Stop! Don't burn those leaves!

Stop! Don’t burn those leaves!

November 2, 2021

By Erin Castillo

The dazzling display of crimson and orange as autumn summons the changes in the colors of leaves will soon fade within a matter of weeks as tree leaves settle to the ground at our feet. To the untrained eye, their purpose seems to have ended only to be tossed to-and-fro by the wind across our yard. But did you know that these fallen leaves from trees can improve your soil? How? Through Leaf Mold.

What is Leaf Mold?  

Leaf Mold is essentially partially decomposed hardwood tree leaves. If you were to go out into the woods and lift up the layer of leaves to reveal the top soil surface, you’d find Leaf Mold. Instead of leaves breaking down with bacteria, like traditional hot composting pile methods, leaf mold uses a slower, but important, fungi-driven process. The result is Leaf Mold—an excellent carbon-rich natural soil amendment that doesn’t cost you a dime. It can be turned and mixed into your soil or simply added to the top of your soil as a surface mulch.

What Leaf Mold is not

Leaf mold is not a source of nitrogen to be applied as a fertilizer. If someone is advising you to just mix freshly fallen leaves into the soil, that's bad advice. Remember, tree leaves are high in carbon and low in nitrogen with the carbon-to-nitrogen (C:N) ratio being typically 60:1. In a traditional hot compost pile, you need an ideal bacterial decomposition carbon-to-nitrogen ratio of 24:1. Because tree leaves have a high carbon-to-nitrogen (C:N) level, it is not recommended to till freshly fallen leaves directly into your soil. Micro-organisms in the soil will use the existing nitrogen in your soil to break down the leaves. You want to retain this nitrogen in your soil, so your plant can instead utilize it.

What are the benefits of leaf mold?

Trees draw up through their roots from deep within the earth, nutrients and minerals and then send to their leaves. The leaves then contain Ca as well as N, K, P, Mg, S, and trace minerals. As the tree begins to go into dormancy, these leaves fall to the ground to redistribute these elements back into the soil where they decompose. Fifty percent (50%) of this decomposition feeds the tree through the soil beneath the tree canopy. These leaves broken down by fungi and added to your garden’s soil, provides a proper habitat for the soil’s micro and macro-organisms and an essential part of the soil’s food web. 

Leaf Mold essentially adds valuable organic matter to your soil improving soil structure (tilth) which, in turn, improves water and air movement within the soil. At the same time, Leaf Mold is able to increase the soil’s water-holding capacity. In addition, research has shown that adding Leaf Mold to your soil can improve plant health, crop yields, and even enhance a plant’s resistance to disease. When used as a top layer mulch, it can moderate soil temperature and limit moisture loss. 

For example, in one particular three-year study, scientists grew onions and amended one plot of sandy soil with Leaf Mold compost and in another identical plot added no Leaf Mold to the soil. The Leaf Mold compost-amended plot had significantly greater yields than the cultivars grown in those not amended. Not only did the onion bulb weight increase, but those same compost-amended plots produced a great number of onions than the non-amended plot. Interestingly, in addition, the compost-amended plot of onions did not exhibit soft rot disease despite receiving higher than average precipitation.

In another study, scientists compared tomato yields from compost-amended plots to un-amended plots and found that the plots that received Leaf Mold had again, a significantly higher yield. Further, tomato plants that received compost developed less blossom-end rot than fruit in all other treatments. 

Making Leaf Mold

If you have hardwood trees, you can make your own free Leaf Mold at home. Find a corner of your yard or garden, simply rake fallen leaves into a pile or place a column of fencing wire into a circle creating a column to hold fallen leaves. Your leaves should be moist, but not soaking wet as to encourage the right level of moisture fungi need to thrive. Let your pile “mold” for 1-2 years before using. Remember this is a slow fungal cold process of decomposition. But, you can speed this process up a bit if you add the simple step of breaking down leaves into smaller pieces using a leaf shredder or lawn mower before placing into a pile or wire column. In chopping up leaves into smaller bits, it allows more space for microbes and fungi to work as well as prevent leaves from becoming compacted layers that slow down the decomposition process.

In fact, if you place a 6-10” layer of dried shredded leaves on your raised bed in November and continue to come back to break them down with a mower or shredder once a month from December to March, you can speed-up the process. You’ll have a layer of beneficial mulch when it comes time to plant in your garden come April! To further help things along, walk out to a wooded area near where you live and dig down below the layer of fallen leaves to the wood’s soil. Scoop up this top soil layer and mix in your newly chopped layer of leaves you’re preparing as leaf mold. The micro-organisms from your soil sample will “inoculate” the chopped bed of leaves you have prepared and kick-start the fungal process.

If piles or columns of wire are not your preference, there is another method to making Leaf Mold. Again, shred dried fallen leaves using a shredder or mower, then scoop them up and fill a large plastic yard waste bag. When you have filled the plastic bag with as many fallen and chopped leaves you can, tie off and create a few slits in the bag with a pocket knife for airflow. Sit this plastic bag of fallen chopped leaves aside in a shady corner of your yard and recheck every few months, adding water to them if they seem too dry. Before you know it, you’ll have free Leaf Mold compost to add to your soil or garden!

Using Leaf Mold

Leaf Mold can be tilled into your soil in the early spring, but consider using it simply as a 2-inch layer as it is an extremely effective mulch for a no-till garden. Worms and micro-organisms will come up to this mulch layer and incorporate the humus and nutrients back into the soil. Using Leaf Mold as a mulch will also reduce weed pressure and slow down any nitrogen in the soil from leeching or the loss of any top soil erosion due to heavy spring rain storms. And if you don’t like the look of Leaf Mold as a mulch, simply add a couple of inches of pine straw or decorative natural bark on top of your Leaf Mold layer to make your landscape look polished. No one will know that you are feeding your soil!

Dried Fallen Leaves Can also serve as Insulation:

Don’t forget that in November, you can also take leaves you've raked and shredded and use to insulate cold-tender plants by applying a 6-inch blanket of leaves around them. This extra layer of dried leaf insulation can be put on fall veggies like carrots, beets, and kale and can extend your harvest well past those first nights of freezing temperatures. And perennials like Lavender, Rosemary, and Scabiosa (Pincushion Flower) also might like a layer of protection for winter’s cold.

Don’t use Pecan and Walnut Leaves

One important thing to mention in closing—do not use Pecan or Walnut leaves. Pecan and walnut leaves actually contain natural compounds that suppress growth in other plants. If you have these trees in your area, leaves from these trees should go into a “hot” bacteria-based compost pile where they can be sure to completely decomposed before adding them to your garden.

In Summary:

• Leaf mold can increase yields, plant health, and boost disease resistance.
• Leaf mold used as a 2” mulch layer on top of your soil can help to regulate soil temperature, moisture and feeds soil life.
• Leaf mold is an amazing free natural resource trees provide every year that build soil health through the addition of organic matter, biological activity, and mineral nutrients.
• Dried shredded leaves can also act as a great insulator and barrier to tender plants and perennials 

References:

• Maynard, A.A. and Hill, D. December 2000. “Compost Science and Utilization: Cumulative Effect of Leaf Compost on Yield And Size Distribution in Onions” 
• Maynard, Abigail. July 2013. “Compost Science and Utilization: Applying Leaf Compost to Reduce Fertilizer Use in Tomato Production” 
• Fuder, J. Cherokee County UGA Extension Oct 8, 2015 “Make Fall Leaves Work for You”