Leaves in the Garden Beds: Free Mulch, If You Shred Them First

It's mid-October. The maples are letting go, the beds along the house are filling up, and you're looking at the rake and the stack of paper yard bags and asking yourself why you do this every year. Mostly, you don't need to. Fallen leaves are about the best free mulch and soil builder you'll ever get. Whole leaves dumped in a bed are the trouble, because they mat into a wet blanket. Shred them, keep the layer to the right depth, and watch for a few exceptions, and the chore turns into a benefit.

The short answer: free mulch, if you shred it

A tree drops a season's worth of organic matter right at your feet. Hauling it to the curb in October and then buying bagged mulch in April means paying twice for the same thing.

The catch is the shape of the leaf. A whole maple or oak leaf is a flat sheet. Pile up a few hundred of them, add some fall rain, and they press into a layer that sheds water like shingles. The Chicago Botanic Garden warns that whole, unshredded leaves can keep water from reaching the roots. Purdue Extension says shredding is what keeps leaves from matting down when wet and smothering plants. Colorado State Extension says it flatly: leaves for mulch need to be shredded, "not left whole."

On a golf course we treat organic debris the same way. Leaves stay where they help, they get chopped, and nobody spends a crew-week blowing every last one off the property. The idea is to set things up so the debris does useful work and you don't have to keep fighting it. Your beds work the same way.

So the real question is how to use leaves without making the problems that leaves can cause, not whether to use them at all. That comes down to three things: shredding, depth and timing.

How to do it: shred, depth, timing

Shred them. The easiest tool is the one you already own. Run a mulching mower over the leaves on the lawn or driveway, bag them, and dump the bags into the beds. Two passes will chop them finer than one. If you only have a small pile, the Chicago Botanic Garden suggests putting the leaves in a garbage can and shredding them with a string trimmer. Wear safety goggles, because that can throws debris everywhere. Shredded leaves let water and air through, stay where you put them, and break down faster.

Get the depth right. How much depends on what the leaves are for:

• General beds around shrubs and established perennials: 2 to 4 inches. The Chicago Botanic Garden says not to go deeper than that, so you don't suffocate roots.
• Tender perennials that need winter protection: Purdue recommends 3 to 6 inches of shredded leaves over the top.
• New or borderline-hardy shrubs: Colorado State suggests a wire cage around the plant, filled with 3 to 6 inches of shredded leaves. The Chicago Botanic Garden recommends loose wire or burlap frames filled with shredded leaves for hydrangeas, rhododendrons and roses.

Wait for cold ground. This is the step most people get wrong. Winter mulch is there to keep the soil cold, so it doesn't freeze and thaw over and over and heave plants out of the ground. It's not there to keep the soil warm. Purdue says to wait until the ground is cold and the plants are dormant, usually after several hard freezes. Across much of the northern US that falls sometime between Thanksgiving and Christmas. The Chicago Botanic Garden says to wait until temperatures stay below freezing, because mulch put down too early holds warmth in the soil. Until then, shred the leaves and keep them in bags or a pile.

Keep leaves off crowns and bark. Pull the mulch back a few inches from shrub stems, tree trunks and perennial crowns. The Chicago Botanic Garden says leaf mulch against bark and stems invites disease and gives rodents a place to burrow right next to the trunk.

When leaves are a problem

Leaves aren't right for every bed. Here are the cases where they cause trouble:

• Matting. Whole leaves, and deep piles of any leaves, pack down into a soggy layer. A layer like that keeps water away from roots, and the plants underneath can't breathe. Shredding fixes most of this. Sticking to the depth limits fixes the rest.
• Evergreen perennials and small bulbs. Some plants keep their leaves through winter, and small early bulbs have to push up through whatever is on top of them. A heavy, wet layer smothers both. Purdue's warning about leaves smothering plants matters most here. I'd keep these spots to a light dusting of shredded leaves, or skip them.
• Diseased leaves. Some leaf diseases spend the winter on the fallen leaves. The University of Minnesota Extension says the apple scab fungus overwinters on fallen infected leaves, and it recommends raking them up before the first snow or chopping them with a mulching mower. Its advice also allows composting them. My rule is simple: if a tree had a leaf disease this year, don't spread its leaves under that tree or other susceptible plants as mulch. Compost them or get rid of them.
• Black walnut. Walnut leaves contain juglone, a compound that injures many garden plants, tomatoes among them. Oregon State Extension says fall leaves carry less juglone than spring leaves and that juglone breaks down in a well-managed compost pile that heats up. That's my opinion on what to do with them too: send walnut leaves to a hot compost pile and keep them out of the beds as raw mulch.
• Voles and mice. A deep, fluffy layer is good winter housing for voles, and they chew bark and roots. The University of Maine Extension says to cut down vole cover by clearing debris and using less mulch. Around raised beds it suggests narrow-gauge chicken wire. Voles breed fast, so Maine Extension recommends using several of these methods together. If you've had vole damage before, keep the leaf layer thin and well back from stems.

What lives in the leaves

There's another reason not to bag every leaf, and it's alive. Leaf litter is winter housing for a lot of insects. Texas A&M AgriLife Extension says ground-nesting bees and wasps depend on leaf litter and loose soil to get through winter. Insects that overwinter as eggs, larvae or pupae come out in spring as an established population that helps with pollination and pest control. The Xerces Society, a conservation group for insects and other invertebrates, recommends leaving leaf litter in place over winter. It points out that some butterflies spend the winter there and that stick insect eggs wait in the litter until spring. Fireflies are part of this too. University of Wisconsin Extension says firefly larvae overwinter in the soil and come out at night to hunt on the soil surface and under vegetation. Bare, raked ground doesn't help them.

None of this means you have to give up a tidy place. The middle path looks like this:

• Shred the leaves for the beds that people see, like the front walk and around the patio.
• In the back beds, under shrubs and along fence lines, leave a thin layer of whole leaves where matting doesn't hurt anything.
• Pick one out-of-the-way corner, behind the garage or under a big tree, for an unshredded pile that stays until spring.

It's the same thinking as a course's out-of-play areas. Not every acre needs to look like a green.

The rest of the leaves: lawn, veggie beds, compost

The beds won't take everything a big tree drops. The extra has three good places to go.

Mow it into the lawn. Colorado State says to set the mower deck to its highest setting, take off the bagger, and make at least two passes. If you can still see grass through the chopped leaves, the layer is thin enough to leave. If you can't, bag the extra for the beds or the compost. Colorado State also cites research on this. Purdue found that mulching leaves into turf at high rates had no effect on turf quality, color, soil pH or thatch, and that it raised soil organic matter. Colorado State also reports a Michigan State study in which lawns that got mulched leaves had fewer dandelions and less crabgrass after three years. Purdue suggests about 1 pound of nitrogen per 1,000 square feet in fall to help the chopped leaves break down. That fits nicely with the late-season feeding a cool-season lawn should be getting anyway.

Work it into the vegetable garden. Vegetable and annual beds are the exception to the "mulch on top" rule. Purdue says shredded leaves left on top over winter can keep the soil too cold and wet to work in spring, so till them in during fall. Colorado State suggests 6 to 8 inches of leaves a year for vegetable or annual gardens, shredded if possible, and recommends a soil test first so you don't over-amend.

Compost the rest. Leaves are high in carbon, so they need nitrogen to break down. Colorado State says to mix them with high-nitrogen materials. Purdue suggests green trimmings or grass clippings, or a fertilizer or manure if you don't have those. A pile of shredded leaves started now becomes next year's mulch.

Fall and spring routine

Now through early November - Mow leaves into the lawn as long as grass shows through after two passes. - Shred the extra with the mower and bag it or pile it near the beds. - Bag diseased leaves and walnut leaves separately for a hot compost pile or disposal. - Till shredded leaves into the vegetable garden. - Set aside a spot of whole leaves for overwintering insects.

After several hard freezes (roughly Thanksgiving to Christmas) - Spread 2 to 4 inches of shredded leaves in general beds. - Put 3 to 6 inches over tender perennials, or fill wire or burlap cages around tender shrubs. - Pull the mulch back from crowns, stems and trunks. Go thin wherever voles have been a problem.

Spring - When new growth starts, remove the cages and the leaves inside them, as Colorado State advises. - Pull leaves back off perennial crowns and bulbs as they come up. Leave the rest of the layer to keep breaking down. - Spread last fall's composted leaves as fresh mulch.

That's one weekend of shredding in fall, one afternoon of spreading after the freezes, and a little tidying in spring. It's a lot less work than raking every leaf to the curb, and the beds come out better for it.

Sources (9)

• Chicago Botanic Garden: Using fallen leaves in the garden
• Purdue Extension: Putting the garden to bed
• Colorado State University Extension: Using leaves in the landscape
• University of Maine Extension: Voles
• University of Minnesota Extension: Apple scab
• Oregon State University Extension: Walnut leaves: Safe for composting?
• Texas A&M AgriLife Today: Leave leaf litter alone for overwintering insects
• Xerces Society: Leave the leaves
• University of Wisconsin Extension: Fireflies

Photo: Autumn Leaves - Caversham Court - Reading by Sumitsurai, CC BY-SA 4.0.