Question of the Week – Number 1: March 12, 2020

“I’m having trouble getting grass to grow beneath my trees. I saw river rock being used in a commercial landscape. Will this hurt the trees?”

River rock, tumbled for centuries in mountain streams, adds a nice and natural element to a garden design.

When we apply a layer of river rock, compost or bark mulch there is good movement of air and water into and out of the soil. That means that the roots will not suffer by having the layer over them. (I will admit, however, that it looks like the root flare of this lacebark elm is too low in the soil, but that’s a different issue.)

Where we run into problems is when we add soil over a tree’s root system. As little as 1 or 2 inches of added soil can compact and prevent oxygen from getting into the soil. Over a period of a couple of years, many of the tree’s shallow roots can die, even to the point that the tree may die entirely.

Continued Below

So, the takeaway from all of this is, if you like the look of the river rock, feel free to use it. It makes a nice contrast when used alongside fine-textured groundcovers. Bark mulch and compost are also excellent, and they look very natural. Of course, they will decay over the months, so you’ll want to add a fresh layer periodically.

Note: The same thing applies to use of interlocking concrete pavers and flagstones beneath trees to build walks and patios. As small as they are, the seams between the pavers and stones will allow ample movement of air and water.

Posted by Neil Sperry
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