Composting Leaves - A Worthwhile Challenge

Composting leaves is an excellent way to give your compost and your garden a boost. However nearly everyone runs into a problem when trying to compost leaves. Here is a quick preview of what's ahead here.

Tree Leaves are Great for Compost

Composting leaves, especially tree leaves is great for both your compost and your garden.

Most trees have long roots extending deep into the subsoil. They draw in the nutrients and trace minerals which have leached out of the upper soil layers.

Fifty to 80% of these nutrients end up in the leaves so you'll find tree leaves rich in trace minerals. They are nature's nutrient recyclers.

Most leaves provide a high carbon source or "browns" for your compost. In other words their C/N ratio is usually over 30, often around 50. Essentially this means they are low in Nitrogen. In a compost they'll need their nitrogen rich green counterparts.

Composting Leaves is Often Difficult

If you were to believe everything you read about composting I have to think you'd feel betrayed by the leaves in your compost. The word out there would have you believe that by mixing your leaves with a few greens, in a couple of weeks you'll be spreading a nicely rotted compost on your garden.

Not, I'm afraid. That huge pile of leaves you're coping with in the fall are tough cookies. They contain varying amounts of Nitrogen, Lignin and Calcium. A whole winter's time in the compost bin and there's a good chance your leaves will look exactly like they did when you added them.

Leaves have two problems in a compost:

  • Leaves have a tendency to mat, especially when not shredded. When matted they will create an impenetrable barrier to air and water.
  • Leaves take a long time to break down. They contain varying amounts of lignin which is extremely resistant to decomposing. Usually a year or two is needed to fully decompose leaves.

Not All Leaves are Alike

Leaves are collectively categorized with a C/N ratio of around 60. This places them firmly in the 'browns' or high carbon category of the compost pile. Their actual C/N ratios range from around 20 to over 100.

It isn't just the C/N ratio that tells how your leaves will perform in a compost. Decomposition is linked to the relative amounts of nitrogen, lignin and calcium they contain.

According to Ken Thompson, author of Compost (whose book I love for its straight forward info and humor), these are useful categories to use when composting leaves.

  • Good Leaves - those lower in lignin and higher is calcium and nitrogen - includes ash, cherry, elm, linden, maple, poplar and willow. Break down in about a year.
  • Bad Leaves - those higher in lignin and lower in nitrogen and calcium - includes beech, birch, hornbeam, oak, and sweet chestnut. I would also add magnolia and holly to this list. Need two or more years usually to breakdown.
For those who don't know the names of your trees or whose trees are not on the list here is a rule of thumb that may work for you.
  • Green Leaves - some trees shed green leaves. These can be added in moderate amounts.
  • Red or Yellow Leaves - These can be used in small amounts.
  • Brown Leaves - Should be avoided but are good for leaf mold.
A last Caution- avoid the leaves of black walnut and eucalyptus tree leaves. These plants have natural herbicides that prevent seed from germinating.

Tips for Successful Leaf Composting

Okay - so your leaves are sometimes slow to breakdown and have a tendency to mat. These are the two problems you want to try to resolve in your compost and here's how.

  • Shred your leaves. This will improve your success composting leaves because:
    • Reduces the bulk of the leaves by about two thirds
    • Reduces the tendency of the leaves to mat.
    • Speeds up the decomposition process as more surface area is bared to the decomposers at work.
  • Mix shredded leaves with a high nitrogen source such as grass clippings. You can mix them by:
    • Setting your mower to bag the clippings and mow the lawn and leaves together. You should get a well shredded and mixed material.
    • Using a shredder pass both the nitrogen rich material of choice and the carbon rich leaves through together. A nice shredded mix results.
    • Mix them by hand, a few forkfuls of leaves, a few of greens and stir.
  • If you are going to use layers make the layers thin so as not to get into big problems with matted leaves.

The Leaf Mold Option

Many experienced composters choose not to mix their fallen leaves into their composts. They instead handle them separately creating a special compost made from almost 100% leaves called leaf mold.

It's simple to make leaf mold. Just follow these steps

  • Shred your leaves with a shredded or your lawn mower. This speeds up the amount of time needed to make leaf mold.
  • Collect them together in either
    • A naked pile (they may blow around though)
    • A wire cage or a compost bin.
    • In big plastic bags.
  • Add water if dry and wait. a year or two until ready.
The process is slow - a couple or three years - but the product - leaf mold is a deluxe mulch well worth the wait.

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