The landscape industry is gradually wending its way to a protocol for lower-water use landscapes. While many of the recommended techniques are non-controversial, the advocacy of massive mulching is one I find hard to endorse. At a recent Bay-Friendly Workshop, one speaker recommended massive mulching (to 5” deep!) for “Lawn Conversion” projects. While I appreciate the concept in theory, in practice it has been my observation that this is a very bad idea, especially when one is using bark mulches, rather than arborists wood chips. Here’s why:
(1) Coniferous bark mulches contain an abundance of polymerized lignin and suberose—both compounds tend to inhibit water penetration, require very long periods to break down into the soil, and can promote brown-rot fungus, leading to increased disease. Here are a few links to studies describing these effects:
The author, Linda Chalker-Scott is THE mulch guru.
(2) Bark mulches are the product of industrial logging—somewhere, the land has probably been clear-cut to produce that mulch.
(3) Too much mulch reduces oxygen available to roots, and when piled up against tree trunks, can promote disease.
(4) Mulched surfaces can’t be used by ground-nesting bees, so deny an important pollinator it’s habitat. These wonderful creatures are also amazing engineers, packing a pollen lunch for each egg/larvae, and then sealing the cell with a clever, breathable waterproof cap.
(5) Bark mulches make an ideal habitat for Argentine ants. These critters —while not themselves harmful—are enablers of serious pests including scale insects and aphids.
Fortunately, there are alternatives. The best (and cheapest) is arborist mulch, which contains bark, wood and leaf material that helps build the soil quickly, suppresses weeds and helps maintain soil moisture. Check to make sure it isn’t from walnut, acacia or eucalyptus—virtually any other tree species is fine. For some people, the appearance is too rustic, so the next best choice for tidier gardeners is Forest Floor Mulch. One limitation to this material is that it tends to blow away, and doesn’t hold well on slopes. Lastly, there is wood chip mulch, some of which is unfortunately dyed to Trumpian orange. Use the brown color if you must.
From my experience, it is better to apply a thin layer of mulch (1-2”) wiat for the first crop of weeds, hoe them out, and then apply another layer. The idea that a garden can be installed like a piece of furniture, rather than as a process that properly takes several years, is a fallacy that underlies most commercial landscaping.