It’s a 66m-year-old decision. Some trees got there much quicker; some took a little longer. But most of the broad-leaved trees that we know and love – the magnolias, plane trees, elms, beech, walnuts, limes, oaks, maples and horse chestnuts – made a calculated decision to drop their leaves come autumn. Large, soft leaves are hard to protect in the winter weather, so the trees evolved to lose them, but not their valuable resources.
Leaf fall is a precision art for a deciduous tree – it’s a salvage operation on the greatest scale as the tree works quickly to bank the resources hidden inside the pigments of the leaves. The greens of chlorophyll go first, then the yellows of the xanthonoids, and then the orange carotenoids, until all that is left is brown – at which point the tree lets its foliage go.
But the spent leaves that flutter to the ground aren’t a waste product. They are rich in carbon and play an essential role for the tree and the ecology it supports. The leaves act as a physical barrier for soil, keeping it and its many microbes insulated, and also for the tree roots, as the wet mats of autumn leaves shelter the fragile top layer from the drying winds.
Many, many things live in these dead leaf layers: caterpillars of moths and butterflies, their chrysalises, beetles, centipedes, springtails, woodlice and spiders … and doesn’t the blackbird know it, rustling through the leaves? Our song and garden birds want these meals far more than they do your imported peanuts.
No one loves wet autumn leaves more than earthworms, though. Sensing one of their favourite things, they start to work on incorporating them into the soil. Earthworms line their homes with autumn leaves, using them for bedding and then, because they are good housekeepers, they eat them as they break down. This act brings all that rich carbon into the soil layer so that it is now digested and available for the rest of the food web – the microbes, fungal networks, the protozoa, amoebas, and all the other millions of weird and wonderful things that keep the soil alive.
A soil made rich by this winter mulching and munching is a better aerated soil, so it can deal with winter and spring flooding – and with summer droughts.
In light of all this work, I’m getting behind the US’s “Leave the leaves” campaign. Doesn’t raking up leaves to put them in plastic bags seem a little ridiculous; doesn’t the deafening roar of the leaf blower sound ludicrous? Do we really think we know better than trees? Leave the leaves be: they are not a mess, a waste or a hindrance – they are life and vital with it.
If you must rake them up because they make the path to your front door too slippery or they have coated the lawn, then return them to the tree. Rake them back up to the base of the tree, tamp them down a little and leave them alone. If they are on a flower bed, leave them for the worms – they will, I promise, be gone by spring.
If you must be tidy, at least make leaf mould. Rake up the leaves, pile them up in a chicken wire cage or old compost bags with a few extra holes in them (the fungi that break them down need plenty of oxygen) and rot them down for a season or two. The resulting leaf mould can be added back to the soil to condition it.
But honestly, as wonderful as leaf mould is, you are still stealing food from the mouths of tiny things. It’s work better done in place. Trust in nature: millions of years gives you time to work out how to get something right.
• Alys Fowler is a gardener and freelance writer
