Fallen Leaves in the City: To Rake or To Leave?

Every autumn, leaf removal sparks heated debates. The opinions of citizens, ecologists, and municipal services often clash. Some demand that nature be left alone, while others insist on perfectly clean lawns. The truth, as is often the case in biology, depends on the conditions. To make the right decision for Yerevan, we need to weigh all the arguments, understand how a healthy forest ecosystem works, and recognize how a tree's life in a modern city differs.

The benefits of fallen leaves

In the wild, fallen leaves are a priceless resource. No one rakes them in a forest, and they perform a tremendous amount of work.

First, they are food. As they decompose, leaves return mineral substances to the soil that the tree drew out during the season. This is a natural fertilizer that maintains soil fertility for years.

Second, leaves make the soil loose and allow it to breathe. They are the primary food source for earthworms. By consuming organic matter, worms create thousands of channels in the ground. Oxygen and water reach the roots through these micro-tunnels. If worms have little food, their population will shrink, and the soil will compact and lose its structure more quickly. Furthermore, processed leaves turn into vermicompost, making the earth softer and more friable.

Third, the forest floor conserves water and protects the soil. A layer of leaves acts as mulch: it retains moisture by slowing evaporation during hot weather and absorbs the impact of raindrops during downpours, preventing the erosion of the fertile topsoil.

Fourth, they form a warm blanket. In northern regions with severe winter frosts, bare ground can freeze to a great depth, which is fatal for many soil organisms. But under a 10 cm layer of leaves, the heat rising from the earth is better retained. The soil under such a "coat" freezes less, helping the biota survive the winter. Although winters in Yerevan are milder, this principle of thermoregulation still applies, smoothing out sharp temperature fluctuations.

Fifth, it is a home for wildlife. Thousands of insect species, beneficial fungi, and bacteria, which together form a healthy forest's immune system, spend the winter in the leaf litter.

Sixth, we cannot forget aesthetics and the psychological well-being of city dwellers. Golden autumn is a time when the city is transformed. Strolling on a rustling carpet of leaves in parks, walking through them, or even just kicking them around—all of this reduces stress, lifts spirits, and allows residents of a metropolis to feel a connection with nature. It is an important element of urban culture and a quality living environment.

Harm and risks in the urban environment

However, a city is not a forest. Different rules apply here, and benefits can turn into harm.

First, the urban environment is "fragmented." While a forest is a continuous ecosystem, a tree in the city is often confined to a 1x1 meter square of dirt surrounded by asphalt. Natural processes are disrupted. The situation is exacerbated by Yerevan's dry climate: unlike in a damp forest, here leaves often don't have a chance to decompose, instead drying out and crumbling. Their effectiveness as a fertilizer is thus reduced, and solitary trees often need comprehensive professional feeding, not just dry leaves.

Second, leaves can become a risk factor due to the poor condition of the trees themselves. In the city, many plants are weakened. They suffer from water deficits and often grow in poor soil made of sand and construction debris. They are rarely treated for pests, and improper pruning (topping) further undermines their strength. The combination of these factors with summer heat leads to a state of chronic stress. Such trees have a weakened immune system. For them, fallen leaves, where pests (moths, mites, fungal spores) overwinter, become not a help but a reservoir of infection that can be fatal for a weakened organism in the spring.

Third, the food chain is broken. It's important to remember that many beneficial predatory insects (like parasitic wasps) lead double lives: their larvae eat pests, but the adults feed on nectar. In the city, there are few meadow grasses and flowers, as they are frequently mown. Beneficial insects struggle to find food, and their populations are critically reduced. As a result, there is no one to control the pest populations that use fallen leaves for wintering. In the absence of these "sanitarians," piles of leaves turn into a perfectly safe incubator. Pests survive the winter in full force and attack the trees unimpeded in the spring.

Conclusions

A paradoxical situation has emerged: what sustains a healthy forest can harm a weakened city tree. Leaving leaves in Yerevan's current conditions is risky. The benefit of potential fertilizer is often outweighed by the harm from pests and diseases that have overwintered in the foliage. At the moment, fallen leaves do less to help nature than they do to preserve sources of infection.

Recommendations

Right now, one of the most reasonable supportive measures is the collection and removal of leaves. This reduces the infectious load and physically removes the wintering forms of pests from the base of the trees.

But our goal is not an endless fight against symptoms but the restoration of the ecosystem. The solutions have long been known: proper tree care, moving away from monocultures (like planting only plane trees or elms throughout the city) in favor of species diversity, and creating conditions for birds and beneficial insects. We need to work on bringing back earthworms and mycorrhizal fungi, without which roots cannot function properly, and replacing parts of mowed lawns with wildflower meadows. These technologies must be gradually integrated into the practices of city services.

Until the ecosystem is restored, it is advisable to collect and process the leaves. The ideal option is industrial hot composting.

The principle is simple: a large mass of organic matter heats up from the inside due to the activity of special bacteria. No external heating is required. The internal temperature rises to 60–70°C and stays there for weeks. These conditions kill the vast majority of pest larvae, fungal spores, and weed seeds. The output is a clean, safe compost that can be returned to city parks.

An important caveat: because city leaves act as filters, trapping dust and exhaust fumes, the resulting compost may have elevated concentrations of heavy metals. Such soil cannot be used for growing food (fruit trees, vegetable gardens). However, it is perfectly safe and beneficial for ornamental landscaping—parks, lawns, and greenbelts. Moreover, the organic compounds in the compost bind harmful substances, preventing them from becoming airborne dust again.

An alternative is incineration in special high-temperature facilities with filtration systems. This generates energy and has nothing in common with smoldering bonfires or dumps.

How to know when the ecosystem has recovered?

The transition from removing leaves to leaving them should not be spontaneous. We need to see real signs that the urban environment is ready to "digest" organic matter and control pests on its own.

The first and most reliable indicator is the soil. If you lift a layer of old leaves, you should see moist, dark earth and earthworms underneath. If all you find is dry dust and no life, the system isn't working yet. The leaves themselves also tell a story: last year's leaf fall shouldn't lie like a dry "herbarium." The leaves should be soft and partially decomposed, gradually turning into humus. This is a sure sign that soil bacteria and fungi are successfully processing them.

Another important signal comes from insects. Predators like ladybugs, lacewings, or ground beetles should be regularly seen on plants. Their presence means that pests have natural enemies, and the balance is being restored. Finally, we need to see that trees are no longer suffering from massive and sudden outbreaks of disease without the constant application of chemical treatments. When these signs appear in Yerevan's parks, the practice of leaf removal can be stopped.

What can I do right now?

Even one person can make a difference.

Start by preventing leaf burning. This is a harmful practice that pollutes the air in our courtyards. If you see janitors or neighbors burning piles of leaves, don't just walk by. Explain to them why it's dangerous for their health and tell them that leaves should be collected in bags for removal.

Contribute to leaf collection. Yes, right now, the leaves collected in bags will most likely go to a landfill. But this is a necessary stage. The city cannot launch a composting facility until a stable and large-scale flow of raw material is established. By collecting leaves today, we are creating the foundation for their ecological recycling tomorrow. Without this constant stream, composting simply won't start.

Help the trees with water. In Yerevan's climate, water is life. A bucket of water poured under a tree near your building during the summer heat will do more good than a ton of dry leaves left on the ground.

And finally, support biodiversity. On private property or in an enclosed courtyard, you can hang birdhouses for tits and shelters for beneficial insects. Try seeding an open patch of ground not with regular grass but with a meadow lawn seed mix—they are easy to find on online marketplaces. This is a feasible but very valuable contribution: by bringing back flowers and shelters, you help restore the ecosystem of the entire neighborhood. The benefit from such "islands of life" is enormous. By the way, we have detailed instructions for building birdhouses and sowing a meadow on our website.

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