How you can boost city trees with leaf mulch

5 JUNE 2011

HERE’S ONE REASON trees in city sidewalks grow poorly: crummy soil that hinders root development. Think about it: the pavement around a tree blocks its fallen leaves from decomposing in the soil and feeding its roots, as happens in nature.

Urban Street Care

You can help sidewalk trees with a layer of decomposing leaves spread directly on the soil. There must be no barrier so the decomposing leaves can trickle down, so you have to pull away all grass and weeds from around the tree first. Think of the leaf mulch layer as a one-time treatment - First Aid for a stressed sidewalk tree. Afterwards you can bandage the site with a 3 inch layer of shredded hardwood mulch (just don’t pile it against the tree, please).

The mulch layer will make weeding and tidying the tree box easier for seasons to come. You’ll increase the chances of having healthier, bigger trees shading the sidewalk and street. Also, a tree with strong roots is less likely to be toppled in storms.

Image: Heart Views

“I struggled trying to grow up until I got leaf-mulched!”

Here are the steps Restore Mass Ave followed in our second leaf mulch project on June 4 and 5. We treated another 20 sidewalk trees, in addition to the 50 we treated last fall; in all we’ve helped 70 DC street trees this way. It seemed worth the work and relatively small expense to help these trees to live a few more decades!

Step 1: Weed a 9’x 4’ area around the tree so the soil surface exposes the flare where trunk meets the main roots. 

Step 2: Snip off any little surface roots; cut any roots “girdling” the base; both will weaken the tree.  

Step 3: Spread shredded leaf mulch over the soil; the new layer will be fluffy with a musty smell like tobacco.

Step 4: Cover the black leaf mulch with shredded hardwood bark mulch to outline a neat box. That’s the “bandage” everyone will see, which will be help against weed growth, mowers, and heavy loads.

Step 5: Stand back and admire your neatly groomed tree and box. Be glad that underground, the roots will be fed naturally for several seasons. Listen.

Maybe you’ll hear the tree saying, Thanks!

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Below: We clear weeds and grass from the tree. (Photo: Restore Mass Ave)

Below: We dump black, fluffy leaf mulch on the exposed soil. You can see the flare where the trunk becomes the tree’s main roots. (Photo: Restore Mass Ave)

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Credits

We recommend you employ a professional if you try to help a whole line of street trees. We found Leprechaun Landscaping LLC of Bethesda. Md. to be experienced and reasonable. 

Shredded leaf mulch is sold as Karbon® Leaf Mulch at Loudon Composting in Chantilly, Va. Some public parks departments, such as that of Arlington County, offer shredded leaf mulch to residents. To find out if DC offers mulch from the leaves it sweeps off our streets and parks, call the Mayor’s Service Line 311.

We thank Dr. Monica Lear of the DC Urban Forestry Administration for recommending we try this for city sidewalk trees. We thank also T. Davis Sydnor of Ohio State University, who devised this protocol.