Leaves as Mulch: How to Use Fall Leaves and How Many Bags You Need

By November most yards are covered in leaves. Instead of bagging them for the curb, you can spread them on your beds for free. The trick is to chop them up first and to rake more than you think you need.

Why leaves work as mulch

Leaves do the same job as bark mulch. They cover the soil, hold in water, and keep weeds down. As they rot, they feed the soil.

N.C. Cooperative Extension calls leaf mulch "compost waiting to happen." Leaves hold calcium, potassium, and magnesium that plants use. They also feed earthworms.

The one catch is that whole leaves behave badly. That is why every extension guide below says the same thing first: shred them.

Whole leaves vs shredded leaves

Shredded leaves win. Whole leaves lie flat and stack up. When they get wet, they press into a mat that air and water can't get through.

Iowa State University Extension says to shred or compost leaves before using them as mulch. Shredding stops the matting, keeps them from blowing away, and helps them break down faster.

Whole leaves still have a job. Colorado State University Extension says whole leaves are better for a compost pile than chopped ones. Finely chopped leaves pack down tight in a pile and choke off the air it needs.

How to shred leaves with a mower

You don't need a leaf shredder. Your lawn mower does the job.

Colorado State University Extension says to set the mower to its highest setting and make a couple of passes over the leaves. That chops them into small pieces.

Two tips from Purdue University make it easier:

• Mow when the leaves are dry. Dry leaves shred more easily. Wet leaves clump and clog the deck.
• Don't try to do it all at once. Purdue says not to shred too many leaves in one mowing. If the pile is deep, mow part of it, then go back for the rest.

If your mower has a bag, use it. You get shredded leaves already bagged and ready to carry to the beds. If not, mow the leaves into a strip and rake them up after.

The shrinkage nobody warns you about

Shredding makes your pile a lot smaller. This surprises almost everyone the first time.

Whole leaves are mostly air. When you chop them, the pieces settle and the air goes away. The Alabama Cooperative Extension System says shredding can shrink leaves to as little as one-tenth of their volume.

How much yours shrink depends on the leaves, how wet they are, and how many passes you make. One gardener on Reddit mowed 23 bags of whole leaves and ended up with 11 bags. That's about half. It's one yard on one day, not a lab test, but it's a fair low end.

So plan on at least a 2-to-1 shrink. Finer shredding shrinks them even more.

How many bags of leaves you need

Work it out in shredded leaves first, since that's what goes on the bed. Then double it to get the whole leaves to rake.

Here is the math, step by step:

1. Find the bed's square feet. Length times width. A 4x8 bed is 32 sq ft.
2. Find the cubic feet. Square feet times depth in inches, divided by 12. At 3 inches, 32 × 3 ÷ 12 = 8 cu ft.
3. Turn it into bags. A 30-gallon lawn bag holds about 4 cu ft. So 8 ÷ 4 = 2 bags of shredded leaves.
4. Double it for whole leaves. Rake at least 4 bags of whole leaves.

If you think in cubic yards, the site's usual shortcut works too. Square feet times depth in inches, divided by 324, gives cubic yards. The 4x8 bed needs about 0.3 cubic yards.

This table uses a 3-inch layer, the middle of the 2-4 inch range. Bags are 30-gallon lawn bags, filled loose, rounded up.

The whole-leaf column assumes a 2-to-1 shrink. If you shred very fine, you may need more. Bags that are stuffed tight also hold more than bags filled loose.

Want your exact number? The leaf mulch calculator runs this math for any bed size and depth.

How deep to spread leaf mulch

For most garden beds, spread 2 to 4 inches of shredded leaves. That is the range from N.C. Cooperative Extension. Iowa State gives the same 2-4 inch range for mulch in general.

The layer will sink. N.C. Cooperative Extension says 4 inches laid in fall may be 3 inches by spring.

The right depth changes with the spot. Here is what extension services recommend.

The vegetable bed row is a special case. Colorado State says to spread 6 to 8 inches of chopped leaves on the empty bed in fall, wet it, and add a bit of fertilizer. By spring you can dig the leaves in or plant right into the spot. Test your soil first so you don't add too much.

Mulching leaves straight into your lawn

You don't have to move the leaves at all. You can mow them right into the grass.

Colorado State University Extension says to set the mower high and make a couple of passes. The small pieces sift down between the grass blades. Here is the rule for how much is too much: if you can still see grass after mowing, you're fine.

The leaves feed the soil and the earthworms, and help the grass roots hold water. Purdue experts found that mulching leaves into the lawn does not add thatch or disease and does not change the soil pH, per Colorado State.

If you still see a solid leaf layer after two passes, you have more than the lawn can take. Rake the extra onto your beds.

Leaf mold: the slow option

Leaf mold is leaves that have rotted down on their own. It's dark and crumbly, like soil. Iowa State says you can use it almost anywhere, from vegetable beds to around trees.

It takes time. Iowa State says a pile of leaves about 3 feet tall and wide turns into leaf mold in 1 to 2 years. It goes faster if the leaves are shredded.

Here is how the three options stack up.

Shredded leaves don't last forever on the bed. The Alabama Extension says last year's leaf mulch will be almost fully broken down by the next fall. That works out well, since that's when the new leaves drop.

Leaves to keep off your beds

Skip black walnut leaves. Penn State Extension says black walnut trees make juglone, a compound in their leaves, stems, and nuts. It can make sensitive plants yellow, wilt, and die.

Tomatoes, peppers, and potatoes are some of the most sensitive. Penn State's advice:

• Don't use black walnut leaves or nuts as mulch.
• Rake them away from your planting areas.
• You can compost them. To be safe, compost them for at least six months or use the result on plants you won't eat.

Oak leaves are fine, but they're slow. Colorado State says oak and cottonwood leaves are high in tannins and break down slowly. Chop them well before you use them.

Leaf mulch step by step

Here is the whole job in order.

1. Measure your beds. Length times width gives square feet.
2. Work out how much you need. Use the table above or the leaf mulch calculator.
3. Rake at least twice that in whole leaves. Shredding shrinks them.
4. Pull out black walnut leaves if you have that tree nearby.
5. Mow the leaves on a dry day. Highest setting, a couple of passes, a bit at a time.
6. Pull weeds from the bed before you spread.
7. Spread 2-4 inches on most beds. Use the depth chart for trees, shrubs, and lawns.
8. Mow the leftovers into the lawn. Stop when you can still see grass.
9. Pile any extra in a corner for leaf mold next year.

For more on mulch in general, see Mulch 101. If you end up buying bags to fill a gap, how many bags of mulch are in a yard shows how bagged and bulk mulch compare.

Sources

• Using Mulch in the Garden, Iowa State University Extension
• Leaf Mulch: Free Soil Amendment for Your Lawn & Garden, N.C. Cooperative Extension
• Recycling Leaves, Alabama Cooperative Extension System
• How to Mow, Compost, and Mulch Leaves, Colorado State University Extension
• Don't Bag It Leaf Management Plan, Texas A&M AgriLife Extension
• Mowing and Leaf Removal, Purdue University Plant & Pest Diagnostic Lab
• Allelopathy in the Home Garden, Penn State Extension