How to properly water old trees: Why even large trees suffer from drought

Old trees often appear powerful and indestructible. With their mighty crowns, deep roots, and thick trunks, they seem to withstand every summer. For a long time, the simple rule was: young trees need watering, and old trees take care of themselves. But this rule of thumb is increasingly no longer true.

Climate change is altering conditions. Increasingly long periods of drought, lack of rain, hot summers, and sealed soils are taking their toll even on trees that are decades old. 

This is a particular problem in cities: asphalt, paved surfaces, and compacted soil prevent rainwater from even reaching the roots. Instead of slowly seeping into the ground, it runs off the surface and ends up in the sewer system. In this article, you'll learn why old trees suffer from water shortages despite deep roots, how to recognize drought stress, and how to properly water large trees.

Key facts at a glance

• Even old trees are increasingly suffering from drought stress.
• Deep roots are only helpful if there is water present there.
• Urban trees are particularly at risk from sealed and compacted soils.
• Large trees need infrequent but intensive watering.
• Slow drip irrigation is significantly more effective than short, shallow watering.
• Mulch, soil life, and a healthy root zone significantly improve water supply.

Why old trees are so valuable

A large, old tree provides shade, cools its surroundings through evaporation, stores CO₂, filters particulate matter, and offers habitat for numerous animals and insects. On hot days, the cooling effect of a large tree can be equivalent to that of several air conditioners. Old trees have a far greater ecological benefit than young trees. Furthermore, they are witnesses to history and have their own stories to tell. It is therefore of particular importance to us that old trees also survive the hot and dry summer months.

Why old trees suffer more frequently from drought today

Many people assume that old trees have no trouble finding water because their roots reach deep into the ground. This is generally true. However, what matters is not just the depth of the roots, but whether there is still sufficient moisture present at that depth.

Previously, deeper [levels] could [become] Soil layers are sufficiently replenished with water by autumn and winter rains. Today, bleThese reserves often dry out for years. At the same time, temperatures rise, evaporation increases, and heat waves last longer.

This is particularly problematic in cities: there, soils are often compacted or sealed. Rainwater can hardly seep in. Instead, it runs off via streets and sidewalks. At the same time, cities heat up considerably. This causes additional water to evaporate from the ground.

Furthermore, old trees have enormous canopies. On hot summer days, they transpire hundreds of liters of water. If the water supply is insufficient for an extended period, the tree suffers from drought stress.

The root dilemma: Deep does not automatically mean moist

Deep roots were long considered a natural protection against drought. However, during extreme droughts, even deeper soil layers now dry out. Furthermore, many important fine roots are not exclusively located deep underground. They often lie in the upper soil layers because that's where oxygen, nutrients, and water are usually available. But it is precisely these upper layers that dry out particularly quickly.

If the soil is also compacted, water cannot penetrate as well. This explains why even large and seemingly healthy old trees can suddenly show damage or deteriorate significantly within just a few years.

How to recognize drought stress in old trees

Drought stress It often develops gradually. Many damages only become visible months or even years later. That makes it all the more important to recognize warning signs early.

• Premature leaf drop: The tree sheds its leaves in the middle of summer, even though it's still the growing season. This reduces evaporation and helps conserve water.
• Small or yellowed leaves: When water is scarce, the tree often produces smaller leaves or leaves turn yellowish and dry.
• Deadwood in the crown: Dead branches in the upper crown area are among the classic signs of chronic water deficiency.
• Bark cracks and sunburn: Dry bark becomes brittle and susceptible to damage. Cracks or dead patches develop, especially on the sides of the trunk exposed to strong sunlight.
• Heavy seed or fruit set: Many trees respond to stress by increasing seed production. Biologically speaking, this is a survival mechanism for reproduction.
• Pest and fungal infestation: Weakened trees are more susceptible to pests and diseases. Bark beetles, wood-decaying fungi, and leaf diseases are particularly common.

Important: Don't wait until the tree shows significant damage to water it. Water at the latest when there has been little to no rain for two to three weeks and temperatures are high.

How much water does a large tree need?

Mature trees require significantly more water than young trees. At the same time, frequent, shallow watering is of little use. What is crucial is that the water can penetrate deep into the soil. As a rough guideline:

• Young trees: 75 to 150 liters per week
• Medium-sized trees: 150 to 300 liters per watering.
• Old, large trees: 300 to 500 liters or more per watering.

More important than the exact amount, however, is the method of watering: the soil should be moistened slowly and deeply. Brief watering with a garden hose often only results in the water running off the surface or evaporating. 

Properly watering old trees

Here you'll find the most important rules for watering trees properly. If you're interested in this topic, you'll find even more information in this article. Watering trees.

Pour slowly, not quickly.

Dry soils absorb water poorly. Experts refer to these as hydrophobic soils. Applying large quantities of water to dry soils in a short period of time often only creates puddles or surface runoff.

A much more effective method is a slow and continuous release of water over several hours. This allows the water to gradually seep into deeper soil layers and reach the fine roots. Irrigation bags or drip irrigation systems are particularly helpful in this regard. irrigation bags They release water slowly over several hours, allowing significantly more water to soak into the root zone. Especially with older trees, several connected watering bags can be useful to ensure even watering of the area under the canopy.

Don't just water the trunk.

Most fine roots are not located directly on the trunk, but in the area below the drip line of the canopy. Therefore, water should be distributed as broadly as possible over the root zone. If you are using tree watering bags, you can secure them with a stake in the root area.

Water infrequently but intensively.

It's better to water infrequently and thoroughly than to water with small amounts daily. Deep watering promotes that water actually penetrates deeper soil layers and remains available for longer.

When should you water old trees?

The ideal times are early morning or evening. At midday, a large portion of the water evaporates from the surface before it can even seep in. Additionally, hot soils can cause temperature stress.

More than just water: Improving the soil

Water is important. But it's part of a system. That's why we recommend considering the larger system as well. We have an article about this topic. Interaction of water, soil and nutrients Written as a combination of factors. Here are the most important steps you can take to support your tree:

mulching

A layer of mulch It protects the soil from direct sunlight and significantly reduces evaporation. Suitable materials include wood chips or leaf mulch. It's important to maintain some distance from the tree trunk.

Promote soil life

Living soil retains water better and remains looser. Compost, Worm compost Organic fertilizers promote earthworms and microorganisms. These improve soil structure and increase water retention in the long term. Organic fertilizers based on worm compost are particularly beneficial, as they not only provide nutrients but also activate soil life.

Collect rainwater and use it for trees

Rainwater is available free of charge, usually contains less lime than tap water, and is part of the natural water cycle. Especially during dry summers, collected rainwater can help supply trees even when there is no rainfall for extended periods.

Even a simple rain barrel can collect surprising amounts of water. Cisterns or other rainwater storage systems offer even greater storage capacity. The collected water can then be used specifically for watering trees. You can find more information in the article. Using rainwater for trees: Why every drop counts.

Frequently asked questions about watering old trees

Isn't there enough rain for my old tree?

During most months of the year, the rain provides enough water for old trees. this article You can find out more about this question.

How often should you water old trees?

Old trees need less frequent watering than young trees, but require significantly larger amounts of water each time. During prolonged dry periods, intensive watering every two to four weeks can be beneficial. The crucial factor is that the water penetrates deep into the soil.

How much water does a large tree need?

Large, old trees often require 300 to 500 liters of water per watering. With very large crowns or during extreme dry periods, the requirement can be even higher.

When is the best time to water?

The ideal times are early morning or evening. By midday, a large portion of the water has already evaporated from the surface.

Where should you water an old tree?

Not directly at the trunk. Most of the fine roots are located in the area below the tree canopy. The water should be distributed over as large an area as possible there.

Do watering bags also help with old trees?

Yes. A slow release of water can be particularly helpful in dry or compacted soils. Several watering bags can be combined for large trees.

Is it possible to overwater old trees?

Waterlogging should be avoided. However, too little water is more often the problem than too much. Slow watering with sufficient intervals between waterings is important.

Which trees suffer most from drought?

Many shallow-rooted trees, as well as urban trees on sealed or compacted sites, are particularly affected. Trees with already weakened root systems are also sensitive.