leaf mulch

Leaves – even if you don’t have a green thumb, you cannot help but produce bushel baskets of these valuable crops, year after year, especially if you have one or two large trees.
This time of year in Los Angeles, leaves are available in abundance. Just like manna, they fall to the ground daily, providing sustenance for the garden.
Like the best things in life, leaves are proof that the best things for the garden are free. If you pile up your leaves and allow them to decompose, you will ultimately be rewarded with a rich, dark, sweet-smelling, crumbly soil-like substance. Ironically, this very same material is the most expensive soil amendment found at the nursery, where it is packaged as leaf mold.
The minerals that a plant takes up from the soil during the spring and summer are sequestered in its leaves. Leaves become repositories of nitrogen, potassium and phosphorus, the three elements contained in what horticulturists call a “complete fertilizer.”
No one has ever fertilized a forest tree, yet forests are where trees grow best. Year after year, the complete fertilizer contained in leaves is recycled from tree to forest floor, to soil, to tree again.
Traditionally, gardeners have been instructed to cultivate around their plants. Once a year, at least, we are supposed to get out the spading fork and shovel, and break our backs turning over every square inch of bare garden soil.
However, many experts no longer believe in cultivation, and not just
because it’s hard work. It is thought that cultivation is disruptive to soil life, that it interferes with the work of beneficial soil bacteria and fungi by physically disrupting the soil ecosystem. Cultivation also contributes to erosion, and it is for this reason that plowing or deep cultivation has been reduced, if not eliminated, on many American farms.
Many people who grow plants now believe that leaving the soil alone is the best way to take care of it. They claim that soil is most effectively improved by layering decomposed plant debris – leaves, bark, grass clippings – on top of the soil, rather than by digging or incorporating amendments into it. The idea is to imitate a forest ecosystem, where a layer of leaf litter is constantly present, slowing decomposing and sustaining both soil and plant life.
The classic local example of leaf litter sustaining plant growth can be found at the Descanso Gardens in La Canada. Under a canopy of oak trees, you will find the world’s largest collection of camellias.
Indigenous to the acid soil of eastern China, camellias, under normal circumstances, are not the ideal plants for our alkaline California soil. As oak leaves decompose on the ground, however, they have an acidifying influence on the soil beneath. Thus the camellias at Descanso Gardens feel as though they never left home, and grow luxuriantly.
All leaves have an acidifying effect on the soil. Still, there is a prejudice against certain types of leaves, such as eucalyptus leaves and pine needles, which some people avoid because of the resins they contain. The truth is that all leaves, once they have decomposed, are beneficial as mulch and will improve the soil.
I have even found fresh pine needles to be useful as a mulch in vegetable gardens. The needles provide a barrier against water loss; as they decompose, their acidity is welcomed by most common vegetable crops, nearly all of which prefer an acid soil pH.
Blowing or raking leaves out of shrub and flower beds is unnecessary. While you don’t want to bury your plants under leaves, you should not be afraid to allow several inches of leaves to accumulate around them.
For several years, during December, I have allowed a large planting of ajuga, the rhizomatous groundcover, to be virtually covered by a layer of leaves dropped by the silver maple growing overhead. As winter comes to an end, the leaves slowly disappear, vanishing back into the earth. The ajuga grows lush, and produces more blue flowers with each passing spring.
In a similar fashion, a large planting of azaleas benefits from a permanent mulch of leaves that continuously fall from a huge, adjacent Australian silk oak (Grevillea robusta).
It’s not only the minerals provided by leaves that justify their use as a mulch or soil covering in the garden. As they decompose, leaves soften the soil, making it easier for you to work and easier for plant roots to grow in.
Finally, the smell of fall leaves – whether from poplars, pines or peppermint eucalyptus – creates a lasting impression, to be savored long after you have finished your visit to the garden.