Fallen Leaves Are Free Soil Food — How to Mow, Mulch and Pile Them This Fall

In 1991, turf researchers at Michigan State University started mowing oak and maple leaves into plots of Kentucky bluegrass at a rate of 100 pounds of dry leaves per 1,000 square feet. That is roughly ankle-deep. Six seasons later, soil pH had not changed, and in one spring count the maple-leaf plots had 4 dandelion flowers while the plots with no leaves had 33 (Nikolai, Rieke and McVay, MSU).

The lawn was fine. The leaves were gone into the soil. That is the whole idea of this article.

Whatever is coming down in your yard over the next few weeks is one of the best soil inputs you will get all year, and it costs nothing. Here is how to use it.

Why leaves are worth keeping

A tree pulls calcium, nitrogen, potassium, phosphorus, magnesium, sulfur and trace minerals up from the ground and sends them to its leaves (UGA Extension). When those leaves fall and break down, that material goes back into the soil it came from.

Leaves are mostly carbon. UGA Extension puts the typical carbon-to-nitrogen ratio of tree leaves around 60:1. That is why they break down slowly, and why fungi do much of the work. Penn State Extension notes that leaf material benefits "fungi, bacteria, and other microbes, as well as tiny mites and insects" (Penn State Extension). Colorado State University Extension adds that mowed-in leaves provide food for earthworms and increase moisture for turf roots (CSU Extension).

Put plainly: leaves feed the fungi and earthworms, those organisms turn leaves into organic matter, and organic matter helps soil hold water. If you want the longer version of how fungi fit in, we wrote it up in The Role of Fungi in Your Soil.

One honest limit. Leaves improve soil, but they do not do the job of a feeding. Virginia Tech's extension publication on leaf mulching says leaves "did not prove to be a substitute for appropriate N fertilization practices" (Virginia Cooperative Extension). Think of them as soil building, not lawn food.

Mow them into the lawn

This is the easiest option and, for most yards, the best one.

How to do it. Colorado State suggests setting the mower to its highest setting and making a couple of passes to chop leaves into small pieces (CSU Extension). Texas A&M AgriLife's Earth-Kind program notes this works best with a mulching mower (Texas A&M AgriLife). The goal is small pieces that fall down between the grass blades.

How much a lawn can take. More than most people expect:

• Virginia Cooperative Extension, citing Michigan State research, says up to 6 inches of leaves can be mulched at a time, though results depend on your mower.
• A Purdue University study mulched up to 4,000 pounds of maple leaves per acre in a single application with no negative effect on turf quality, thatch, soil pH, weeds or disease (both via Virginia Cooperative Extension).

How to check your work. After you mow, look down. Colorado State's rule: if you can still see grass, the layer isn't too thick (CSU Extension). Penn State puts it at a layer up to about one-half inch of chopped leaves being beneficial to the lawn (Penn State Extension).

Keep up with it. Purdue's Zac Reicher recommends regular mowing through the fall so the leaves get chopped and filter into the turf before they pile up (Purdue Turfgrass Science). Mowing once a week while leaves are dropping is far easier than one big job at the end. Slightly damp leaves make less dust, but very wet ones strain the mower's engine, according to Virginia Cooperative Extension.

Shred them for bed mulch

When the lawn has had all it can take, move the extra to your beds.

Shredded leaves break down faster than whole ones (Penn State Extension notes shredding speeds decomposition). Texas A&M's Earth-Kind guidance suggests 2 to 3 inches of shredded leaves in flower beds and 3 to 6 inches around trees and shrubs (Texas A&M AgriLife). Penn State's range is 2 to 4 inches around trees, shrubs and perennials.

The easy way to shred: run the mower over a pile with the bagger on, then empty the bag into the bed.

Start a leaf pile for leaf mold

Leaf mold is simply leaves that have broken down slowly, mostly by fungi. It's dark, crumbly, and good for soil structure and water retention (Penn State Extension).

Penn State's method is about as simple as gardening gets: pile the leaves, pack them down, water them, and wait. A wire or wood bin 3 to 6 feet across and at least 3 feet tall helps keep it tidy. Left alone, it takes a couple of years. You can bring that down to 6 to 12 months by shredding the leaves, turning the pile now and then, watering it occasionally, or covering it with a tarp.

Be patient with it. UGA Extension calls it "a slow fungal cold process" and suggests letting the pile sit 1 to 2 years before use. Finished leaf mold can go on as mulch or be mixed into potting soil.

What not to do

Don't leave thick, wet mats on the grass. Purdue's Zac Reicher warns that heavy leaf layers can smother and kill grass and encourage snow mold where winters are cold (Purdue Turfgrass Science). A light layer you can mow is soil food. A soggy blanket isn't.

Don't bag them for the curb. Every bag at the curb is organic matter your soil paid for and won't get back. Texas A&M's Earth-Kind program built an entire leaf plan around the message "Don't Bag It."

Don't burn them. Purdue Extension's Rosie Lerner notes that leaf smoke carries fine particles that can stay in the lungs for years, that burning moist leaves releases irritating hydrocarbons, and that smoldering piles give off carbon monoxide (Purdue Extension). It also sends the carbon into the air instead of into the ground.

Optional helpers for beds and new piles

Leaves will break down on their own. Nothing here is required. But if you're building up a bed with shredded leaves, two MicroLife granulars add biology and soil conditioning, and both come in 9 LB jugs that ship:

• MicroLife Humates Plus 0-0-4, 9 LB Jug — $35.99. MicroLife describes it as "concentrated compost" in granular form: humates, kelp, molasses and sugars plus a bio-inoculant that includes mycorrhizal fungi. Their label rate is 5 to 10 lb per 1,000 square feet, 1 to 2 times a year, so one jug covers roughly 900 to 1,800 square feet (MicroLife).
• MicroLife MicroGro Bio Inoculant, 9 LB Jug — $38.99. Beneficial microbes that colonize around plant roots, with humates included. MicroLife's rate is 10 lb per 1,000 square feet, so one jug covers about 900 square feet (MicroLife).

A light scatter over freshly mulched beds is all it takes. And best of all the shipping on the se jars is free!  Larger formats are shipped at standard UPS rates but if you live within 20 miles we will deliver for free.  

And if you'd like the lawn itself on a steady liquid program, MicroLife Liquid Lawn Monthly runs March through November and ships free in the lower 48.

The short version for this weekend

Halloween lands on Saturday, and for a lot of yards that's peak leaf week. Mow what the lawn can take. Shred the extra into beds. Pile what's left in a back corner and let the fungi have it.

None of it shows up overnight. Leaf mold takes a year or two, and the benefit to the lawn builds season after season. That's how soil works.

Questions about your yard? Call us at (281) 317-7919, or read our Soil Biology Guide.