Why clay is harder than it looks

Heavy clay holds water and nutrients well, but it drains slowly and compacts easily. In winter a planting hole can fill like a bucket, drowning fine roots before they have a chance to grow; in summer the same soil bakes hard and cracks. The good news is that clay is fertile, and once a tree's roots get through it, they often grow strongly.

Most British gardens have clay of some description, often with a layer of compacted subsoil left behind by builders. A quick test: dig a spade-depth hole, fill it with water, and see how long it takes to drain. If it is still full the next morning, take drainage seriously.

Choosing your moment

Bare-root trees are lifted and sold between late November and March, while they are dormant. Plant as soon as the ground is workable rather than waiting for spring.

• Avoid planting when the soil is frozen solid or standing in water.
• Cool, damp, still conditions are ideal — a mild day in December beats a dry one in March.
• Keep roots damp from purchase to planting: heel the tree into a spare patch of ground or pack the roots in damp compost.
• Soak the roots in a bucket of water for an hour or two before planting — never longer than that.

If the ground is hopelessly wet, either wait for a drier spell or plant on a slight mound and let the soil settle around the roots over winter.

Improving the soil before the tree goes in

Do not simply dig a comfortable hole and fill it with compost. In clay, that creates a sump: water gathers in the soft material and roots circle within it rather than pushing out. Instead, improve a wider area.

• Mark out a planting square roughly three times the width of the root spread. Square holes beat round ones in clay, because roots are less inclined to circle.
• Fork the base and sides of the hole to break the smeared, glazed surface a spade leaves behind. That glazing is what stops water and roots moving sideways.
• Mix the excavated clay with a bucket or two of well-rotted manure or garden compost, plus a handful of grit if the clay is truly sticky.
• If there is a hardpan beneath, fork it if you can reach it, but do not dig deeper than the root system needs.
• Improve the surrounding soil too — a metre-wide area of better structure will do more than any additive placed in the hole itself.

Mycorrhizal fungi, sold as a dusting powder, can help roots explore poor soil, but they are no substitute for drainage.

Setting the roots

Look for the soil mark on the trunk where the tree grew in the nursery, or the point where the roots flare outwards. That line should sit at, or a fraction above, the finished soil level. Planting too deep is the most common way to kill a young tree.

In heavy clay, plant slightly proud — set the root flare a couple of centimetres above the surrounding ground — and let the soil settle around it. Spread the roots out in their natural direction. Backfill in layers with the improved soil, firming gently with your heel as you go and working around the tree so it does not shift. Aim for firm, not compacted: the soil should not feel like concrete, but nor should it wobble.

Water as you fill, letting it wash soil around the roots and close any air pockets.

Watering the first year

Establishment depends far more on water than on feed. Give the tree a thorough soaking — ten to fifteen litres, poured slowly — immediately after planting, even if it is raining.

• Water in the morning, at the base, slowly enough that it soaks in rather than running off a baked surface.
• Through the first spring and summer, water deeply once a week in dry spells rather than little and often. Deep watering encourages deep roots.
• Check moisture by pushing a finger or trowel into the soil a few inches from the trunk. Damp, not sodden, is the aim.
• Never water into a waterlogged hole in winter. If the tree is sitting in standing water, the drainage has not worked.

Mulch, stakes and patience

Once planted and watered, spread a mulch of well-rotted manure, compost or bark chippings five to seven centimetres deep over the whole improved area. Keep it well clear of the trunk, since a mulch collar touching the bark invites rot. Mulch moderates temperature, holds moisture and, over the years, slowly improves the clay beneath it.

Stake only if the tree is tall or the site exposed. A short stake driven in at an angle, tied low, allows the trunk to move and thicken with the wind. Check ties each year and loosen them as the trunk grows.

Weeds and grass compete hard with young trees, so keep a clear metre around the trunk for the first three years. Beyond that, clay will reward you: it holds moisture and nutrients, and a tree that gets through its first year in it often needs very little from you afterwards.