Charles Square is being revitalized, and Wenceslas Square is also undergoing changes, with new tree-lined avenues returning there. How does Prague generally approach urban greenery today?

A shift has been visible in recent years. The city no longer views greenery merely as an aesthetic accessory, but increasingly as part of the infrastructure that influences the climate, quality of life, and the functioning of public space. It’s good that projects are emerging where trees are returning to places that were previously mainly paved or dominated by traffic. At the same time, however, planting alone is not enough. What matters most is whether the tree is given a chance to thrive in the long term.

What do you see as the biggest shortcomings in urban tree care?

We often focus on what is visible above ground—that is, how many trees we plant or how a park looks after renovation. But what is not visible is often much more important. In the city, common problems include insufficient root space, poorly designed surfaces, conflicts with utility lines, or inadequate post-planting care. The tree may stand in place, but in the long term, it lacks the conditions it needs to grow. It then either dies or simply stops growing.

In your opinion, what should a park look like that will still be thriving decades from now?

Such a park should be based on three fundamental principles. The first is quality root space—that is, soil that allows trees to grow, absorb water, and get enough air. The second is smart water management. In the future, it will become increasingly important to retain rainwater right where it falls and divert it from paved surfaces into green spaces instead of letting it flow uselessly into the sewer system. The third key point is species diversity. Today, cities often rely on just a few tree species, which is a risk in the long term. All it takes is a change in conditions or the arrival of a disease, and a large portion of the plantings can begin to fail simultaneously. A more diverse mix of trees therefore means greater resilience to climate change, pests, and diseases. If we succeed in combining these principles, urban greenery can function stably over the long term while significantly improving the environment in which we live.

Preparatory work has begun at Charles Square. What exactly is happening there now, and why are you using a so-called air spade?

Most of the work takes place underground, so passersby hardly notice it, but it is crucial for the future of the trees. The goal is to improve conditions for the trees, specifically to get more air and water to the roots. The problem today is that the soil here has been compacted over the long term by the intense movement of people and machinery. Water runs off quickly after rain, and only a small portion reaches the roots. The current work on Charles Square is intended to systematically change this situation.

The air spade is a technology based on a very powerful stream of compressed air that literally “blows out” the soil from the space around the roots. The air naturally finds its way between the firmer structures, so it blows away the fine material, but the roots remain undamaged. This is a fundamental difference compared to traditional digging.

Why is compacted soil such a critical problem?

Soil compaction is one of the biggest barriers to tree growth in cities, even though it’s rarely discussed. If you imagine a tree in nature, say in a forest, there’s a layer of loose soil under the canopy filled with leaves, organic debris, earthworms, and other soil life. The soil is aerated and alive. In the city, however, the soil gradually becomes compacted by human traffic, maintenance equipment, and sometimes even cars. Inside the compacted soil, there is a lack of oxygen; its structure breaks down, and the soil hardens. Roots often cannot penetrate it, and water has nowhere to seep. The tree then essentially has no access to basic resources.

At Charles Square, some trees have even been relocated. What does such an operation look like?

Transplanting a mature tree is a relatively intense and time-sensitive process. Trees in the city are usually not prepared for transplanting, as they are in ornamental nurseries, so it is necessary to proceed quickly and precisely. Here, too, we used an air spade—to expose the roots so that the tree could be transplanted in what is known as a bare-root form, that is, without a root ball. For example, one mahaleb tree, a type of cherry, about ten–twelve years old, was moved about 50 meters. It was growing in a spot where an intersection is set to be expanded, so it had to be relocated. The entire move must take place very quickly, within a single day at most. The roots must not dry out, so they are watered intensively throughout the process. Even so, there remains a certain risk.

Economically, it would be easier to plant a new tree, but the goal is to preserve as many valuable specimens as possible. That is why trees are selected that have a high chance of surviving the transplant, and they are subsequently given intensive care.

What makes the trees on Charles Square so special?

It’s not just about individual trees, but about the whole. Charles Square has an exceptionally strong and historically layered composition—the park’s green framework—which has evolved over time and remains visible to this day. Here we find striking solitary trees, such as the historic plane tree, which captivates with its size and shape. At the same time, however, there are older layers of plantings that the casual visitor won’t recognize at first glance. These include, for example, the elms from the period before František Thomayer’s renovations—trees that are more than a century old. Today, these do not appear as massive as other trees, partly because they were partially buried in the past, which slowed their growth. Yet this is precisely what makes them interesting; they have survived for a long time under adverse conditions and are proof of just how resilient certain species can be.

How do trees influence a city’s microclimate?

Very significantly. Trees are among the most effective tools for mitigating urban heat islands. They shade surfaces that would otherwise heat up, while actively cooling the surrounding area through water evaporation. When converting water into water vapor, they absorb solar radiation that would otherwise be reflected as heat radiation. This effect is not limited to the area directly beneath the tree; it also affects the wider surroundings, and in larger parks, it extends into the surrounding streets.

How does climate change factor into this? Which trees have a future in the city, and which do not?

Climate change is fundamentally transforming how we think about urban plantings today. The conditions that existed thirty or fifty years ago simply no longer apply—there is greater drought, higher temperatures, and weather extremes are more frequent. It is therefore becoming apparent that some traditionally used species in urban environments are gradually failing, even if you improve their soil conditions. A typical example is the sycamore maple, which is often dying in streets and parks today. Conversely, it is becoming clear that some tree species are significantly more resilient to these stresses. These include, for example, plane trees, oaks, elms, and hackberries. Urban greenery is always planned well in advance. A tree planted today will continue to shape the appearance of streets and squares for generations to come. This makes it all the more important to make decisions with a long-term perspective, not just based on current needs.