Like many living organisms, a tree spends the summer preparing for winter, and indeed for the year ahead. Nutrients gathered from sun and rain and soil have been packaged and are now falling to the ground around the tree, contributing to a layer of leaf litter that performs a variety of essential functions.
As the litter builds up, it is occupied by an army of organisms that will assist in decomposition. Bacteria, fungi, algae, protozoa and nematodes are among the abundant and invisible life-forms that break it down. This process releases essential nutrients like nitrogen, phosphorus and sulphur in forms that are available for use by the tree and other plants. Other minerals leach down with precipitation to return to the soil.
But the litter is much more than a nutritional storehouse precisely tailored in content and in timing of release to the needs of the tree that produced it. It is a protective blanket, insulating the roots from the extremes of weather - frost in winter and drought in summer. The blanket absorbs rainfall, allowing the moisture to filter through slowly and guarding against soil erosion by wind or water. It suppresses the growth of grass and other plants that would compete with the tree for nutrients and moisture. And it increases the rate of soil respiration, an important measure of soil health.
This is the sum of it: Annual storage of nutrients in plant tissues + replacement of losses from litterfall and leaching = the amount of uptake in an ecosystem. But there’s so much more: the tree is generous.
In summer, it shelters birds and small mammals and provisions pollinators. Its litter provides year-round shelter and forage for reptiles, amphibians, mammals and birds. Salamanders live in this damp underworld.
It’s in the months of bitter cold that the litter atop the soil becomes vital to the survival of our familiar garden creatures. Many non-migratory species of butterflies and moths overwinter in leaf litter. All sorts of predators, the ones we need in summer to keep plant-eating pests under control, spend the cold months under the protective layer – ladybugs and many other insects and beetles.
In spring, all this diversity comes to life, offering returning migrants much-needed sustenance early in the year when pickings are slim, and providing breeding bird populations with the protein almost all hatchlings need, even the ones that as adults eat mostly seeds and fruit. So here’s my prescription for dealing with leaves. Don’t give them away.
-Allow each tree to keep its own leaves, or at least some of them. Rake them off the areas that you use - driveway, paths and lawn, and create a circle of leaf litter around the tree, to kill off the grass and provide the “protective services” nature intends. Don’t pile the leaves up against the trunk, though – this would promote rot and damage, or even kill the tree.
-If you have huge trees and more leaves than you can make space for, make leaf mould, an excellent mulch for your ornamental and vegetable beds. Leaf mould is what results after the leaves have been left to sit around for at least a year (I plan each spring to use the leaf mould that was started in the fall 18 months earlier). Pile the year’s leaves into a bin or cage. Turn occasionally and water occasionally. Expect the volume to reduce by half by next spring, and half again by fall; the leaf mould can be moved into progressively smaller bins as it goes through the seasons. The final product is dark and moist and fragrant, and won’t blow away the way fresh leaves tend to.
-But, as you’re spreading all this lovely mulch around, do remember to leave some areas clear for native ground-nesting bees, like bumble bees. The queens emerge in the spring and need bare ground so they can start digging their underground burrows.
-It may be necessary to remove the leaf litter of certain types of non-native trees that are prone to disease - because the disease can carry over in the leaf litter. But a protective layer of mulch will still be beneficial. Another important thing not to do at fall clean-up time: Don’t send bundles of nesting bees to the dump.
Stems are another place where many bee species hibernate. When tidying up your perennial garden, leave 18 inches or more to stand as pollinator nesting sites over winter, and don’t worry about them in spring, because the perennial’s foliage will grow up to conceal the dead material that has been home to the year’s bee populations.
Where you have to cut for the health of the plant – raspberry canes – or because you’re putting something new in the space - sunflower stems – save these and any others with pithy or hollow stems. Cut them into lengths, bundle and store somewhere outside where they are protected from excess moisture but exposed to cold – these are inexpensive bee hotels and your plants will appreciate the pollination next year. More on leaves More on pollinator nesting sites