10.3 Soil for trees and shrubs
Planting a tree, a hedge plant or a shrub in open ground. The best soil for the hole is the soil that came out of it.
At a glance
- Difficulty
- 1 of 4
- Time
- 1–2 hours per tree
- Tools
- Spade, garden fork, tape measure, wheelbarrow or tarp for the dug soil, hose or buckets
- Parts
-
- 1 Undug soil under the ball
- 2 Backfill of the dug-out soil
- 3 Root ball
- 4 Mulch on top
What you need per tree, root ball 500 mm wide
| Material | Kind | Type | Amount |
|---|---|---|---|
| Backfill | Existing soil | The soil you dug out of the hole, clods broken up | |
| Compost | – | Not in the hole. Use it as mulch on top if you like | |
| Mulch | Wood chips or bark | Coarse, organic, over the whole dug area | 0.15 m³ |
Why the dug-out soil, not compost?
Roots must grow out of the hole into the soil around it. If the hole holds the same soil, nothing stops them.
Compost or bought soil in the hole makes a boundary. Water and roots move poorly across it, so the hole acts like a pot in the ground.
Most tree roots grow in the top 300–450 mm and spread far beyond the crown. A wide, loose hole helps them; a deep one only lets the tree sink.
Put organic matter on the surface as mulch. It feeds the soil from above without making a pot.
Step by step
-
1
Dig a wide, shallow hole.
Make it 2–3 times as wide as the root ball and no deeper than the ball is tall. Give it sloping sides, like a saucer. Pile the soil on a tarp.
Saucer-shaped hole, soil on a tarp -
2
Check the drainage.
Fill the hole with water and let it drain. Fill it again. If water still stands 24 hours after the second filling, drainage is poor: choose a plant that tolerates wet soil, or another spot.
Water standing in the hole -
3
Set the root ball on undug soil.
Stand the ball on the firm bottom. Its top should sit 25–50 mm above the surrounding ground and the root flare, where the trunk widens into roots, must be visible.
Root ball standing slightly proud of the ground -
4
Backfill with the soil you dug out.
Break up clods and put the soil back in. Add no compost, peat or bought soil. Firm it gently with your hands. Do not stamp on it.
Backfilling by hand around the ball -
5
Water to settle the soil.
Soak the whole hole slowly. Water settles the soil and fills air pockets without packing it hard. Top up any low spots with more of the dug soil.
Hose soaking the planting hole -
6
Mulch, but keep it off the trunk.
Spread 50–100 mm of wood chips or bark over the dug area. Keep the mulch at least 100 mm away from the trunk.
Mulch ring with a bare gap at the trunk
Specifications
| Part | What | Value | Note |
|---|---|---|---|
| Hole width | – | 2–3 × root ball | Purdue. Colorado State and Utah State say 3 ×. |
| Hole depth | – | = root ball height | Ball on undug soil. |
| Top of ball | – | 25–50 mm above ground | Colorado State. Root flare visible. |
| Backfill | Dug-out soil | no additions | Arizona, UC, Washington State, Utah State. Colorado State accepts up to 5 % organic matter by volume. |
| Mulch | Wood chips, bark | 50–100 mm | Illinois 50–100 mm, Utah State and Purdue up to 100 mm. |
| Bare ring | – | ≥ 100 mm | Mulch kept this far from the trunk (Illinois). |
Where the roots are
80–90 % of tree roots grow sideways in the top 300–450 mm of soil. They usually reach 2–3 times as far as the crown; one older extension source gives 4–7 times.
Trees in paving
A tree surrounded by paving needs loose, rootable soil under it. A stormwater manual gives about 0.6 m³ of soil per m² of the tree's mature crown, and cities ask for 10–30 m³ per tree.
Shrubs and hedges
The same rules apply to shrubs and hedge plants from containers: wide hole, no deeper than the ball, dug-out soil back in, mulch on top.
Sources
- University of Illinois Extension, Proper mulching techniques: mulch 2–4 in (50–100 mm) deep, compost 2–3 in, coarse mulch 3–4 in; keep mulch at least 4 in (100 mm) away from the trunk
- Purdue University Extension FNR-FAQ-18, Trees need a proper start: hole two to three times the width of the root ball; do not compact the soil by stomping; water to remove air pockets; mulch 75–100 mm, not against the bark
- Colorado State University Extension, The science of planting trees: saucer-shaped hole three times the root ball diameter; up to 5 % organic matter by volume in the backfill is acceptable, 25–50 % is a common mistake
- Colorado State University Extension, Tree planting steps: top of the root ball 25–50 mm (1–2 in) above grade, on undug soil; water settles the soil without packing it; avoid stomping
- Utah State University Extension, Tree planting rules: hole at least 3 × the root ball; backfill with native soil, no amendments; root collar at or just above the soil
- University of Arizona Extension AZ1022, Planting guidelines: container trees and shrubs: hole no deeper than the root ball; native soil backfill without amendments; loosen soil 3–4 times the hole width; drainage is poor if water stands 24 hours after the second filling
- University of California ANR, No need to amend soil when planting new trees or perennials: research shows no significant advantage from amending planting holes; the amended soil makes an interface roots and water cross poorly
- Washington State University, L. Chalker-Scott, planting instructions: amending native soil for woody plants is not a sustainable practice; topdress with organic mulch instead
- University of Maine Cooperative Extension, Ask the expert: tree roots: 80–90 % of tree roots grow in the top 300–450 mm (12–18 in); roots typically extend 2–3 times wider than the drip line
- Iowa State University Extension, Tree root systems (1992): root spread 4–7 times the drip line radius
- Minnesota Pollution Control Agency, Minnesota Stormwater Manual: tree soil volume: 2 ft³ of rootable soil per ft² of mature canopy (about 0.6 m³ per m²); Toronto 30 m³ and Langley 10 m³ per tree
Mulch amount: a 1.5 m ring (3 × a 500 mm root ball) is about 1.8 m²; at 75 mm deep that is about 0.13 m³, rounded up to 0.15 m³.
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