‘Leave the Leaves’ — Yes, But Why and How?
A million.
That’s how many microorganisms can live in a soil sample that would fit in a bottlecap.
They’re nature’s decomposers: microscopic, beneficial fungi and bacteria that form the foundation of the food web. They make soils fertile, providing various food sources for plants and animals alike.
But they don’t live in soil unless they, too, have something to eat.
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Enter “leave the leaves.” Some homeowners employ it as a low-impact mulching method — harvesting free soil-enriching resources by doing almost nothing.
Fallen leaves can insulate roots, trigger nutrient cycling, and create shelter for a wide range of insects and invertebrates. Birds and squirrels use the litter as nesting material, and whatever’s left can help conceal seeds from them.
But Maas Verde wanted to go deeper. So we asked:
- What effects does leaf litter cause in soils and wildlife communities?
- How does it impact the environment on larger scales?
- What do we lose when we remove leaf litter from the landscape?
And if we do “leave the leaves,” how can we tell that we are causing a positive effect?
Researchers are still studying the impact leaf litter makes on soils and biota at large. But we do know that this unassuming natural byproduct can build biodiversity, create soil resilience, and increase ecosystem function.
“The leaf layer protects and nourishes the soil, enables rainwater to filter into the ground, harbors seeds, and provides hiding places for woodland animals,” the American Museum of Natural History writes. “Without it, the woodlands would be unable to exist and reproduce.”
Not bad for a renewable resource that ends up in landfills at rates up to 10 million tons each year.
Natural Leaf Recycling in the Landscape
When a tree loses a leaf, it starts a recycling process.
(Photo/Marc Opperman)The first step in this process is senescence, which is the aging of leaves. As sunlight on a tree decreases, it begins to reabsorb nutrients from its photosynthesizing structures. Proper tree maintenance, including pruning and trimming, can help ensure that trees receive adequate sunlight and nutrients, promoting healthy leaf growth and senescence. The tree first pulls nitrogen and minerals back from its leaves. This triggers a hormonal response that causes the leaf to start detaching from the tree.
Fall colors start popping as the tree reabsorbs chlorophyll from its leaves. Finally, the leaf detaches, usually influenced by wind and other disturbances.
Once the leaf drifts to the soil, it joins an unsung but vital group of the food web: the decomposers. These heavy-lifting fungi and bacteria complete the food chain.
Decomposers break down organic materials like decaying plants and leaves into simple compounds like phosphorus and nitrogen, creating viable, nutrient-rich soil. Many varieties of microbes and fungi belong to this group.
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Along with nutrients, they also expel CO2 and water into the soil for future plant and animal generations.
They and their leaf litter habitat are existential in forests and landscapes.
“Forest litter, including fallen leaves, twigs, seeds, and other woody debris, is the link between forest and soil systems. [T]he leaf litter is the main component, accounting for more than 70% of the litter.”
Guizhou Normal University, ‘Leaf litter chemistry and its effects on soil microorganisms in different ages of Zanthoxylum planispinum var. Dintanensis,’ 2023
The nutrients they produce can act as fast-uptake food sources. Nitrogen and phosphorus are cornerstone ingredients in many fertilizers. They generally belong in the landscape in abundance.
Why Not Mulch?
Is leaf litter mulch? No, but Maas Verde recommends adding leaf litter along with wood mulch. Soil organisms need varied food sources to thrive, and leaves contain key nutritional components that wood does not.
Imagine a cyclist. The athlete needs a platform of robust, protein-rich meals for long-term strength and stamina. But during a ride, their needs are completely different — here, they require simple sugars for fast-burning energy.
Basically, this is the difference between wood mulch and leaf litter as food for soil organisms.
Wood mulches contain very high amounts of lignin. This is the compound that reinforces plant cell walls, making them sturdy enough to create structures like trees.
Lignin helps create fibrous woody tissues, like this oak stem; (photo/Fayette A. Reynolds, M.S., via Flickr)Soil bacteria and fungi can digest lignin — but they can’t do it very fast. On the other hand, leaf litter is a highly bioavailable food source for these creatures. The nitrogen and phosphorus in fallen leaves can provide quick bursts of growth.
This helps diverse bacterial and fungal colonies propagate, which increases ecosystem resilience.
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How Leaf Litter Shelters Insects and Protects Roots
Why should you leave your leaves? So the plants that were put in during landscaping can thrive.
But leaf litter also creates critical habitat for a wide range of invertebrates. Many insects like cicadas, worms, and beetles depend on leaf litter for larval habitat and overwintering.
That’s also the case with bumblebees, which are the only pollinators for potatoes, blueberries, and tomatoes.
Bumblebee “gynes,” or young queens, spend winter burrowed under warm leaf litter. When one emerges, she can produce a colony of up to 800 pollinating workers.
Some key functions the leaves provide for these animals are soil decompaction and temperature insulation.
Leaf litter insulates the soil and the roots and organisms in it, creating more stable temperatures. This helps plants develop healthy roots, which loosen the soil. It results in better conditions for insects and increases water absorbency, which mitigates pollution, erosion, and floods.
The opposite conditions — bare, compacted soils where water can’t penetrate — often host damaging chain reactions.
In these soils, the American Museum of Natural History writes, “it is difficult for rainwater to filter into the soil. When soil is compacted, the pore spaces in the soil collapse, making it difficult for water to penetrate and making air less available.”
Bare soils also fluctuate in temperature more readily than covered ones. And dry soils like those in Central Texas can heat and cool faster than wetter soils.
Consistency in soil temperature supports healthier plant and animal populations. Biodiversity strongly depends on these animals which, like soil fungi and bacteria, play a key role in the food web.
Generally, biodiverse ecosystems are healthy ecosystems — capable of supporting a wide range of life.
Ground-nesting birds like killdeers protect and camouflage their eggs with litter components, and feed on insects and worms; (photo/Marc Carlson via Flickr)How To Apply Leaf Litter to the Landscape
Of course, the easiest way to use leaf litter on any landscape is to simply not move the leaves from where they fall.
You may want to use them more strategically. You can use them to cover bare soil in beds or lawns, jump-starting the process of improving soils there. You can also insulate plants with leaves, including trees. (Protective structures around trees should be rings — never mulch volcanoes!)
Keep in mind that each tree on your landscape is actively using its falling leaves to support its own health. So Maas Verde wouldn’t recommend transporting all of your leaves between spots. From a land management perspective, “leave the leaves” should literally apply on each landscape.
Several inches of cover should be plenty for trees. Planted beds can take less.
Deeper leaf cover can yield higher soil benefits, depending on conditions.
Finally, try to make sure your soil cover doesn’t get compacted. Mulches or leaf litter can turn into matted surfaces that won’t allow water and air to reach the soil.
Simply choose a small area of soil cover and dig it up with your fingers or a trowel. It should release under light pressure and not come out in clumps.
To loosen your soil cover, use a twist tiller. Work it moderately, several inches deep across the whole surface until you’ve broken it up.
“What you’re doing is infusing oxygen into the system that probably wasn’t there before,” said Maas Verde president and founder Ted Maas. “Thick mulch thatch can prevent infiltration — so what you’re doing when you aerate these materials is kick-starting the process for the microbes inside.”
How Can We Tell if Leaf Litter is Working?
Like many creative ideas, enriching a landscape with leaf litter is only as good as its results.
One of the central goals of this method is to cultivate soil microbes and fungi. But these organisms are tiny, and even huge communities of them leave very little visual trace.
On top of that, they also need time to cause their effect. How can you measure the impact of using leaf litter?
For one, Maas Verde recommends long-term seasonal monitoring. You’ll want to do this over multiple seasons or years. To start, set up a camera spot. This should be a convenient, out-of-the-way place that overlooks your landscape or yard — like a fence corner or deck.
Mark it with a flag, T-post, or similar item and start snapping a few photos each season. Keeping track of every plant on a landscape can be harder than you might think — but plant spread can be a general marker of soil health.
Sometimes a desired species will propagate in one area, then pop up in another one nearby (Maas Verde recommends native plants under virtually all circumstances). When desired plants “move” around a landscape like this, it can indicate improving soil conditions.
Hot conditions inside this leaf compost pile indicate microbe activity; (photo/Marc Opperman)However, the information you’ll get from these observations is anecdotal and may not pertain directly to healthier soil. Changing light conditions, seasonal variances, and species competition are factors, too.
A targeted soil test is the only way to measure bacterial and fungal content in soils. Multiple groups provide these resources. Choose a regional specialist, like Texas A&M or Austin’s Rhizos.
Or take the short route — which, in this case, is letting nature take over. Without leaf litter to provide ecological services like nutrient cycling, soil will soon lack nutrients and plants will suffer.
The American Natural History Museum simplified it:
“New plants sprout from the nutrient-rich soil. The cycle begins again.”
Ecologically-maintained landscapes can serve a function, like this planted dry creek, and look neat and tidy.