News

A New Way to Mulch: Wood Chips

Wed., Jul. 20, 2022

By Rob Maganja, Horticulturist

Earlier this year, when still in the throes of winter, we found ourselves with many Lilac, Forsythia, and Viburnum branches lining the Display Gardens from recent prunings, and we wanted to put their chipped remains back into the beds from which they came instead of carting those chips off to the dump.  Not that storing woodchips at the dump is terrible, but weed and invasive seeds getting mixed into the woodchips is an inevitability. Another consideration is the expenditure of gas and time to cart things to and fro, as our dump is located way at the other end of Sperry Road (where it intersects with Kirtland-Chardon Road).

Starting in October 2021, through a collaboration with the Conservation department that involved clearing woody invasives out of the Core Natural Areas—forest fragments within the 600-acre horticultural core of the Arboretum—we began chipping those woody species back into the edges of the sites.  This was because we didn’t want to cart their associated seeds off to the dump, where they’d likely create whole new populations, but it also represented a “Circle of Life”-vibe where sustainability was heightened and no material left the forest. 

So, it seemed only fitting that we’d do something similar in the Display Gardens—and with Sandi Cesarov’s (Display Gardens Horticulturist) blessing, we got to it.  And, eventually, Jes Burns (Corning Lake Gardens Gardener) gave a name to the concept—ramial woodchips. 

So, what are ramial woodchips? 

Although most mulch comes from larger-diameter tree trunks and branches, ramial woodchips come from branches that do not exceed 3” in diameter.  Gilles Lemieux and others of Quebec City’s Laval University did some of the primary work in this field, and they

“Concluded that ramial wood is indeed the tree’s “factory” for producing wood, lignin, polysaccharides, “oxides” (glucose, saccharose, fructose, mannose, etc.), and proteins.  It therefore represents an important source of nutrients and energy for living things at all levels.”

(as quoted by Tom Roberts in Maine Organic Farmers and Gardeners, 2003)

And, as per the “Ramial Woodchip production and use” bulletin from the UK’s Organic Research Centre from 2020, ramial chips created in winter and immediately spread on the soil

“Encourages fast entry of soil microorganisms, enabling both nutrients and energy to be transferred more quickly to the soil humus complex.”

Now, the amount of chips we were actually creating and applying per bed was minimal and way less than would actually be necessary to cover the bed to a depth of 3” or so.  But, in the event of a large winter storm, this practice may prove to be exceptionally beneficial. 

In fact, according to the Organic Research Centre, winter is the best time of year to do this.   Though, if you have wet soils in the winter, this becomes very tricky.

Arborist Rick Anielski extricating the chipper from the mud

But, in the Display Gardens, it’s nice because there are plenty of pathways that the chipper can sit on.  And, from those paths, we can simply angle the chute to evenly spray the chips over a wide area. 

In the long-term, the benefits of adding woodchips to the soil are similar to those of adding compost to the soil: better crop health, using less fuel, reduced irrigation, and improved soil structure (Organic Research Centre, page 5).  It acts as a slow-release fertilizer for nitrogen, phosphate, and other nutrients. 

NutrientsGreen Waste Compost (kg/t)Ramial Wood Chips
Nitrogen as N8.14.6-11.5
Phosphate as P₂O₅3.31.4-5.3
Potash as K₂O6.63.0-13.0
Magnesium as Mg20.26-1.1
Sulfur as S1
(Organic Research Centre, Ramial Woodchip in agricultural production, page 3)

The biggest concern I’ve heard regarding the use of ramial woodchips applied right into garden beds—as opposed to going through a 3-6 month curing process—is the concept of nitrogen immobilization.  According to the University of Minnesota Extension, certain high-carbon, low-nitrogen plant materials—such as ramial woodchips—are more difficult for soil bacteria to digest, so they need extra nitrogen to digest the material.   And this extra nitrogen comes from the soil, thus making it unavailable to growing plants. (Understanding nitrogen in soils (umn.edu)).  The counterpoint is sending the mulch through a curing process, with benefits that include lowering the carbon to nitrogen ratio. 

My theory is that well-established plants should have little issue with nitrogen immobilization.  I would think that the most susceptible plants would be annuals, followed by relatively new perennials.  If using ramial woodchips on a bed that’s intended for annual or new perennial crops, it might be best to add a nitrogen fertilizer as a precaution. 

But, as Sandi Cesarov can attest, the established perennials, shrubs, and trees in the beds that we applied ramial woodchips to in her garden so far look like they weren’t impacted at all negatively. 

A Hydrangea bed, leafed-out with no sign of ramial wood chips.

Personally, my only main concern was about the aesthetics.  Because ramial woodchips look much different from the double-shredded mulch we use in the “higher-aesthetic” garden areas, and because the nature of the chips flying out of the chipper’s chute—even when expertly aimed—can lead to the area looking slightly haphazard, I figured our boss might not approve.  But, lo and behold, she had no issue with the practice and found it quite sustainable.  And, as you can see from the two sets of before-and-after photos below, once leafed-out, you can barely see any of the ramial woodchips. 

a lilac bed in winter
a lilac bed in late spring
A Magnolia bed in winter
A Magnolia bed in late spring

Rob Maganja

Rob Maganja

Horticulturist

Rob Maganja graduated from Hiram College in 2013 with a BA in Environmental Studies. He started as a seasonal at the Holden Arboretum in 2013, and in the 8 years since, has worked several more seasons in all the different gardens. Additionally, he’s spent time with the United States Peace Corps in Cameroon and was an IPM intern at Longwood Gardens. He currently is the horticulturist of the Rhododendron Discovery Garden and is also one of the stewards of Holden’s Core Natural Areas. He looks forward to the Core Natural Areas becoming a bona fide agroforestry demonstration.

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