Much has been written about how to plant a tree: dig a proper hole for trees that’s at least twice the diameter of the rootball and don’t amend the soil. Mulch is good. Using a weed whacker near the trunk is bad. What’s often overlooked is how important it is to plant a tree at the right depth.
A tree is planted too deeply if the bottom section of the trunk where it begins to widen — called the trunk flare — is below soil level. Two problems arise when this happens: roots begin to grow up and around the base of the trunk that’s underground, girdling the trunk and preventing it from widening. Secondly, if the trunk flare is below soil level, it can’t do its job, which is to stabilize and anchor the tree to the site. A tree needs this support to thrive and live a long time. Without it, a tree is vulnerable in strong winds. (Incidentally, staking may hinder the development of trunk flare, too. See Just say no to tree staking.)
When shopping for a tree or planting a new one, look for a slight flare at the trunk’s base and keep this above ground level at planting time. Dig the hole not much deeper than the rootball so it doesn’t settle deeper after planting. If you don’t see the flare on a tree you’ve purchased, gradually scratch away the top surface of the rootball until it’s exposed. Sometimes nurseries aren’t careful when moving a freshly dug tree into a container and the flare gets covered.
The photo on the right shows a dark section at the bottom of a young katsura tree that was planted too deeply. It had struggled for about four years before it was replanted at the proper depth. After digging it up (maintaining as wide a rootball as two young men could lift), the top few inches of soil were carefully removed. The tree was replanted about four inches (10 cm) higher. A few feeder roots were lost in the process, but the tree has a good chance of survival and long-term health.
This katsura needed to be replanted. It was planted too deeply, and the tree never got established. Note the ring of dark trunk at the base, showing how much of the trunk was buried underground.