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PERMACULTURE: The Breakdown On Mulch

By Diane C. Kennedy, for Let’s Talk Plants, October 2022.

Diane Kennedy
Author's daughter on a load of free arborist's mulch in a driveway.

The Breakdown On Mulch

Mulch and compost are different. Compost is organic material that has gone through the digestive systems of worms, bacteria, fungi and other soil inhabitants, providing all the nutrients. Mulch is an organic material that hasn’t yet broken down. Its purpose is to cover and protect the soil. Mulch covers compost, which in turn covers the soil. Over time, most mulch will become compost because the soil organisms see it as new food and will chow down, so mulch should be replenished.

The most common mulch is the layer of the leaves, twigs, flowers, seeds and fruit of the plant that drops them. When a plant defoliates, the leaves are not waste; quite the contrary. The plant is covering its roots with organic material that is imbued with site-specific microbes. Leaves decompose adding the right amount of nourishment that the tree needs. Raking away leaves, twigs, flowers and fruit away is interrupting the feeding cycle of the tree. Many think it’s right to trash those perfect foods and dose the tree with chemical fertilizers, insecticide, and herbicide. It’s a crazy cycle only humans selling chemicals could have thought up.

Another common mulch is wood chips, or shredded bark. Some plants don’t want a lot of nutrition in the soil from decomposing materials so rocks can be mulch (NOT gravel!). The worst mulch is plastic, which kills soil and decomposes into small bits that contaminate the ground. Even using cotton sheets or untreated burlap on the ground would be far better than plastic or gravel. Laying plastic weed barrier topped with gravel is the antithesis of water harvesting and soil health.

Mulch should be several inches thick to be effective. If you can see dirt between chips then the mulch is working in reverse. It is heating and soaking moisture up from the ground. Layer mulch at least three inches thick. On the hottest day at soil level it should be cool and moist.

Pine needles make great mulch, particularly around acid-loving plants such as berries, camellias, azaleas, etc. Eucalyptus, pepper, walnut, melaleuca, and some other trees are allelopathic; they produce a substance that retards the growth of plants around them. Chips from allelopathic trees can be used fresh on pathways, or on top of cardboard or newspaper (sheet mulch). If the chips have aged about eight months they should be safe to use under trees.

Diane Kennedy
If you see a foamy yellow mass, don’t worry about it! It’s called, aptly, Dog Vomit Slime Mold.

Those white threads in mulch are fine. They are a cellulose-eating fungi (Basidiomycetes) slowly decomposing the bark. Mushrooms are also fine. They are the fruiting bodies of fungus alive in the mulch (don’t eat them!). If you see a foamy yellow mass, don’t worry about it! It’s called, aptly, Dog Vomit Slime Mold, Physarum polycephalum. It is an amazing type of semi-intelligent cooperative effort between unrelated organisms, and it shows healthy decomposition in the bark.

(There is a great TEDtalk about it, and here’s a great article you can bring up when there is a lull in conversation during holiday dinners: ).

If you mulch, you are protecting the soil and keeping raw materials out of the waste stream. Keep mulch five feet away from your house for fire regulations and use different sized rocks there instead. Keep mulch four inches away from plant trunks or stems so they won’t rot. Sign up for Chip Drop, listen for arborists shredding trees and ask them to drop off their load, leave your leaves or run them and twigs through a small chipper/shredder, and check out your neighbors to see if they have chips to spare. Mulch will protect your soil from the coming heavy rains and high heat, so do it now.


Diane Kennedy has certificates in Permaculture Design, Irrigation, QWEL, and an AA in Landscape Architecture.

She has been designing, consulting, writing and lecturing about permaculture since 2011.

She and her daughter, Miranda, own and operate Finch Frolic Garden Permaculture, a food forest through which they give educational classes. They both volunteer with the Fallbrook Land Conservancy’s Native Plant Restoration Team.


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