The root of the matter

Walking my dog at a beautiful park along a stretch of the Charles River, I’ve been alarmed to see heavy construction machines demolishing a small playground surrounded by giant mature oaks and pines. My worry is that digging so near the trees to remove the footings of the play structures, the excavators will destroy enough roots to kill the trees.

    Remember that old diagram of a tree sending roots underground as deep as the tree’s height?

   That turns out to be completely wrong. Roots stay near the soil surface, the majority in the top 1 to 2 feet of soil. That’s because they need the oxygen that’s there for respiration. 

The exposed roots of this tree form a shallow disc, not a deep wedge

    You can kill a tree or shrub if you dig too much in its root zone, even superficially. I once assassinated a lovely mature Japanese andromeda (Pieris japonica) by carving a narrow trench all the way around it for plastic edging that I fondly imagined would keep the sweet woodruff (Galium odoratum) I’d planted as groundcover from spreading (it did nothing of the kind).
    That gave me a healthy respect for how far feeder roots extend from the base of a shrub or tree. I’d thought the roots stayed approximately inside the drip line, the outer circumference of the plant’s branch tips. In reality, when unencumbered, roots extend to two to three times the width of the tree. 

    Not that you can never dig a hole near a tree, of course. You just have to think about how much of the root mass you’re going to interfere with. Since the roots radiate out from the base of the tree, the closer you dig to the trunk, the greater percentage of roots you’re going to kill. If a tree loses 40 percent or more of its roots, it will die. 
    So now I don’t like to see a power shovel digging close to the trunk of a tree. Sure, it saves a lot of time and manpower, but the potential environmental cost is too high. 
     It doesn’t help, either, that a heavy dump truck has been parked next to trees at the site I visited. This will compact the soil and reduce air spaces needed to allow roots to take up oxygen, water and nutrients. There’s more sobering information about tree damage caused by construction at the Minnesota Department of Natural Resources website.

 
    The white pines (Pinus strobus) and red oaks (Quercus rubra) along the river must be more than a hundred years old, judging from trunk diameters of almost 4 feet. They shade the playground and the many people who enjoy running, bicycling and strolling along the river bank. They stabilize the soil.

     As native trees, they’re foundational species for communities of plants and animals. They would be a terrible loss, irreplaceable in our lifetime. 

     New playground equipment is a nice idea. I very much hope the trees will survive its installation.
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For a story of utterly unsustainable gardening, don’t miss this recent post at The Massachusetts Spy