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The Truth About Mulch

If I’m consulting with a client who is on a very tight budget to improve the look of her yard, I often say the biggest bang for your buck is simply applying a layer of mulch. That’s it. (Well, a bluestone patio, fresh plantings and some dramatic up-lighting would be great, too, but we’re talking budget upgrades here, folks.) The benefits of mulch, however, go beyond just aesthetics. Organic mulches help retain moisture during California’s hot summer months, improve long-term soil fertility and microbiology, reduce weeds and, of course, give the yard a tidy, finished look.

In this post, I’m going to address what mulches I like best and why, followed by some words of caution about certain mulch products and application styles.

Organic Mulches

Bark Mulch: Tried and True Favorite

Mini bark mulch is by far the mulch we use most often in our landscape projects. On the supply side, it is a by-product of timber and wood production, so we can pat ourselves on the back for making full use of the trees cut down for lumber. And because it’s sourced directly from trees, there is no fear of contamination by weed seeds or unusual chemicals. The natural color of bark is a reddish-brown that will fade to gray – all attractive in my opinion - and natural, by common definition of the word.

Bark mulches usually come in multiple sizes. We like the mini bark for aesthetics. The next size up will also work just as well if you prefer the look and has the advantage of taking longer to break down. Mulch breaking down is not a problem, however. The decomposition of mulch is responsible for adding organic matter to the soil and feeding the soil microorganisms – read “free soil conditioning.”

Bark mulch is the only product that I will 100% recommend 100% of the time. Continue reading for critiques and analyses of the other products out there:

Mini bark mulch acts as a soil conditioner, weed suppressant and water conservation agent on this newly installed landscape

Arbor Mulch: No Such Thing as a Free Lunch

Arbor mulch in a newly planted park
Arbor mulch in a newly planted public park

For a group of people who seem to tout loving trees and respecting the green canopy, arborists do a lot of cutting trees down. We all know arborists by the chippers they haul around, those powerful tree-grinding machines made quite infamous by the Cohen brothers once upon a time. When used for their intended purpose, chippers almost instantly turn a tree into ready-to-use mulch.

Generally speaking, ground-up trees and tree-parts make excellent mulch – perhaps arguably better than bark alone. The different materials of wood, bark, leaves, twigs, etc, provide a varied source of nutrients for the soil. Aesthetically, they are not as neat as the bark and have a more rustic feel. If that doesn’t scare you away, beware of the following:

1) Insects and diseases

Why is this tree being pruned or removed? It might be dying from a disease or parasite that will now be carried and delivered directly to your property, under your trees. If possible, ask the arborist for information on the source of the mulch. Ask what kind of tree it was, what kind of health it was in, and if it displayed any diseases or insect pests. If you decide to take the risk, a good rule of thumb is not to use mulch from one species under the same species. In other words, don’t put Monterey Pine mulch under a Monterey Pine. Many (but not all) pests are species specific, so you can reduce the risk somewhat by following this guideline.

2) Allelopathy (Botanical speak for "obstructionist")

Some trees have a marvelous adaptation where they release chemicals that inhibit the growth of all plants, except themselves. The most common allelopathic plants grown in California are eucalyptus and pines. Typically, eucalyptus mulch is not a good idea for this reason. Pine mulch can be beneficial if you are trying to grow plants that prefer acidic soils, such as azaleas, rhododendrons, or hydrangeas.

3) Weeds

The process of dragging the tree trimmings from the property to the chipper opens up opportunity for weeds to tag along. There are cultivated trees in California that are weedy themselves. Two that come to mind first are privet and acacia, but there's certainly many more. If seeds are present in the mulch, those will be deposited right onto your bare soil, and let the germination games begin.

Arbor mulch was a viable option for this rustic project

Leaf-Litter Mulch: Wait, there IS such a Thing as a Free Lunch

There is free mulch being produced on your property on a daily basis by the power of senescence and gravity. Shrubs and tress are constantly dropping leaves, twigs, and flowers onto the ground below. These materials can do everything that purchased mulches can – retain soil moisture, inhibit weeds, and increase organic matter. Many maintenance gardeners are in the habit of blowing all this loose material into piles and taking it offsite. However, if you can tolerate the look of it, consider leaving all this material in place and focusing the blowing and raking to paved surfaces only.

Bare soils are prone to erosion and compaction
Bare soils are prone to erosion and compaction
Leaf litter left in place has all the benefits of mulch
Undisturbed leaf litter has all the benefits of mulch

A few words of caution: in some cases, removal of fallen materials is beneficial. Certain diseases live in the discarded leaves and flowers and if left on the ground, will re-infect the healthy plant. The most common plants you’ll want to clean-up after are camellias and roses. Fruit, especially larger fruits like apples, pears and peaches, can get messy if left on the ground and attract vermin.

A final caution is for those living in fire-prone areas: mulch is a combustible material, and any proliferation of dry material can serve as fuel during a fire. If fire is a concern, limit the amount of dead plant material around the house, or consider an inorganic mulch (see below.)

Gorilla Fur Mulch: Faux fur is for fashion, not landscapes

Technically a bark mulch, “gorilla fur” is derived from shredded redwood bark and thankfully not gorilla or any other simians' backs. Purity wise, it compares well to regular bark mulches. But aesthetically and functionally, it pales. Dramatically. It is difficult to spread by rake and does not allow water percolation as well as traditional barks. It looks furry and clumpy. The only application I would make an exception for is on a slope. If you’ve installed a planting on a very steep slope and need something to cover the exposed soil until the plants fill in, gorilla fur and jute netting will resist erosion better than other mulches. Otherwise, better to be used as Halloween costume supplies.

Colored Mulches: Politically, environmentally, and aesthetically incorrect

Because the landscape industry is so in tune with nature and her ability to recycle materials, many by-products of other industries make their way into our supply chain, as you have already seen with the previous mulches. Colored mulch is no exception. Colored mulches are derived from recycled wood, usually palettes, broken into mulch-sized pieces. The dyes used, usually red or black, are made from natural colorants, but the source of the wood, or the materials held by the palettes, is more difficult to determine.

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The Mulch & Soil Council certifies landscaping products, and colored mulches with their seal of approval are guaranteed only to use wood not treated with CCA. (CCA was used prior to 2003 to treat wood and contained arsenic).

Aesthetically, I abhor the red color. The black also looks quite unnatural to me, but often clients are drawn to the dark contrast. These colors won’t fade the same as natural products. However, my general rating is a thumbs down.

Inorganic Mulches

Gravel and Decomposed Granite

If you’re going for a minimalistic desert or Mediterranean look, you’re likely drawn to the use of gravel and decomposed granite (DG) as a ground cover. These materials will not break down over time like organic mulches, so there’s less long-term maintenance. Used in the right setting, they can provide a clean backdrop to architectural plantings, but be sure to install correctly and use with the right type of plants.

Gold colored gravel is a suitable ground cover around this drought tolerant landscape of agaves, olive and succulents
Gold colored gravel is a suitable ground cover around this drought tolerant landscape of agaves, olive, and succulents

Soils shift and move over time, as anybody who’s ever tripped on an old stepping stone knows. For the best long-term results, the ground beneath should be compacted and a layer of landscape fabric installed before the material is placed. Wherever there will be plants, however, do not compact the soil. Just like us, plants prefer a soft, forgiving bed to a slab of concrete. Landscape fabric provides a layer of weed protection, as well as keeping the stones from mixing with the native soil.

Unlike organic mulches, gravel and DG will actually heat up and dry out the soil rather than retain moisture in hot weather. They do not provide any organic matter to boost soil health. If you’re using it around plants that are used to growing in arid, hot climates with little shade or leaf litter, this will not be an issue. Succulents and cacti, yucca, olive trees, lavender, and rosemary are some examples of plants that will thrive with a gravel mulch. If this is not your aesthetic, inorganic mulches are not a viable option for long-term success in your garden.

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