Used correctly, mulch builds and improves your soil like nothing else. But don’t make these common mistakes.
Mulch provides so many services in the garden, from retaining soil moisture to suppressing weeds. But we often forget the most important one.
“Mulch plays a big part in growing better soils,” said William C. Fonteno, a professor emeritus of horticultural science at North Carolina State University. “Mulching is a way to continue that process on a natural scale.”
A longtime mulcher, I understood that instinctively, if a little imprecisely. As the mulch I applied each spring gradually broke down into the soil, I thought of it as “passive composting.”
But I didn’t know the science behind what goes on until a recent conversation with Dr. Fonteno, a former technical adviser to the Mulch and Soil Council, an industry association.
Rather than the analogy to composting, “which is not a natural system, but a man-made, human system,” Dr. Fonteno said, a better way to think about mulching is that it mimics the process of soil-making in a forest. Nature’s way is slower. It doesn’t pile up its duff layer of fallen leaves and other plant debris, or turn it, to accelerate the process.
“In a forest,” he said, “there’s no big heap, just a layer of an inch or two or three, breaking down and returning to the system” — as in mulching.
“It’s future soil, a slow-motion amendment,” concurs a new mulch fact sheet that is available as a PDF download from Brooklyn Bridge Park, where the landscape is managed with an ecological focus. (In some situations, that includes not mulching.)
In our gardens, more of us are leaving autumn’s fallen leaves to degrade naturally wherever we can, in support of the beneficial organisms of the leaf-litter habitat. But in some garden areas — including formal ornamental beds and vegetable beds — there may be no such material doing the job naturally. Or what falls may be too heavy for delicate plants to push up through. Or it may just not look right.
That’s where mulch comes in. Americans buy about $1 billion of mulch and soil amendments every year, according to recent industry numbers. It could be bagged from the garden center, or delivered from a local source in bulk. We may get ours free at the municipal leaf-collection facility, or make it ourselves, by piling up wood chips to age after tree work, or by shredding and partly composting autumn leaves.
But we need to use mulch correctly in order to confer the greatest advantage to the underlying soil. Too often, it is used almost decoratively — like carpeting or area rugs — and an expanse of mulch gets more real estate than the plants do. It’s as if the mulch is the point.
Mulch as a Multitasker
There is a lot happening in and under that workhorse layer.
Mulch contributes to weed control by blocking the light that some unwanted species need to germinate. It also forms a barrier that prevents weed seeds from getting a foothold. Successful weeds could steal as much as 25 percent of the moisture that would otherwise reach the plants you’re trying to grow, Dr. Fonteno said.
Mulch can mitigate soil compaction, and it is better than bare soil at capturing water and fostering water penetration, so it reduces runoff. It also slows water evaporation from the underlying soil by up to 35 percent, Dr. Fonteno said, and it moderates soil temperatures in the summer and winter.
It does all of this while also growing topsoil, as the degrading organic materials are processed by saprophytic organisms, from bacteria and fungi to protozoa, nematodes and even earthworms. This process releases humates, which Dr. Fonteno describes as “black, gooey liquid” filtering down to coat the mineral particles of soil: sand, silt and clay.
The coated particles bind together in small aggregates, creating spaces for water to penetrate, improving the soil’s tilth.
Even in tough, urban settings, where there is no topsoil layer and only subsoil to start with, “the use of mulch alone can build soil in three or four years,” Dr. Fonteno said. “Every year that you are adding mulch, you are creating additional soil, and your soil will be better.”
Mulch materials can include grass clippings and leaves, straw, bark or wood chips in varying sizes and ground or shredded wood products. The speed at which humates form depends on the mulch: Grass clippings and leaves are processed faster than wood chips, and pre-aged materials break down faster than raw ones.
“The only difference in mulches, as long as you use organic materials, is the rate at which they decompose,” Dr. Fonteno said, adding a caveat about using gravel, which is popular in some gardens. “Putting rocks on top of the soil surface will retard water loss and moderate soil loss, but it won’t grow soil.”
What Makes for a Good Mulch?
We’re not talking plastic sheeting or rubber “mulch” made from recycled tires — materials incapable of building soil. Desirable mulches derive from plant-based materials, “things designed by nature to degrade and go back into the soil,” Dr. Fonteno said.
Identifying a mulch source offering bulk delivery locally can eliminate all those plastic bags. Nurseries can often recommend suppliers if they don’t sell it themselves.
In formal beds, I seek a fine- to medium-textured material: finer than mini-bark nuggets, but not so fine as finished, screened compost, which isn’t substantial enough to stay put and do all of the jobs that mulch is capable of. If a bed needs compost, spread an inch before mulching.
I also prefer a mulch that has been aged. That, in combination with the not-too-coarse texture, means it will need to be topped up every spring, and occasionally in the fall, too. So it’s no surprise that many gardeners favor big chips — but there is a trade-off.
“Most people don’t want to put down something that breaks down quickly, because you have to do it again and again,” Dr. Fonteno said. “But if you take the attitude that you’re growing the soil, maybe you’ll think differently. It’s hard for soil microbes to chew on big chips.”
Save the big stuff, especially the chunks fresh out of the arborist’s chipper, for pathways. Or at least pile them up to mellow before you use them.
The worry that gardeners often voice — that mulch ties up some of the nitrogen from the soil — comes in here: If supplemental fertilizer isn’t applied when you’re piling on coarse, fresh, carbon-rich wood chips, or even fresh sawdust, Dr. Fonteno said, it can cause some drawdown in soil nitrogen. Using aged material eliminates that problem.
Other requirements in my choice of mulch? That it is not a source of contaminants, pests or diseases, of course. And if it’s being used for ornamental planting, that it’s brown in color, like the local soil. Again, I don’t want to feature the mulch; let’s skip the dyed black and orange hues that look fake.
For vegetable beds, my favorite materials are leaves that have been shredded by the mower and then piled up to age. Or straw — the stems that remain after threshing grain plants like oat or barley — so that, unlike hay, it’s not full of seeds.
The Dreaded Volcano Mulch
Generally, mulch is applied in ornamental beds at a depth of about one to three inches, depending on the type of plants beneath it. Mulch at planting time, and then establish a routine to keep the layer replenished.
“Simply top-dress what remains from your last application to reach the former depth,” Dr. Fonteno said, “offsetting what has broken down.”
In a vegetable garden, mulch right after planting, too. The following spring, “pull the remaining mulch aside and plant into the spaces,” Dr. Fonteno said, and “then maybe add a little more mulch.”
Of course, there can be too much of a good thing.
One of the worst examples of mulch gone wrong began as a workaround for trees planted in urban settings where the soil was thin and digging was difficult. “They simply placed the plants on the soil surface and added mulch around them,” Dr. Fonteno said.
This misguided practice — where a ring of mulch is piled high around the bark of a tree trunk, assuming the shape of a volcano — has a name: volcano mulching.
In addition to promoting bark decay, it causes the tree’s roots to grow up into the mulch layer, rather than down into the soil. The tree may eventually die, and even topple.
Keep the mulch at least several inches away from tree and shrub trunks.
And don’t invite rot by smothering the crowns of perennials, or expect tender spring ephemerals and bulbs to find their way up through a pile of wood chips. If you’re awaiting the emergence of self-sown seedlings, like poppies or larkspur, mulching too early could stop them in their tracks.
After all, the plants are the point of all this effort — plants made happy by improved soil conditions, evidence that our work in applying all that mulch has paid off.
Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.
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