In Autumn the leaves fall: the importance of the undergrowth

What happens to leaves when they fall? Why has nature devised this mechanism of seasonal renewal? What life is hidden within the undergrowth?

We are used to considering falling leaves as a peculiarity of autumn, a sign of the changing seasons, but we never question their function, or why lush trees shed their canopies. We don't ask what happens to them once they land on the ground, or we are used to considering them waste to be removed from our gardens, parks, and groves, because they appear unsightly. But are they really purposeless, or do those leaves, which cover the undergrowth, have a role that we ignore but which is actually fundamental?

1. Why leaves fall
2. The formation of the undergrowth
3. The importance of the undergrowth for the health of the garden
4. If nature has its rules, there must be a reason

Why leaves fall

Every autumn, leaves fall. What seems to us like an ordinary event that we almost take for granted, along with the warmer colors that replace the green in the tree canopies, is actually based on a complex biological mechanism within ecosystems.

In particular, the decrease in daylight hours and the drop in average temperatures trigger a series of hormonal changes within deciduous trees and shrubs, which first cause the leaves to change color—sometimes spectacularly—and then the petiole to detach from the branch.

However, although we are used to seeing the red and yellow leaves typical of autumn fall, not everyone knows that evergreen plants also lose their leaves. Their renewal is continuous, occurring in other seasons and as a result of different phenomena. The leaves of evergreen trees age and fall at different times depending on the species, and their fall—partial or total—can also be caused by traumatic events, such as excessive drought, a pest attack, or a hailstorm.

The formation of the undergrowth

Whatever species inhabit them, our gardens are always full of dry leaves, as are any natural woods or forests. However, they are not waste to be removed: it is precisely this layer of decomposing organic matter, which characterizes the undergrowth, that provides nourishment and protection to billions of living beings, both plants and animals, which through their incessant work create humus, extremely rich in substances ideal for the life and development of plants.

We still know little about the thousands of interactions that occur within the soil and in this layer of just a few centimeters, among the roots and all these industrious creatures. However, we know for certain the relationships that are established between certain symbiotic fungi, which in exchange for nourishment provide nutrients that they are able to synthesize.

The abilities and potential of these fungi, although still to be studied, are fundamental both for the large forest ecosystem and for the smaller ecosystem of a garden. In fact, it is thought that mycorrhizal associations (fungus-plant) are able to convey a great deal of information from one plant to another.

The undergrowth, therefore, and the soil beneath it, are fundamental elements within our gardens. For enthusiasts of the subject, an essential scientific reading to deepen your knowledge is “A Plant Is Not an Island,” by Paola Bonfante, published by Il Mulino.

The importance of the undergrowth for the health of the garden

The advice we think it appropriate to give you, from a practical and agronomic point of view, is to leave all the leaves that fall at the base of trees and shrubs alone.

Of course, this technique often does not mesh well with the precision and cleanliness required by paths, sidewalks, and patios, which will inevitably need to be cleared. In this case, it is possible to use the leaves as mulch in other areas of the garden; the excess could be shredded with pruning waste and piled in compost corners where, over time, it will turn into a precious, highly fertile soil that you can use to enrich other areas of the garden or as a component of potting mixes for pots and planters.

However, too often, during our visits and inspections of gardens, even very prestigious ones, we notice the perverse habit of removing everything organic that falls on the ground, almost exposing the roots of trees and shrubs. Some, the most extreme, are not satisfied with raking; they even have the surface swept or blown, making it clean, but also poor, dry, and arid.  

Think that some plants, such as Cistus and Phlomis, for example, protect themselves from competitors by dropping their leaves, rich in anti-germination substances, at their base, to prevent the growth of other plants with which they would have to compete for water and nutrients. Doesn't that seem incredible to you?

Then, there is one thing that few people think about: the scent. The undergrowth smells of yeast, of mushrooms, of something good: try picking up a handful of humus from your garden and smell it. It smells of life! And it will be even more evident when it rains, just as it happens in a natural forest.

However, as with all things, you must be careful not to overdo it. Some types of shrubs, especially those typical of the garrigue, an area poor in organic matter, do not like to have a moist and highly organic layer around their collar. They can often become ill, affected by molds and pathogenic fungi. For them, a gravel mulch, rich in calcium and quick-drying, is more suitable.

If nature has its rules, there must be a reason

Accustomed as we are to intervening in everything in the world, we sometimes forget that trees, plants, and the tiny, complex ecosystems that inhabit the soil existed before us, and could continue to survive even without our intervention

In fact, letting nature take its course, even in the case of the undergrowth, has its advantages. For example, economic advantages in managing our garden, saving not only on cleaning, but also on water for irrigation, and on the need for fertilizers and pesticides.

It is clear that a complex system like this is not achieved in a few weeks after planting the garden: it takes several years for it to form, and only a few minutes are enough to destroy it with inappropriate radical cleaning.

It is therefore essential to be careful!