How to water a mature tree in its first year

A newly planted mature tree has a large crown and roots limited to the rootball. During the first year, watering needs to keep that volume functional while the new roots start to occupy the surrounding soil.

There is no universal calendar. Frequency depends on the species, the size of the rootball, the soil texture, drainage, exposure and climate.

The safest way to adjust watering is to check the moisture in the ground. The surface alone does not show what is happening next to the roots.

After backfilling, a deep, soaking watering is given.

The water helps to settle the soil, reduce air pockets, stabilise the rootball and create contact between roots and ground.

The volume should pass through the whole root mass. A quick pass over the surface does not do this job.

Even where there is an automatic system, the initial watering by hand may be needed to make sure that every side receives water.

Small volumes every day keep only the top few centimetres wet. The roots of a mature tree lie well below that layer.

It is better to apply enough water to wet the rootball and let the soil drain and recover oxygen before the next application.

The interval varies. In sand and heat, it can be short. In clay and cool weather, it can lengthen considerably.

‘Deep’ does not mean flooding continuously. The aim is to reach the roots without keeping the soil saturated.

At Diaplant we recommend opening small observation points next to the rootball, about 20 cm deep, to see whether the water is getting there and how the ground drains.

The check should be made on more than one side. An emitter may wet only one area.

You can also look at the soil taken out: whether it holds its shape, whether it is sticky, dry or has an anaerobic smell.

The assessment does not need to be daily throughout the year, but it should be frequent when the system is being adjusted or when the weather changes.

Wilting, dry leaf margins and leaf drop can indicate a dry rootball. The same symptoms can appear when the roots are saturated and without oxygen.

Before increasing the programme, the cause is confirmed.

Soil that is dry at the surface can hide water held below. A wet surface can hide the inside of a rootball that is dry because of a difference in texture.

Watering by symptoms without checking the soil is one of the most common ways of making the problem worse.

Birches, magnolias and other trees with high water loss need particular attention. Evergreens keep their leaves in winter and go on transpiring.

A dense crown, or one very exposed to wind, needs more water than a leafless deciduous tree of the same size.

The response also depends on root preparation. A well-rooted rootball uses water differently from a mass with few fine roots.

In sand, water infiltrates quickly and the reserve is small. It may be necessary to water more often or to split the volume to avoid loss below the root zone.

Organic mulch reduces evaporation.

The emitters should distribute water over several points, because a single spot creates a narrow wet column.

Clay holds water and drains more slowly. Frequent watering can keep the rootball saturated.

The interval tends to be longer, but the application still needs to be long enough to wet the whole mass.

Compaction and impermeable layers should be checked, because they change the behaviour a great deal.

Mature trees should have their own irrigation zone or some form of independent control.

Using the same run time as the lawn or the perennials is rarely suitable. Sprinklers can wet the crown and the surface without supplying enough volume to the rootball.

Drippers or rings need a known flow rate, distribution and inspection. As the roots grow, the wetted area is widened outwards from the base.

Rain does not automatically replace watering. A short shower may not get through the crown and the rootball.

After prolonged rain, the programme should not be kept running without checking drainage either.

Sensors and weather data help, but observing the soil still decides.

In hot spells, the need increases. The response should not begin only when the leaves wilt.

Watering at cooler times of day reduces losses and allows greater infiltration. Water next to the trunk should not keep the bark permanently wet.

Wind can dry the crown even at moderate temperatures.

In winter, frequency is generally lower, but the rootball should not dry out.

In spring, bud break increases demand. In summer, heat and wind make monitoring critical. In autumn, watering is reduced in line with rain and activity.

The programme should change through the season and not stay fixed from planting onwards.

The roots have already begun to explore the soil, but the tree is still vulnerable in hot periods.

The volume can be applied further from the base, encouraging expansion. Frequency is reduced gradually, without going from regular watering to abandonment.

Autonomy arrives progressively and depends on the soil and the species.

Noting volumes, frequency, rain and symptoms helps you to understand what works.

In projects with shared maintenance, the record prevents contradictory changes by different teams.

The best watering is not the most frequent. It is the one that supplies enough water, allows oxygen in and keeps pace with the real expansion of the roots.

• Post-transplant stress
• Waterlogging
• How to schedule irrigation for a garden

1. European Arboricultural Standards. European Tree Planting Standard.
2. Watson, G. W. — literature on soil moisture and water uptake by roots.
3. Diaplant’s method for post-planting aftercare.