How to plant a tree in Irish soil

When to plant

November to March is the window, for one simple reason: a dormant tree spends everything it has on roots, because it has no leaves to keep alive. Plant the same tree in June and it has to do both at once, in the driest weeks of the year.

Container-grown trees can technically go in at any time, and sometimes a job leaves no choice. If you plant between May and August, accept that you have signed up for watering — properly, every week, until the autumn rain takes over.

Avoid planting into frozen or waterlogged ground. If the soil smears rather than crumbles when you squeeze it, wait a week.

Digging the hole

Wide and shallow. This is the part most people get backwards.

Width: three times the diameter of the root ball. Roots spread sideways in the top 30–60cm of soil, not down. A wide hole gives them loosened ground to move into in their first season.

Depth: exactly the height of the root ball. Not a centimetre more. If you dig deeper and backfill underneath, the tree settles over the following months and ends up too deep — the fault you were trying to avoid.

Square, not round. In clay, a spade glazes the sides of a round hole into a smooth wall that roots will circle inside rather than cross. Cut the sides square and rough them up with a fork before you plant. It takes thirty seconds and it is the difference between a tree that establishes and one that sits there.

Setting the tree

1. Find the root flare. Scrape the top of the compost away until you can see where the trunk stops being a trunk and starts widening into roots. On container-grown stock it is often 3-5cm below the surface of the pot. That flare is your ground level, not the top of the pot.
2. Tease the roots out. Any root circling the inside of the pot will keep circling once it is in the ground and can eventually strangle the tree. Pull the outer roots free with your fingers. On a badly pot-bound tree, cut three or four vertical slices down the side of the root ball with a knife.
3. Set it in and lay a cane across the hole. The flare should sit level with, or a couple of centimetres above, the surrounding soil. Never below.
4. Backfill with the soil you dug out. If you fill the hole with rich compost in the middle of clay, you have built a sump. Water drains in from the surrounding ground and sits there, and the roots have no reason to ever leave the comfortable pocket you made. A tree with a compost-only root ball is a container tree that happens to be in the ground.
5. Firm as you go, in layers, with your heel, not by stamping once at the end. You are removing air pockets, not compacting the soil.
6. Water in, 20-30 litres, slowly. This settles the soil around the roots more effectively than any amount of treading.
7. Mulch. A 5-7cm layer of bark or well-rotted compost over a circle a metre across. Keep a clear gap of 10cm around the trunk. Mulch piled against bark keeps it wet and rots it.

Staking — low, not high

A stake is there to stop the root ball rocking in the wind, not to hold the tree upright. The tree needs to sway: movement is what tells the trunk to thicken.

Drive a single stake in at an angle of about 45 degrees, into undisturbed ground outside the root ball, leaning into the prevailing wind. Tie once, low, no more than a third of the way up the trunk, with a proper adjustable tree tie and a spacer between stake and bark.

Check the tie twice a year. A tie left to bite into the bark will kill a tree that survived everything else.

Remove the stake after two growing seasons. A tree still dependent on a stake after three years has been prevented from developing a trunk that can stand on its own.

Planting on wet ground

If your drainage test left water standing after four hours, planting at ground level will drown the tree. There are two honest options.

Mound planting. Build a low mound of soil 30–40cm high and about 1.5m across on top of the existing ground, and plant into that. The root ball sits above the water table. It looks unusual for the first two years and then disappears as the mound settles and grass grows over it.

Choose a tree that wants wet feet. Alder, willow, birch and native oak all cope with ground that would kill a Japanese maple or an olive. There is no shame in changing the plant instead of the site.

What does not work: digging a deeper hole and filling it with gravel or compost. On clay, that is a bucket with no outlet. It is the most common piece of bad advice in circulation and it kills trees reliably.

The first two years

This is where trees are won and lost, and it is the part nobody budgets for.

Water

One deep soak a week from April to September, 20–40 litres depending on the size of the tree, for two full summers. Not a daily sprinkle — light watering keeps the roots at the surface where they dry out fastest. Slow and deep, at the base, so it reaches the root ball rather than running off.

Rain does not count for the first year. A newly planted root ball is a narrow column of compost in the middle of soil it has not yet grown into, and a canopy sheds rain outwards like an umbrella. Ground a metre away can be soaked while the root ball is dry.

Keep grass off it

A metre of clear, mulched ground around the trunk for three years. Grass competes for water far more aggressively than most people expect, and a strimmer against young bark does in ten seconds what nothing else will.

Feed in spring, not autumn

A general slow-release feed in March or April. Feeding late pushes soft growth into the first frosts.

Watch for rock

If the trunk moves independently of the soil at its base in wind, the root ball is still loose and the fine roots are being torn each time. Firm the soil and re-check the stake.