at the bottom sit in it. That is how a newly planted tree dies in its first winter, and it looks exactly like drought stress, so people water it more.
The backfill is mostly the soil you took out
The instinct is to buy soil for the hole and to treat what came out of it as spoil. That is backwards. The soil you removed is the soil the tree will be growing in for the next fifty years, and replacing it with something different creates exactly the interface problem that makes roots circle. The backfill should be the excavated soil, broken up, with a modest addition of compost.
Ten to twenty percent compost is enough. It improves structure and gives the young roots something to grow into without creating a pocket of material so different from the surrounding ground that nothing crosses the boundary. On very poor or very heavy ground the same logic applies: improve the whole area rather than just the hole.
The point where the trunk widens into the roots — the root flare — should sit at or just above the finished soil level. It is under the soil in most container-grown trees and has to be found by hand.
Planting with the flare buried is the single most common cause of a tree failing in its first five years, and it is invisible once the tree is in.
How much to buy
Five holes at 0.9 metres across and 0.6 metres deep is about 1.9 cubic metres of backfill, of which twenty percent — around 0.38 cubic metres, or 380 litres — is compost. That is ten bags of forty litres, and the rest is the soil on the spade. Buying nearly two cubic metres of soil for five trees would be both expensive and counterproductive.
Where the soil really is unusable — pure builder's rubble, or ground heavily compacted by machinery — the answer is to improve a wider area rather than a deeper hole, or to choose a species that tolerates the conditions. Replacing two cubic metres with bought topsoil in five isolated holes is the worst of both worlds.
| Tree | Root ball | Hole width | Hole depth | Note |
|---|---|---|---|---|
| Bare-root whip | n/a | 0.6 m | 0.3 m | Wide enough to spread the roots |
| Bare-root standard | n/a | 0.9 m | 0.4 m | Stake before backfilling |
| Container, 10 L | 0.3 m | 0.9 m | 0.4 m | Three times the pot |
| Container, 40 L | 0.5 m | 1.2 m | 0.5 m | Two to three times the pot |
| Root-balled, 60 cm | 0.6 m | 1.5 m | 0.6 m | Heavy; handle by the ball |
| Multi-stem shrub | 0.4 m | 1.0 m | 0.45 m | Same logic, smaller hole |
A worked example
- Volume per holeHalf a diameter of 0.45 m squared, times pi, times 0.6 deep is about 0.38 m³.
- Across five trees1.91 m³ of backfill in total.
- Compost shareAt twenty percent that is 0.38 m³, or 380 litres — 10 bags of forty litres.
- The rest1.53 m³ is the soil you dug out, broken up and put back.
- Shape check0.9 m wide against 0.6 m deep is a ratio of 1.5, which is still narrower than the recommended two to three times — widen it to a metre and a half.
That last line is the useful output. The volume tells you what to buy; the width-to-depth ratio tells you whether the hole is the right shape, and it is the shape that decides whether the tree lives.
Planting and aftercare
Water the tree in the pot before planting, and soak the hole before it goes in. Backfill in layers, firming each one gently with a boot — not so hard that you compact it, not so loose that there are air pockets against the roots. Leave a shallow basin around the tree to hold water, and mulch it.
The mulch matters more than the compost. A ring of fifty to seventy-five millimetres kept clear of the trunk suppresses the grass that would otherwise take the water and the nitrogen, and the mulch figure sizes it for the whole planting. Water for the first three years: roughly twenty litres a week in dry weather in year one, tapering after that.
Stake only if the tree needs it, and low, with the tie loose enough to let the trunk move. A tree staked tight and tall never builds the taper it needs and snaps the moment the stake rots. Where several trees are going in, the plant spacing figure sets the distances between them.
Do
- Dig wide and shallow: two to three times the ball, no deeper.
- Find the root flare and plant it at finished ground level.
- Backfill with the soil you dug out, plus ten to twenty percent compost.
- Leave a water basin and mulch the ring, clear of the trunk.
- Water weekly for the first three years.
Avoid
- Digging a deep hole and improving the bottom of it.
- Burying the root flare under several inches of soil.
- Replacing all the excavated soil with bought topsoil.
- Adding more than twenty percent compost to the backfill.
- Staking tight and high, so the trunk never builds taper.
Where the numbers stop being true
The hole is treated as a cylinder. Real holes are rougher and slightly conical, and hand-dug holes in stony ground are bigger than the nominal dimensions because of the way the sides break away. Treat the volume as an estimate to within about ten percent.
Nothing here accounts for soil that is genuinely unusable, or for a planting pit in a paved area, where the whole volume may need to be engineered growing medium with proper drainage — a different specification entirely and one where the raised bed figure is closer to the right model.
Nor does it account for spoil. The soil you take out occupies more volume loose than it did in the ground — typically twenty to thirty percent more — so digging five holes produces more material than goes back into them, and the surplus has to go somewhere.
How the figures are produced
The hole is treated as a cylinder: pi times the radius squared times the depth, multiplied by the number of trees. The compost share is that volume times the percentage you entered, converted to litres and divided by bag size. The soil to reuse is the total minus the compost. The width-to-depth ratio is the diameter over the depth, which is the shape check rather than a volume.
Questions people ask
How deep should a tree hole be?
No deeper than the root ball is tall — usually 0.3 to 0.6 m. Deeper holes fill with water and drown the roots.
How wide?
Two to three times the width of the root ball. For a container tree in a 40 L pot, that is around 1.2 metres across.
Should I add compost to the backfill?
Ten to twenty percent. Much more and you create a pocket of different material that the roots circle inside rather than leaving.
Can I put the tree deeper for stability?
No. Burying the root flare is the most common cause of failure. Stake it instead, low and loose.
What do I do with the surplus soil?
Spread it, or use it elsewhere. Excavated soil occupies twenty to thirty percent more volume loose than it did in the ground.
Do I need to stake it?
Only if the tree is tall enough to rock in the wind. Stake low, with a loose tie, and remove it after two growing seasons.