I have a mature black oak tree in my backyard, I’d guess around 70′ tall with about a 26″ diameter trunk. Directly under the tree, installed by the previous owners is a roughly a 6′ radius of medium to large beach stone, resting atop 12″-16″ (depth) of limestone screenings or some other type of fine stone aggregate, and then heavy clay below that. There are also numerous potted hosta resting amongst the beach stone. Further beyond that for another 12′ are paver stones also resting on limestone screenings atop heavy clay. I can only guess that the pavers, beach stone and aggregate were installed at least a couple of decades ago.
I would like to remove the beach stone and also as much of the limestone screenings as I can around that 6′ radius directly next to the trunk, and replace it all with a couple of feet of rich topsoil in order to plant the hostas directly in the ground beneath the tree as opposed to leaving them potted.
My question then is how close to the tree trunk do feeder roots start growing? Are they mainly just large structural roots for the first few feet away from the trunk? I’m worried that I’ll damage the tree severely if I start cutting feeder roots that close to the trunk while trying to shovel out the limestone screenings. I’m also wondering if the oak tree roots will take over that area if I replace the rock and screenings with rich soil, leaving my hostas constantly struggling for water. I’m hoping that this project will not only look nicer than what is there, but also somehow make life easier for the tree.
The most frequent cause of tree death is compaction because it closes the pores in the soil that are needed for oxygen and water absorption. I suspect that the beach stones were placed around the tree to provide a porous ground cover. While Black Oaks prefer a moist, rich well drained acid soil, they can grow in poorer sandy or clay soils and this ones seems to have thrived in clay soil.
What you have to consider is that Black Oaks are particularly sensitive to disturbances to their roots. They have deep tap roots as well as shallower feeder roots that extend beyond the dripline and are typically less than 20-30 cm (8 – 12 in) below the surface. The feeder roots do not sprout just at the ends of the lateral roots so will be found in the soil closer to the trunk. Unlike many other tree species, the oak roots are connected to its leaves in a straight ascent rather than an interlocking or a spiral ascent. This means that if you cut the roots on one side of the tree you will kill the leaves on the same side that are connected to those specific roots.
Although you plan to replace the screening and stones with rich top soil, do not add more than 5 – 8 cm (2 – 3 in). Use plants that do not require summer watering. Plant ones that are shade-loving and drought tolerant and preferably at least 6 feet from the tree’s base. Wood ferns (Dryopteris) and coral bells (Heuchera) are some that can grow near a mature oak and try to disturb the tree roots as little as possible (see the link below to University of Minnesota site).
There is a link below: