Watering well: less often, but for longer

A garden that gets a little water every day is a thirsty garden. It's better to water less often, but for longer: the water goes deep, and the roots follow it.

Why water long rather than often

A light watering only wets the top 3 or 4 centimetres of soil. But roots grow where the water is. If the water never reaches 20 cm, the roots stay near the surface, where the soil heats up and dries out fastest. The result: you water every day, and the plant still wilts in a heat wave.

With deep, less frequent watering, the water goes down. The surface dries, and the roots go down after the water, to 20 or 30 cm. Aim for that depth every time: for example, 10 to 20 litres poured slowly for a shrub planted this year.

Add mulch to keep that water in the soil. 5 to 8 cm of mulch cuts evaporation in half and keeps the surface from crusting over, which makes water run off. Leave 5 to 10 cm free of mulch around the base of perennials and the trunk of trees: mulch piled against the stem makes it rot and attracts voles in winter.

Where should you water a tree or shrub? The first year, water on the root ball, at the base. Once the plant is well established, its roots spread all around it, up to a metre beyond the tips of the branches. That's the area to wet, not the trunk: a trunk that's always damp rots.

How to tell if you need to water

Before watering, the finger test. Push a finger into the soil up to the second knuckle, about 5 to 7 cm. If the soil is cool and moist at that depth, don't water.

After watering, the screwdriver test. It tells you how far the water went: a 15 cm screwdriver slides in almost by itself in wet soil and gets stuck in dry soil. In clay, it gets stuck even in moist soil: first try it in a spot you've just watered, to compare.

In an established flower bed, water until the screwdriver goes in easily.

• In ordinary soil (neither sandy nor clay): in July, one long watering every eight to ten days.
• In sandy soil: two shorter waterings in the same period.

Don't overestimate the rain. A 5 mm shower barely wets a centimetre of soil: it doesn't count, even if the pavement is soaked. Only count rains of at least 15 mm. And under an overhang, along a wall or under a cedar hedge, much less rain gets through.

Water in the morning

In the middle of the afternoon, some of the water evaporates before it soaks in. More importantly, foliage wet in the evening stays wet all night, and several leaf diseases (black spot on roses, septoria, anthracnose, downy mildew) need a few hours of moisture to take hold. Watered in the morning, the foliage dries within an hour or two.

Contrary to a common belief, water droplets in the sun don't burn leaves.

Powdery mildew (a white powder on the leaves, common on phlox and lilac in August) is the exception: it develops in humid air, not because of watering. It's more related to poor airflow between plants and to the variety chosen.

If you can only water in the evening, water at the base, without wetting the foliage.

The first season: keep an eye on the root ball

A newly planted plant has all its roots in its root ball, which is made of potting soil. That potting soil doesn't take up water at the same rate as garden soil: the root ball can stay dry in the middle of a watered bed. That's why you make a basin over the root ball when planting: the water has to land right on it.

• The first four to six weeks: forget the calendar. Check the root ball with your finger every two or three days, and water as soon as it's dry 5 cm down. At 30 °C, that will often be every other day.
• Once new roots grow out of the root ball, toward the end of summer: one watering a week if it doesn't rain, two in a heat wave.

Amounts to plan for:

• 5 to 10 litres for a perennial;
• 10 to 20 litres for a shrub;
• for a tree, about 10 litres per 2.5 cm of trunk diameter, so 20 to 40 litres for a 5 to 10 cm trunk.

Plan on about two seasons of this care for a shrub, three for a tree, and more if it was big at planting.

After that, it depends on the plant. Once well established, coneflower, yarrow, liatris, fescue and juniper get through a normal summer in zone 5 without watering. Hostas, astilbes, hydrangeas, phlox and dogwoods need water even when mature: a single thirsty day in a heat wave can scorch the leaves of an astilbe or hydrangea for the whole season. Coral bells, on the other hand, mostly fear standing water.

A droopy plant isn't always thirsty

A plant can also wilt because it has too much water: in soaked soil, roots run out of air and stop absorbing anything. The symptoms look like thirst. So always put your finger in the soil before watering:

• the soil is soaked: stop watering. Yellowed lower leaves and a sour smell confirm there's too much water;
• the soil is dry 5 cm down: water deeply. If the plant only started wilting recently, it should perk up within the day.

Don't stop watering in September

The watering season ends when the ground freezes, not when the heat ends. Conifers, boxwood and rhododendrons lose water through their leaves all winter, while their roots, stuck in frozen ground, can't take any in. A dry root ball in November means scorched foliage in March. The "winter burn" people blame on cold is almost always caused by a dry fall.

So keep watering in fall if it doesn't rain, then give one slow, deep watering in late October or early November, while the water can still soak into the soil:

• 20 to 40 litres for a shrub or young tree;
• about 10 litres per metre of cedar hedge.

This matters most for plants that keep their leaves in winter and for everything planted that year. Once the ground is frozen, you're done until spring.

In short

• Water rarely, but for a long time: roots only go deep if the water does. Mulch keeps that water in the soil.
• Your finger at 5 cm tells you whether to water; the screwdriver tells you how far the water went. A 5 mm shower doesn't count.
• Water in the morning, so the foliage doesn't stay wet overnight.
• For the first four to six weeks, check the root ball every two or three days. Once established, many full-sun perennials get through summer without watering.
• One last deep watering before the ground freezes protects conifers and the year's plantings.