How often should you water newly planted trees and shrubs?

Most newly planted trees that die have in fact been watered. They have just been watered a little and often, so the top few centimetres got wet while the root ball stayed dry a hand's breadth further down. Water deeply and rarely instead: 30 to 50 litres (8 to 13 US gallons) per tree, 10 to 20 litres per shrub, once a week through the growing season. And keep it up for three seasons, not one.

Gardian is a Swedish garden app, and the advice here is the same wherever you garden. Only the end of the season moves: where the ground freezes, the watering stops when it does, and where it never freezes, a dry winter can need a can or two as well.

Why isn't a little every evening enough?

A newly planted tree has almost no roots outside its root ball. It grew up in a nursery, the ball was dug up, and a large part of the root system stayed in the ground there. What now stands in your garden can only drink from a volume of soil about the size of a removal box.

That volume is what you are watering, not the border round it. Pour a splash over the surface every evening and you wet the soil next to the ball, and a tree can perfectly well die of thirst in wet soil. Give the whole dose at once and the water instead soaks down through the ball and a little below it, and the roots follow it down. That is the whole point of watering rarely. You teach the roots to search.

Build a low ring of soil just outside the root ball, ten centimetres (4 inches) high. Without it a good part of the can runs off over the surface while you stand there watching, and you think the tree got twenty litres when it got ten. The ring is the one thing I always rebuild when it has collapsed.

If 20 to 30 millimetres (about an inch) of rain falls during the week, skip that week's watering. Less than that does not count.

How do you know if the root ball is dry?

Don't guess. Dig. Push a trowel in ten centimetres right next to the ball and feel the soil you bring up. Dark and moist means wait. Pale and crumbly means water now.

It takes ten seconds and it is the only reading that means anything, because the surface can be bone dry while the ball is soaked. A layer of bark mulch or grass clippings on top, five centimetres thick but never right up against the trunk, keeps the moisture in much longer between waterings.

Warning: If the tree looks limp while the ground is wet, it is not thirst. It is lack of oxygen at the roots, and more water makes it worse. Stop watering and let the soil dry out before you do anything else.

How long do you have to keep going?

Two to three growing seasons. That is where almost everyone gives up, because the tree looks healthy after the first summer and so they stop. The second summer the roots are still mostly inside the ball, and that is the summer the tree dies if it turns dry.

One thing worth knowing before the drought comes: a watering ban usually stops hoses and sprinklers, not the watering can. In Britain a hosepipe ban leaves the can allowed, and the Swedish water company VA SYD says outright that watering with a can is fine during its bans too. The rules are set by your water company or town and they differ, so read theirs before you need them. Five cans for a newly planted tree is water well spent that summer.

Do not stop when autumn comes. A tree or shrub planted in late summer or autumn should get its weekly dose as long as the ground has not frozen, unless the rain does it for you. Evergreens like arborvitae (thuja), yew and rhododendron need it most, because they keep losing water through their needles and leaves all winter, and a dry autumn is what turns them brown in March. What else a young tree needs before winter is in the article on protecting young trees.