026-Using Leaves as Mulch & Compost (and Why Leaves Change Color & Shed)

I consider leaves to be Mother Nature’s greatest gift to gardeners. Beyond their unparalleled beauty when they peak in a kaleidoscope of fall color we all enjoy; real gardeners know the greatest gift is still to come when we use those leaves as mulch or in compost.

While there is no dispute over the spectacular beauty of leaf color change each autumn, I’m the guy who can’t wait for the leaves to drop so I can collect them and put them to use in the garden, landscape, and compost bin.

Collecting leaves each fall is a ritual I actually look forward to. I know that the effort I put forth now will pay big dividends for years to come.

Now, before I share with you all the reasons I love leaves after they’ve hit the ground, I think the timeline of what happens before their fall is fascinating as well.

So here is an abbreviated timeline of a deciduous tree’s transformation from a full green canopy in summer to the point where we get to use them in our gardens.

Why Leaves Change Color (and Why Some Years are More Vivid than Others)

Broadleaf deciduous tree leaves change color each fall due to three factors: day length, pigments, and weather.

The Three Main Factors Responsible for Leaf Color Change

1. Day length: As autumn approaches, shorter days reduce the amount of available sunlight plants need to photosynthesis.  It’s natures’ way of signaling winter is on the way. Food and energy production shift to storage and reserves. This also impacts the chlorophyll production beginning to decrease and eventually stop. The green chlorophyll is the dominant pigment that we see during active growth of spring and summer. In fact, it’s so dominant, it’s the only pigment we notice, even though it’s not the only pigment present during this time.
2. Pigments: Along with chlorophyll, carotenoids are also present. This is the pigment group most responsible for red, yellow, and brown. But during the active growing season of spring and summer, carotenoid pigments are masked by the more dominant chlorophyll. But once photosynthesis stops, and chlorophyll ceases, then carotenoid pigments take center stage. The third primary pigment in this equation, anthocyanin, isn’t even present until autumn. But as days shorten and nights get cooler, trapped sugars and bright light trigger the production of anthocyanins. They produce red, purples and everything in between.
3. Weather: The third factor in what impacts leaf color change is the weather. Fall leaf color is most brilliant as the result of warm wet springs and mild summers, combined with bright autumn days and cool but above freezing nights. When all these factors come together, the result is the most vivid leaf colors.

Why Leaves Shed From Trees

Leaves falling from trees is part of a complicated process that culminates at the end of the growing season.

Most leaves can’t handle freezing temperatures, so trees shed dead tissue (leaves). At the same time, trees seal off the place at the base of each leaf stalk. This is known as the abscission layer.

As conditions change in fall, hormone ratios, primarily due to auxin, change significantly enough to signal this physiological change. In fact, it’s the key trigger.

As days shorten and temperatures cool, auxin production starts to decrease. This results in fracture lines and scarring at the abscission layer. This, combined with wind and rain eventually cause the leaves to get knocked off the trees.

The exception is marcescent leaves (commonly noted in most beech trees and some oaks). They cling on longer but are finally pushed off as newly emerging growth occurs in spring.

What to do with Leaves Once They Fall

Use fallen leaves as a soil amendment or mulch. Leaves contain 50-80% of nutrients extracted from the soil, so they are high in nutrients and break down into a wonderful soil amendment.

Shredding leaves will allow leaves to break down faster because there is a more exposed surface area. The more exposed surface, the faster decomposition occurs. Also, as a mulch, whole leaves are more likely to blow away. Shredded leaves tend to interlock and clump together better and don’t end up blowing away nearly as much.

Accessing Large Quantities of Leaves

When I lived in Mount Airy, North Carolina, the city would collect the leaves each week and store them here. After several years of composting, they transformed into rich garden soil, the source for my highly productive vegetable garden.

City or municipal collection services – Contact your local government solid waste department to find out if you have such a service and if access to these leaves is an option.

Neighborhood social networks, social media, market bulletins, friends, and neighbors.

What About Questionable Leaves

Oak leaves – Oak leaves and other leaves with high lignin levels and/or low nitrogen levels take longer to break down. The reason is some leaves have more lignin. This is a natural substance that occurs in leaves and woody material that causes woodiness and stiffness. As a result, these leaves take more time to decompose. Shredding or reducing the size of high lignin material is a good way to help speed up the decomposition process.

Oak leaves are also higher in acidity. Not a big deal. Assuming you don’t till them in, it doesn’t impact soil pH at the surface enough to be an issue.

Some tree leaves have less lignin and more nitrogen than others. These October Glory maple leaves are a good example. They will break down much faster than lignin heavy oak leaves.

Diseased leaves – Generally diseased leaves are not an issue if you give them enough time to decompose fully. However, if you want to play it safe, then keep them out of your compost and mulch piles. But according to Dr. Lee Reich and other authorities, diseased leaves are generally not an issue.

Black Walnut leaves containing Juglone –  Juglone is a natural herbicide. All walnuts and especially Black Walnut have a higher concentration of Juglone than other trees.

Some plants are more susceptible than others. But when leaves containing Juglone are exposed to air, water, and bacteria, the toxin breaks down over the course of just 2-4 weeks, and it’s not an issue anymore.

The bottom line is that if you shred leaves containing juglone and store them over time, these leaves should not be a concern.

Also, I have a personal observation and hypothesis: concentrations of juglone in leaf compounds tend to peak early in the growing season during leaf expansion. Therefore, at the end of the season, when the growing season is waning and leave production and expansion have ceased, juglone concentration should be waning as well.

Accessing Leaves

My favorite way to shred a lot of leaves is with a mulching mower. If you choose this route, spread leaves over a large flat surface such as a lawn or driveway and simply run over them with your mower. Then rake, blow or disperse them in the landscape or garden beds. Or collect them and add to compost or store separately.

Another option is a reversible blower. Some blowers convert to a vacuum, and the leaves can be sucked up, chopped, and collected into a large bag attachment.

These days I no longer own a walk behind mulching mower. So now I rake up whole leaves, place them into a plastic trash can, and plunge a string trimmer into the can to chop them up. It’s like a hand blender. It works very well. Just don’t pack the leaves in too full as it makes it harder to access the leaves at the bottom for chopping.

Another idea is to compost the whole leaves by placing them into a black trash bag. Poke holes in them, wet them first, and set them aside and forget about them. Perhaps each month you can turn them to speed up the process a bit. This is a very passive, hands-off way to compost your leaves.

Lastly, simply collect and store whole leaves in a bin. Perhaps a circle ring of snow fencing, other wire fencing or several pallets to form a corral where you store them. When all leaves are composted down, it’s known as leaf mold.

Putting Shredded Leaves to Work

My preference is to use newly shredded leaves and use them as mulch in my vegetable garden or elsewhere. They’re very easy to work with. And I love knowing they’re free of chemicals and a totally clean feedstock source for mulch. In the spring, you can turn them into the soil or leave them in place.

If I had one single choice for a mulch that becomes a soil amendment, I would choose shredded leaves. Ultimately, they break down to improve the soil, so it’s the best of both worlds.

Leaves are a great carbon source for compost, but you can use shredded white paper from your house as a great carbon input substitute so you can use your leaves as mulch if you’re worried about depriving your compost pile of a carbon source.

Did you Know…

Studies have shown that trees in an acre of forest can shed up to two tons of leaves each year.

Resources & Links

The Complete Guide to Home Composting

Episode 212, Growing a Greener World; The Gift of Trees (watch me join the city leaf collection crew to capture those amazing leaves. And more about collecting and using leaves for mulch.)

Articles I’ve written:

Why leaves change color

Why leaves fall

Put Fall Leaves to Work to Improve Any Soil in 3 Simple Steps

Related Podcasts: Five of the Biggest Mistakes in Tree Care

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