Is there enough rain for my trees?

You look out the window in the morning. The street is still glistening, the flowerbeds look dark and damp. Your young Norway maple stands in the garden and you think: "I don't need to water it today." This intuition is understandable, but often wrong.

Two factors prevent rainwater from reaching the tree as we expect: 

• Dry soil absorbs water poorly: When it rains on dry or compacted soil, only a small portion of it seeps deeper into the ground. The rest runs off the surface or evaporates.
• The umbrella effect of the tree canopy: The dense canopy of an established tree can deflect a considerable portion of the rainwater. This then drips onto the ground outside the trunk area. 
  

Rainfall doesn't automatically mean your tree has enough water in its root zone. What's much more important is whether enough water actually reaches the root zone of your tree. 

When do I need to water the water despite the rain?

The answer depends heavily on whether you are dealing with an established tree or a young tree, and on how much rain has actually fallen.
Established trees with deep, extensive root systems can better withstand moderate periods of drought. Their roots can reach water layers stored deeper in the soil. Nevertheless, they too suffer during prolonged dry spells and need support.

Young trees However, plants typically grown within 3-5 years of planting have not yet developed a robust root system. Their root space is small, and their buffering capacity is low. Even a short dry period can become a problem for them.

Here is an overview of different types of rain and whether trees still need watering:

How much rain does a tree actually need?

Meteorologists measure rainfall in millimeters, but what does that mean for your garden? 1 mm of rain means 1 liter per m², measured on a flat surface without runoff or evaporation.
A newly planted sapling needs roughly 75–100 liters of water per week during its growth phase (April to September). With an irrigation area of ​​approximately 1 m² around the trunk, this equates to 75–100 mm of rain that actually soaks in.

A brief shower with 3 mm of precipitation delivers just 3 liters to this square meter. That's not even a tenth of the weekly requirement. And this rain still has to overcome the canopy effect of the treetops, ground resistance, and evaporation.

Here's an example:
Weekly requirement for a young tree: approx. 80 liters
Rainfall this week: 12 mm
Of this, actually available (approx. 50% after crown & drain): ~6 liters
Deficit: ~74 liters – the tree urgently needs support.

Accurately estimating rainfall: How do you measure rainwater in the garden?

There are three common methods for estimating how much rain has actually fallen in your garden. They differ significantly in accuracy and effort.

1. The spade test: Dig up the soil 10–15 cm deep and feel it with your hand: Is the soil still dry and crumbly? Then the water hasn't penetrated deeply enough. Quick and intuitive, but inaccurate.

2. Weather app & rain radar: Apps display measurement data from the nearest weather station, often several kilometers away. Local microclimates and shading from buildings and tree canopies are not taken into account.
This solution is also quick and intuitive, but inaccurate.

3. Own rain gauge: A simple measuring cup or rain gauge placed in the garden in an open area will show you the exact amount of rain that has fallen, down to the millimeter. The measuring cup should be placed in an open, uncovered spot in the garden, ideally at or just above lawn level. After each rainfall, you can note the reading and compare it to your tree's weekly water needs. If the deficit is too great, you can water the tree.

Watering trees correctly: Slowly and thoroughly instead of superficially

Ideally, trees are poured slowly and deeplySmall amounts of water often remain in the top few centimeters of soil and evaporate quickly. The deeper roots are often not reached.

The most important thing is the slow release of water: If you simply pour with a watering can, you risk the water running off the surface before it can sink in, especially in compacted or dry soil.

This is exactly what it's suitable for. irrigation bags What's particularly good is that they release the water drop by drop into the soil over several hours. This allows significantly more water to actually reach the root zone instead of running off the surface. Simply fill it once, attach it to the trunk, and the water will drip into the tree's root zone.

The interaction of water and soil

How much water reaches the tree also depends heavily on the soil. Loose, living soil helps to absorb water better and keep it available for longer. At the same time, active soil life supports the supply of nutrients to the roots.

Therefore, it's worthwhile to consider water and soil as a system. In this article, you'll learn more about this. Interaction of water, soil and nutrients. 

Collect rainwater

Collecting rainwater makes it much easier to bridge dry periods. Rain barrels, cisterns, or other storage systems help to capture precipitation and use it later for targeted irrigation. Another advantage: rainwater is usually lower in calcium than tap water and follows the natural water cycle. At the same time, valuable drinking water is conserved.

You can find more information in the article. Using rainwater for trees: Why every drop counts.

Conclusion & Checklist: Watering trees after rain

Whether the rain is sufficient depends on factors you can't assess with the naked eye: amount of rain, rain intensity, soil type, crown volume, and the age of your tree. The simplest solution: measure instead of guessing.

Your checklist to check if the rainwater is sufficient

• Note the crown: Did the tree canopy shield the rain? Especially with dense canopies, hardly any water should reach the underside of the tree.
• Check rainfall amount: Read the rain gauge. How many millimeters actually fell?
• Assess rain type: Was it a short, heavy downpour (runoff) or a long, persistent drizzle (seepage)?
• For young trees: Less than 10–15 mm of effective rainfall? Then water again today – at least 30–50 liters directly at the base of the tree.
• Spade test as confirmation: Dig 10 cm deep and feel the soil. Is it still dry and crumbly? Then the tree needs additional water.