Leaf litter, not wood chips, under the canopy

Wood chips are the standard advice for mulching a tree, and they are far better than turf or bare soil. But no forest has ever grown on wood chips. It grows on its own litter, and in our experience a yard tree given a few years of leaves, twigs and fruit under its canopy comes through a dry August in visibly better shape than the same tree on a bed of chips. This is why, and how to make the switch without the yard looking abandoned.

What a forest floor is made of

Push your fingers into the ground under an old oak and you pass through three layers. On top is this year’s litter: whole leaves, twigs, bark flakes, acorn caps. Under it is last year’s, half broken down and matted with white threads of fungus. Under that is duff, a dark crumb of humus that no longer looks like anything, grading into mineral soil. The whole thing is a few inches deep, and it is the most biologically active material on the property.

Two things make it different from a chip bed. The first is mixed sizes. Leaves, stems, twigs, fruit and small branches pack loosely, leaving the air spaces that roots and animals need, and they break down at different rates, so something is always releasing. The second is who does the work. Fungal networks knit the layers together and connect to the tree’s own roots; springtails, mites, millipedes and worms shred and mix; and the result is a living layer that rebuilds itself every fall.

Nutrients: litter returns, chips withhold

A leaf falls carrying a large share of the minerals the tree spent the summer gathering: calcium, potassium, magnesium, and a good part of the nitrogen it did not pull back into the twig before letting go. Most of that is released within a year or two, in step with the tree’s own demand, right where the fine roots are. Raking it away and replacing it with a bag of fertilizer is a poor trade.

Wood chips are mostly carbon. They break down over several years, mainly by fungi, and while they eventually add organic matter, they return few nutrients along the way. Chips do not rob an established tree of nitrogen, whatever the folklore says; they simply do not give much. Fresh chips from a tree crew, with leaves and bark mixed in, are the best of the chips; bagged bark nuggets and dyed chips are the worst.

Water: where the drought insurance comes from

The forest floor is a sponge with a lid. Litter breaks the impact of raindrops so the soil surface does not crust and seal, and its loose structure lets water soak in instead of running off. The duff beneath holds several times its own weight in water. The layer shades the soil and blocks the wind, so what soaked in stays. And as litter becomes humus, the soil itself holds more water year after year. A chip bed does the shading part well, but a thick layer of fresh chips can absorb a light rain and pass none of it to the soil, and it builds organic matter far more slowly.

The difference shows in a dry summer: trees over litter hold their leaves longer, scorch less, and go into fall with more stored energy, which is insurance against next year too.

How to do it in a yard

• Leave the leaves where they fall under the canopy, out to the drip line if you can. That is the whole method.
• Shred only if you must. Mowing the leaves makes them lie flat and look tidier, but whole leaves shelter more overwintering insects and last longer. Where they blow, a few branches laid on top hold them.
• Add the twigs and small branches that come down in storms, and the fruit and nuts the tree drops. Cut anything thicker than your thumb into short lengths.
• Pull it back from the trunk. Keep a hand’s width of open ground around the root flare so the bark stays dry.
• No plastic or fabric underneath. Fabric stops the litter from ever reaching the soil and turns into a shredded mess within a few years.
• Aim for two to four inches, and let it settle. It will look like a lot in November and half that by May.

Whatever the material, do not pile it against the trunk. A cone of mulch holds the bark wet, invites rot and rodents, and encourages roots to grow up into the pile and around the stem. Spread it flat and keep the flare in the open. Mulch volcanoes are the fourth item on our list of planting mistakes for a reason.

Where wood chips still belong

Chips are fine on paths and anywhere feet and wheelbarrows go, where litter would be scuffed away. A thin scattering on top of leaves holds them in place on a windy corner. They are the right first layer when a new tree goes into a lawn and there are no leaves yet. Under an established tree, though, the leaves already falling on it are a better mulch than anything you can buy.

What to expect when a lawn becomes a forest floor

Grass under a mature canopy is usually thin already. Stop mowing and let the leaves lie, and the first year looks unfinished. By the second fall the litter has knitted together, the fungus threads show when you lift a leaf, and the ground gives underfoot the way a woodland path does. Edge it cleanly with a mowed strip or a line of stones so it reads as a choice. Once the layer is established it is the ideal place to plant shade-loving native shrubs such as spicebush, which is how a lawn tree turns back into the center of a habitat.