Why young trees need a helping hand

A newly planted tree is a bit like a house guest who has just arrived: it is upright, presentable, and quietly panicking underneath. The rootball you eased into the ground occupies only a fraction of the soil around it, and the fine feeding roots that do the actual drinking are still confined to that small volume of earth. Until those roots venture outwards, the tree depends almost entirely on the moisture you provide.

A mature oak or ash will ride out a dry spell by drawing on water metres down and metres out. A whippet-thin sapling planted last November has no such reserves. That is why the first two or three summers after planting are the make-or-break period, and why a long, slow soak does far more good than a quick flick of the watering can. Deep watering pulls roots downwards, into cooler, damper soil that holds its moisture long after the surface has turned to dust. Little and often does the opposite, coaxing roots upwards into the very layer that dries out first.

How much water, and how often

For most garden and hedging trees, 10 to 20 litres per tree, twice a week in dry weather is a sensible starting point. In a genuine heatwave, or on light sandy ground, you may need to water every other day. On heavy clay, which holds water stubbornly, once a week may be plenty.

• Small trees and whips: around 10 litres per watering.
• Standard trees (2–3m): 20 litres or more.
• Larger specimens, or trees in pots before planting: up to 30 litres, applied slowly.
• Sandy or gravelly soil: water more often, in smaller but still generous doses, as water drains away fast.
• Bare-root trees planted in autumn: they establish quicker and need less fuss the following summer, though they are not indestructible.

Container-grown trees are the thirstiest of all. Their roots were trained in a pot and often circle rather than spread, so they are slower to leave the planting hole and explore.

The slow soak method

The trick is not the volume but the pace. Water delivered faster than the soil can absorb it simply runs off across the surface, which is why you so often see a puddle at the base of a tree and bone-dry soil an inch away.

• Water with a can fitted with a rose, or with the hose on a trickle, letting it run for several minutes rather than seconds.
• Make a shallow ring of soil, a few centimetres high, around the tree about 40cm out. It acts as a basin and stops water escaping.
• Forget the can altogether if you like: pierce a few small holes in the base of a bucket, set it beside the trunk, fill it, and walk away. It will empty itself over half an hour or so.
• Water the rootball area, but also the soil a little beyond it, so roots are tempted outwards.
• If the ground has baked hard, prick it over gently with a fork first, then water. Otherwise most of it will sheet straight off.

Timing, mulch and the things people forget

Water in the early morning or evening, when evaporation is at its lowest and the soil has a chance to drink properly. Midday watering wastes a surprising amount to the air.

A mulch of bark chippings or well-rotted compost, spread 5–7cm deep and kept a clear 10cm back from the trunk, will cut your watering by half. It keeps the soil cool, suppresses weeds and slows evaporation. Do not mound it against the bark, as that invites rot and disease.

Then there is competition. Grass, nettles and weeds will happily take the water you meant for the tree, so keep a metre-wide circle clear around each young trunk for the first few years. Tree guards and stakes are useful, but check ties in midsummer, when a tight tie can strangle a swelling trunk.

How to tell whether you have watered enough

Push a finger, or a trowel, about 10cm into the soil beneath the mulch. If it feels cool and damp, you have done your job. If it is dry at that depth, the water has not got through, and another slow soak is called for.

Watch the leaves, too. Wilting, curling, browning margins and premature leaf fall all point to drought stress. That said, a permanently soggy rootball can produce similar symptoms, so always test the soil before reaching for the can again.

Watering through the first few summers

Plan on watering attentively for the first two summers, and keep an eye on trees for a third. After that, most will have rooted widely enough to fend for themselves, though a newly planted tree in a hot, exposed spot may still appreciate a soak in a prolonged drought.

If a hosepipe ban is announced, check the rules: watering cans are usually permitted, and newly planted trees are often given an exemption for their first year or two. Rainwater butts and bathwater are reasonable substitutes, so long as you go easy on anything heavily soapy or bleached.

Above all, be patient. A tree planted well and watered deeply through its early summers will reward you for decades, long after the watering can has been put away for good.