Planting a tree too deep is a major contributor to trees failing to thrive. And, in fact, if it is a sensitive species, it can contribute to establishment failure and death in a relatively short time.
What does “planting a tree too deep” mean? Trees have a root flare. This is where the main roots “flare” out away from the trunk. This junction is a delicate area where the trunk bark meets the root bark. You want it to be just above the level of the soil. In fact, if the tree settles after you have planted it and you have a slight depression, leave it. It is better to have a well than to bury the root flare. Planting the tree a little higher than you might think reasonable solves this problem.
When purchasing a bareroot tree, the root flare is easy to see as the roots are not covered in soil.
However, on the balled and burlapped (BnB) and container trees, this is not the case. Often soil accumulated during the digging and wrapping process in the BnB or too much soil was added in the container.
Prior to planting, it is mandatory to determine where the root flare is. For the BnB, if it is in a wire basket, cut or bend out a portion of the wire, carefully open the burlap and gently start excavating around the trunk to see where the roots are. It is very common to find the first major roots 3 to 6 inches below the top of the soil, shown in photo below.
When I say first major roots, I mean roots at least the size of your finger, sweeping away from the trunk. They could be much bigger, depending on the size of the tree. These roots, which will become the buttress flare, generally do not abruptly erupt from the trunk at a 90 deg angle. The term “flare” is very illustrative. Depending how long the tree has been in the burlap, you may encounter small, adventitious roots. These are not what you are looking for. You want to find the bigger roots. Locating these roots also gives you the opportunity to see if there are girdling or circling roots which will become damaging to the tree in the future. And, yes, need to be removed prior to planting.
Many trees are grafted which can produce a bulge on the trunk. The root flare is below the graft. Many people mistake that bulge for the depth to plant the tree which can lead to the tree being planted several inches too deep.
Once you have found the roots which will become the buttress or structural roots, you now measure from that point to the bottom of the root ball. That is how deep your hole needs to be, no deeper. In fact, shallower is better than too deep.
If you are purchasing a container tree, you will also need to determine where the root flare is.
In addition, circling roots are extremely common in container plants. Not only may they be hiding where the actual root flare is, those roots will stay circling unless they are straightened out. Simply making a vertical cut through the roots doesn’t work but you may end up having to cut some to get them to straighten out.
Have your hose or water buckets handy because this can take a while and you do not want the roots to dry out. You need to physically straighten them. If you are fortunate to be able to identify the root flare immediately, you can go ahead and measure how deep the hole is to be and have one of your helpers start digging while you are straightening. Straightening the roots is often a two-person job, so hopefully you invited two friends to help. If you do not straighten them out you can have a tree fail to thrive or even survive.