Root Crowns

Most people never get to see what a tree looks like underground. Today we did some root crown excavations so I took a few pictures. This first one shows clearly what can go wrong and why we sometimes excavate.

The tree is about 10 inches in diameter and the width of the hole is about two feet. We blew out the top six inches or so of soil to get a look at the primary trunk flares. When soil gets added around the base of a tree, it loses some of the deepest roots that no longer get enough air and/or water. It responds by sending new roots upward toward the surface. Sometimes a small root crosses over a flare and chokes it off like a noose. The dark line across the center flare in this picture is such a girdling root. We cut it at either end to relieve the pressure, but left it in the groove that had formed around it so as not to damage the bark underneath.

Another problem with added soil is that it traps moisture against the stem of the tree. Roots are well equipped to keep out water they don't want, but stems and branches are not. Trapping moist soil against the bark can lead to decay, disease, or insect infestation. The second picture shows a tree that was about 4 inches too deep. In this case, it wasn't from raising the grade of the lawn, but rather it is a recent transplant that was buried too deep. It may have been the fault of the installer, but just as likely could have been that way in the container. Nurseries tend to add a little soil to the top of the root ball each time they repot, so often container-grown trees are several inches deep before they are ever purchased. The next shot is from the other side of the same tree. You can see a dark line where the soil was before we started at the bottom of the gray bark . The hole is a little deeper than it needs to be because we like to inspect thoroughly for problems when we have the opportunity. When all is finished, we will fill the deepest parts of the hole, leaving the arching trunk flares exposed.The last one shows a tree that has been excavated and had the soil aerated to about 10 feet from the trunk. There are bits of grass still poking through, but it has been pretty dried out and may not survive if we left it like this. The final step is to spread compost over the soil, then top it all off with a three-inch layer of wood chips as mulch. That combination will choke out almost all of the remaining grass. Any stragglers that poke through will be easily pulled out. By creating this buffer between tree and lawn, we avoid weedeater damage and make more water and nutrients available to the tree (grass is a very good competitor).

I'm often asked how far out a tree should be mulched. I always say, some is better than none, and more is better than less. In a forest, there would be little or no grass and everything would be mulch. The closer we can get to that ideal, the better off our trees will be. For more information about mulching, visit the ISA consumer site.