Should I put anything in the planting hole?

The instinct to give a new plant the best possible start by filling its hole with rich compost is understandable and, on heavy soil, actively harmful. What a new plant needs is roots that leave the hole, and everything in this guide follows from that one requirement.

Safety

What you'll need

• A spade and a fork
• A watering can or hose
• Mulch material
• Mycorrhizal fungi granules (optional)
• Garden compost for the surface

Step by step

1. Dig a wide hole rather than a deep one.Two to three times the width of the rootball and no deeper than its height. Roots spread outward far more than downward, and a hole dug deeper than the rootball lets the plant sink so the stem ends up buried, which kills trees and shrubs slowly.
2. Break up the sides and base.A smooth-sided hole in clay, particularly one dug with a spade in wet conditions, is glazed and roots will not cross it. Score the sides with a fork and break the base. This matters more than anything you could put in the hole.
3. Do not fill it with compost.A pocket of rich, open compost in surrounding heavy soil creates a sump: water drains in and cannot get out, so the rootball sits in water all winter. Worse, the roots have no reason to leave a luxurious pocket and never venture into the surrounding soil, so the plant is permanently in a pot it cannot see.
4. Backfill with the soil you dug out.Possibly with a modest amount of compost mixed evenly through it — a quarter at most — but not a distinct layer or a hole full of it. The aim is for the soil in the hole and the soil around it to behave the same way.
5. Consider mycorrhizal fungi on bare-root planting.The evidence is reasonable for trees, shrubs and roses planted bare-root, where the granules can be sprinkled directly onto damp roots so there is physical contact. The fungi need to touch the roots to colonise them — sprinkling them into the bottom of a hole under a container rootball achieves little. They also do nothing for brassicas, beets and a few other families that do not form mycorrhizal associations.
6. Skip the bone meal in most cases.Bone meal supplies phosphorus, which encourages root growth — but most long-cultivated British garden soils already have ample phosphorus, and phosphorus is immobile so it barely moves to where roots are anyway. It also attracts dogs and foxes, which dig up new plantings. If a soil test shows phosphorus is genuinely low, it has a place.
7. Do not add fertiliser at planting.A newly planted plant has damaged roots and cannot use it. Soluble fertiliser around damaged roots scorches them. Wait until the plant is growing away — usually the following spring for autumn planting — and feed then.
8. Water thoroughly at planting, then keep watering.A full can poured slowly settles the soil around the roots and removes air pockets, which is what actually kills newly planted shrubs. Then water regularly through the first two summers. This is the single highest-value thing you can do and it costs nothing.
9. Mulch the surface, keeping it off the stem.Fifty to seventy-five millimetres of compost or bark over the root area but not touching the stem. It holds moisture, suppresses competition and feeds the soil from above, which is where soil feeding should happen.

Tips

• On heavy clay, plant slightly proud — the top of the rootball 25mm above the surrounding soil — and mound soil up to it. It stops the plant sitting in a puddle.
• The commonest cause of a new tree or shrub failing is drought in its first two summers, not anything to do with what went in the hole. Water thoroughly and infrequently rather than a little often.
• Tease out circling roots on a pot-bound plant before it goes in, or cut through them. Roots that circle in the pot continue to circle in the ground and eventually strangle the plant.
• Do not stake unless the plant genuinely needs it, and if you do, stake low and remove the stake after two years. Movement builds stem strength.
• Autumn planting for hardy trees and shrubs gives the roots a whole winter and spring to establish before their first summer, which is worth more than anything you could add to the hole.

Common mistakes

• Digging a neat, smooth-sided hole in clay and filling it with compost — It becomes a sump that fills with water in winter and a comfortable pot the roots never leave in summer. The plant is dead within two or three years and nobody connects it back to planting day.
• Planting too deep — Burying the stem or the graft union causes rot and slow decline. The top of the rootball should sit level with, or slightly above, the surrounding soil.
• Feeding at planting — Fertiliser around damaged roots scorches them, and the plant cannot use it until it is growing. Feed the following season instead.

Questions people ask