Leave Fallen Leaves Alone

leaf litter

leaf litter

In our city, one of the reasons people create gardens or buy houses with back yards is to stake out a quiet retreat from the noisy world around them.
Some might argue that this is an absurd idea, that if you want real peace and quiet you should go live in a less densely populated area, certainly not in urban-suburban Los Angeles. Thank goodness we live in a world with assurances that “you can have it all.”
Horticulturally, Los Angeles has it all. In few other cities do apples and oranges literally grow side by side, as do papayas and pistachios. Because the ground never freezes, you can plant every day of the year. And along Sherman Way between Reseda and Van Nuys, you will find the only boulevard on Earth where cedars of Lebanon and Mexican fan palms alternate as street trees, chasing each other skyward in surrealistic frenzy, dodging telephone wires as they go.
In this world of living contradictions, the leaf blower enters as just another case in point. The garden is supposed to be a retreat or refuge from noise, gas fumes and dust clouds, and the blower produces all of these. Yet, once a week, the property owner expects every fallen leaf between backyard fence and street curb to be hygienically removed by the gardener. This could be done with a rake and broom, but the time involved would be staggering compared to a blower clean-up. Not many property owners would be willing to pay double or triple what they are paying now for gardening service.
Removal of leaves from a property, whether by blower or rake, is a blunder of staggering proportions. There is no better mulch and fertilizer than fallen leaves. Yet people of all stripes – including the so-called environmentally conscious – insist on leafless ground.
Most leaves will decompose completely within four months. If you insist on hastening their decomposition, mix them with straw and alfalfa pellets (both available at the Red Barn in Tarzana) and with grass clippings. Be aware that certain leaves, such as those from magnolia and loquat trees, are quite slow to decompose unless they are shredded first.
In their natural surroundings, trees and shrubs grow under a layer of leaf litter that, as it decomposes, provides a constant source of slow-release fertilizer. Beneficial fungi known as mycorrhizae live in association with plant roots beneath this leaf litter. Mycorrhizae are of crucial importance to plant health, easing the uptake of water and minerals. In fact, it may well be that the primary importance of mulch is not to conserve water, control weeds, or keep roots cool, but rather to sustain mycorrhizal growth. And without mulch – as in the form of fallen leaves – mycorrhizae cannot exist. In the Amazon, where heavy rains result in leached, mineral-poor soils, mycorrhiza provide the only vehicle for storing minerals – needed by plants – that otherwise would be lost.
More than 20 years ago, botanist Helena Curtis reported that forest tree seedlings raised on nutrient solution in the greenhouse will die when transplanted to a foreign environment – such as a prairie or grassland – even when the soil in their new home is full of minerals. However, if such seedlings are transplanted to the same spot with a handful of forest soil around their roots, they will grow just fine. The explanation for this success is to be found in the mycorrhizae that live in the added forest soil.
Most mushrooms encountered when walking through a forest live in association with certain trees; the kinds of mushrooms living under oaks will be different from the mushrooms living under pines. Most forest mushrooms are the fruiting structures of mycorrhizal fungi, whose own filamentous bodies live underground in close association with the roots of particular trees. Roots secrete sugars and amino acids that allow the mycorrhizae to grow, while the mycorrhizae make it possible for roots to utilize soil phosphorus and other minerals that would otherwise be unavailable.
Plant species may only be found where their mycorrhizal fungi are also present. For instance, the brilliant heathers native to the acidic soils of Oregon and Washington are virtually impossible to grow in the alkaline soil of Los Angeles. Since heathers are highly dependent on mycorrhizae, the absence of these fungi in our soil may be the ultimate reason that heathers won’t grow here. Experiments where mycorrhizae are inoculated into the soil offer the possibility that, some day, by introducing the appropriate fungi into your own backyard soil, you will be able to grow almost any plant your heart desires.

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