Guide to Leaf Mold

There is only one way to get leaf mold: make it yourself. Leaf mold is not produced or sold commercially. The good news is that you only need 3 ingredients to make it with: fallen leaves, moisture, and time.

Instead of bagging leaves and putting them on the curb to be transported away, take the freebie that nature has given you and recycle your fallen leaves to create a valuable soil builder called “leaf mold” for use in your garden and landscaping.

What is Leaf Mold?

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Leaf Mold is the organic material left behind after leaves have undergone fungal decomposition. Leaf mold is the term used to describe the result of only decomposed leaves that haven’t had any other ingredients added.

Letting leaves decompose separately from other organic matter yields a wonderfully beneficial soil conditioner that is dark brown to black, crumbly textured soil amendment with a pleasant earthy aroma. Leaves mixed in with compost take longer to decompose via aerobic bacterial action than other organic materials because they are generally too dry, acidic, and low in nitrogen.

Factors Affecting Leaf Decomposition

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Left completely to nature, the process of making leaf mold can take anywhere from 1 to 3 years, depending on your climate.  If you have the time and patience, you can let nature do all the work. Just rake your leaves into a pile in a sheltered corner of your yard (so that wind doesn’t blow them back around your yard), and a few seasonal cycles later, the pile will be reduced to rich humic matter.

If you want to turn this fall’s leaves into next summer’s soil amendment or mulch, then you will need to take the “managed” approach. A managed pile’s time to leaf mold can be shortened substantially. The rate of decomposition is largely determined by four key factors:

  1. lignin content of leaves used – lignin is cellulose, and the higher the lignin content, the longer the leaves take to break down. Combining different types of leaves balances lignin content and improves the quality of the finished leaf mold.
  2. moisture – the fungi that do the work to create leaf mold need a moist environment.
  3. nitrogen level – Freshly fallen leaves have a carbon-to-nitrogen ratio in the range of 30 to 1, which is ideal for quick decomposition. Old leaves, including those that have been on the ground for just a few weeks, will have already lost most of their nitrogen content. If you can gather fresh leaves and get the process underway, there will still be a good amount of nitrogen to speed up the initial decomposition.
  4. exposed surface area – shredding leaves increases the amount of surface area, which makes it easier for fungi to do their work. Shredding the leaves also prevents them from packing together into stacks that repel moisture and seal out air. It also makes it easier to fit a large quantity of leaves into a relatively small space.

Leaf Mold Uses

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Leaf mold is best described as a “soil conditioner”. Among the things leaf mold does to condition soil is increase water retention, improve soil structure, and provide a robust habitat for earthworms and beneficial bacteria. Though leaves are not high in nitrogen, phosphorus and potassium, tree roots do mine calcium, magnesium and many other trace minerals from the soil and your garden will also benefit from these nutrients. but overall, leaf mold does not provide much in the way of soil nutrition, so you will still need to add compost or other organic fertilizers to increase soil fertility. Leaf mold is an ideal ingredient for potting mix and in container planting where it performs a similar function to peat moss. Leaf mold is an effective moisture retaining weed barrier (mulch) for landscape and border gardens. Another bonus of leaf mold is that it is essentially weed-free. Leaf Mold can be used a variety of ways in your lawn and garden:

  • Peat substitute. Use leaf mold in place of peat because it has similar qualities and it’s a renewable resource.
  • Moisture-retaining mulch. Leaf mold can hold up to 500 times its own weight in water. Place it around (but not touching) the crowns of annuals, perennials, and vegetables to help them maintain moisture during summer. When applied to the soil surface as mulch at a depth of 3 inches, leaf mold prevents extreme fluctuations in soil temperature, keeps the soil surface loose so water penetrates easily, and retains soil moisture by slowing water evaporation. There’s no need to dig the material in at the end of the season. You can just pile more on top year after year.
  • Soil conditioner. It’s easier for roots to penetrate soil and take up nutrients when the soil is not as dense. Garden soil amended with leaf mold has a 20 percent lower bulk density than soil to which leaf mold was not added, which is a heavenly environment for plant roots.  Leaf mold also stimulates biological activity in the soil, creating a microbial environment that helps thwart pests.
  • Drought-proof soil. Soils amended with leaf mold increased their water-holding capacity by almost 50 percent. Leaf mold amended soil can hold nearly a two-week supply of water for vegetables which is great for long hot summers. But beware that this water-holding capacity can be a problem for seeds planted in early spring, because it can promote seed rot in cool, wet soil.
  • Seedling mix. Mix one part leaf mold with one part well-aged compost or worm castings for a nutrient-rich potting mixture for seedlings. Or substitute it for peat moss in potting mixes by combining it with equal parts of soil and perlite or, for a soilless mix, with an equal part of only perlite. Before using in potting mix, you should screen the leaf mold through half-inch hardware cloth (steel mesh).
  • Root feeding solution or Foliar Spray (diluted). Brew into a “tea” to use in this way.
  • Vegetable Gardening Tip: Do not add raw leaves to your vegetable garden. The nitrogen needed for their decomposition will compete with the nitrogen needs of vegetable crops.

    How to Make Leaf Mold

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    Start with the fall leaves from around your (and/or your neighbor’s) yard. Only use leaves from yards that don’t use synthetic chemicals on their lawns for contamination reasons.  Some people don’t consider chemical contamination to be a significant concern with leaf mold because the lengthy decomposition time allows for chemicals to break down as well. But why take chances when leaves are so plentiful? Beyond chemical contamination, leaves that have been raked into the street for pickup may contain sand, fuel, or oil residues, so it’s not a good idea to just drive around and pick up leaves that have been left by others on the curb. That said, if possible, you may want to gather a variety of leaf types for your pile since different leaves bring different concentrations of minerals to the mix. It might be helpful to think of your leaf pile as a mixed leaf salad.

    Now ensure that the leaves are thoroughly moistened (but not soaked). Dehydrated leaves have lost most of their nitrogen, which hinders the decomposition process. If you can, rake up your leaves and get them started in your pile before they have time to dry out and lose the activating nitrogen!

    You can also speed up the rate of decomposition by starting with smaller leaf pieces. A quick and effective way to do this is to run a lawn mower over your leaf pile a few times.

    Leaf Mold Bin Method
    Make a leaf mold bin or “cage” from stakes and chicken wire. It should be a minimum of 3 ft square (or diameter), but larger is better for leaf mold — a 5 or 6 foot square allows for good moisture and heat retention, and will take about 25 bags of leaves to fill. Line the sides (not the bottom!) with recycled cardboard and/or black plastic.   Fill the bin with leaves: Pack them as tight as you want. Moisten them, and then cover the top of the pile with plastic, a tarp, or a piece of carpet (though I’ve heard there are some nasty chemicals that can leach out of carpet, so take that into consideration). The cardboard, plastic (or tarp) make it easier to maintain the moisture level of the leaf pile and keep it from the drying effects of wind. Contact with the soil is helpful to the process  (worms will move into the pile while it’s working) which is why you don’t want to line the bottom of the bin. Check the moisture level occasionally during dry periods, add water if necessary, and turn the pile periodically to keep the moisture evenly distributed and increase contact with the soil.  The volume of a full bin will reduce considerably over time. After 6 to 12 months, the bottom of the pile will be useable leaf mold but the rest of the pile can take more than a year to complete, so you may want to plan for multiple cages or bins and turn the piles by transferring from one bin to another.

    Leaf Mold Plastic Bag Method
    Place wet/moist leaves into black plastic bags. Seal the bag and then cut some holes or slits in the bag to allow some air flow (poking the bags with a pitchfork works well).  Check the bag every month or two for moisture, and add water if the leaves are dry. Once in a while, turn the bags over and/or give them a rough shake to keep moisture evenly distributed. When the leaf mold freezes, break it up as much as possible. Since you may have several years worth of leaf fall in various stages of the process, it can be helpful to mark the bags somehow to determine which bags will be ready, when. A handy side benefit of this method is that you can also use the stockpiled leaves that are partly decomposed as a “brown” component of your regular compost.

    Some sources suggest adding a few grass clippings now and again to boost the nitrogen level of your leaf piles to speed the leaf mold process. But in doing this you are actually changing the decomposition method to “composting” (bacterial) from the intended molding process which is fungal. Add any greens whatsoever to your leaf pile, and you have instantly switched from making leaf mold, to making compost…

    Your leaf mold is “done” when it is  soft, crumbly, and the individual leaves aren’t recognizable. Harvesting leaf mold is as simple as shifting your cage or bin and moving the rich humus to the place of your choice. You will end up with an amount of leaf mold that is approximately 20% of the original volume of leaves that you started with.