Why dry spells hit trees harder than they look

The UK is not, on the whole, a dry country, and our native trees are well adapted to the sort of summer that delivers a bit of rain most weeks. What causes real damage is not a single hot afternoon but a run of four, six or eight weeks without meaningful rainfall, especially when it arrives alongside drying winds and high temperatures. Lawns turn brown and recover. Trees often show nothing at all until the following season, when you notice smaller leaves, a thinner canopy, dieback at the tips and early autumn colour.

By then the damage was done the previous summer, deep underground. Roots are doing the work of a tree all year round, and the fine feeding roots that take up water sit mostly in the top 30cm of soil. That is precisely the layer that dries out first. Clay soils can crack and bake hard, sandy soils lose moisture within days, and chalky soils drain almost as fast as you pour. Whatever your soil, the practical question is the same: how do you get enough water down to the roots without turning the ground beneath your tree into a bog?

Deep and infrequent beats little and often

The single most useful principle in summer tree care is to water thoroughly and then leave the tree alone. A quick daily sprinkle from a hose does very little except wet the surface, where it evaporates or encourages shallow rooting. You want the water to travel down 20 to 30cm, which takes time and volume.

As a rough guide for a tree in its first three years, aim for around 10 to 20 litres per week in dry weather, delivered in one or two slow soakings rather than a daily dribble. During a genuine heatwave, with hot wind and no rain, a newly planted tree may need 40 to 50 litres a week to stay ahead of losses. Established trees, once their roots have spread beyond the original planting hole, usually manage without intervention, but a mature tree under stress will respond well to a proper soak every fortnight during a prolonged drought.

Check before you reach for the hose

Overwatering kills more young trees than drought does, and it is easily done. Before watering anything, push a trowel or your fingers into the soil about 10cm deep, a hand's width out from the trunk and again at the edge of the canopy. Squeeze a handful.

• Dry and crumbly: water slowly and generously until the soil is damp well below the surface.
• Cool, damp and holding together: leave it alone, even if the surface looks pale.
• Soggy, sticky or smelling sour: stop entirely and let it drain. Roots need air as much as water.

Check two or three spots, because one side of a tree can dry out while another stays wet. Mulched ground holds moisture far longer, so always check beneath the mulch rather than on top of it. Clay soils may still be damp 10cm down a fortnight after rain, while a sandy bed can be bone dry two days later.

Getting water to the roots rather than the lawn

Water needs to arrive slowly, so it soaks in rather than running off. A hose on a gentle trickle, moved around the root zone every twenty minutes, works well. So does a watering can with the rose removed, poured in a ring around the trunk rather than against it. A pierced plastic bottle or a couple of small holes in a bucket, left to empty itself over an hour, is a simple and effective trick for young trees.

For anything planted in the last couple of years, a shallow soil ring or berm around the tree helps enormously, holding water over the root ball until it soaks away. Keep the trunk itself dry and clear. Mulch afterwards, 5 to 7cm deep, but leave a 10cm gap around the bark to prevent rot and damp-related problems.

Young trees, established trees and woodland planting

Prioritise. A tree planted this year is your first concern, followed by anything under five years old. After that, trees generally look after themselves unless the drought is exceptional.

• Newly planted: water weekly, sometimes twice weekly in extreme conditions.
• Three to ten years old: water during any dry spell lasting more than a fortnight.
• Mature trees: leave them be unless they are showing clear signs of stress, such as flagging leaves, heavy coning in conifers or premature leaf drop.
• Woodland plantings: watering is rarely practical at scale. Focus instead on clearing competing vegetation, mulching around new trees and avoiding planting into bone-dry ground in the first place.

Small habits that make a big difference

Timing matters. Early morning is best, when the soil is cool and evaporation is slow; early evening is the next best option. Avoid watering in full sun, and avoid splashing the leaves, which encourages fungal problems without helping the roots at all.

Hold off on feeding during drought, since fertiliser encourages growth the tree cannot support, and avoid heavy pruning while it is stressed. Weeds and grass around the base compete directly for moisture, so a clear ring of bare soil or mulch pays for itself many times over. Water butts, bathwater and washing-up water are all fine for established trees and shrubs, though keep soapy water away from anything newly planted and never store it for long.

Above all, resist the urge to do something every day. A tree given a deep, occasional soak and left to dry out between waterings will grow stronger roots, stand up better to the next dry spell, and need less of your attention with every passing year.