The most spectacular thing about autumn is its colors. The palette ranges from green to yellow, orange, red to a deep brown. The leaves of the trees are mainly responsible for this firework of colors. But why do they change color and eventually fall?
Soest – Well, it’s just about preparing for hibernation. Deciduous trees evaporate most of the water through their leaves. In winter, however, the water supply can falter when the ground is frozen. The leaves are also at risk of freezing. Therefore, the trees first remove all important nutrients from the leaves, above all the nitrogen-rich green leaves, the chlorophyll. Then other leaf pigments appear, such as the reddish carotenes, which are otherwise covered by the chlorophyll and which then cause the discolouration. This burst of color, world famous in North America as “Indian Summer”, also tells us that shortly after the leaves will come to an end: the trees will cut them down. And already the problem begins for many of us: where to put all the leaves?
The easiest way is to throw it in the green bin. But this only works with small amounts of leaves. The baskets set up by the city may only be filled with leaves from trees that are on public land, never with leaves from private gardens. But the foliage is actually far too valuable for us to just throw it away.
Garden series: Composting leaves
First, let’s look at the possibility of composting the leaves. This is problematic when we want to compost large layers of leaves. Then it rots badly, can rot and mold and block our compost for months. So if you want to compost leaves, you should always mix them with other garden waste so that thick layers cannot form. Decay can be accelerated by chopping the leaves. If you don’t have a shredder, the lawnmower will do as well. It even helps to collect the leaves if it delivers them cut into small pieces in the collection bag ready for compost, so to speak.
An alternative to this is to use this clippings, which are also mixed with lawn clippings, as mulch material on beds. The soil is protected by the soil layer, moisture does not evaporate as quickly, wild herbs do not germinate or only with difficulty, and the humus content in the beds is improved. We also don’t have to worry about this layer of leaf litter remaining on the beds for months. Notching increases the surface area of the material. Microorganisms thus have a larger “attack surface” and thus break down the biological material much faster.
We can also “disappear” large quantities of leaves under bushes and shrubs and in hedges. There it does not disturb, forms a good habitat for many creatures in the winter and in the long term improves the soil structure between the trees. Nothing else happens in nature in the forest. There, the old leaves form a thick layer of humus over the years. However, not all leaves are the same. There are unproblematic leaves. This includes the foliage of all fruit trees, almost all shrubs and bushes, as well as linden, maple, hornbeam, ash, alder, birch and hazel.
Garden series: differences in rotting
On the other hand, the leaves of German oak, walnut, common beech, chestnut, sycamore and poplar rot very slowly and with great difficulty. Sometimes it can take years for them to break down. This is due to two factors. Above all, the leaves of especially oak and walnut have a high content of tannic acid, which makes the decomposition process more difficult. In addition, these sheets have a high CN ratio. C stands for carbon, N for nitrogen, basic building blocks for plant life. A CN ratio of 20 means there is 1 part nitrogen for every 20 parts carbon. 20 is considered a favorable CN ratio for the formation of humus.
If the microorganisms now want to break down the foliage, they need nitrogen, which they extract from the decaying material if there is enough of it. With a high CN ratio (straw has about 100, wood even 200, i.e. very little nitrogen in relation to carbon, oak leaves 50, needles of various conifers from 60 to 110), the microbes must draw the nitrogen out of the soil and thus make the more barren during the duration of the decomposition process. This process also takes much longer.
If you still want to use the leaves of oaks and co for mowing, it is best to spread them out on the lawn and beat them with the grass. In addition, we can enrich the mixture with cornmeal, which contains a lot of nitrogen. This greatly speeds up the rotting process.
Oak and walnut leaves in particular should be used as mulch for plants that like it acidic. These include rhododendrons, blueberries, but also heather (Erika), raspberries and strawberries. So this “problem foliage” also helps us to do without peat products in the garden.
You can find more tips from our garden expert Klaus Fischer here