I recently had two separate requests to plant Norfolk Island pine trees (Auracaria heterophylla) that had been left in their containers way too long. In both cases the trees had been used as Christmas trees and were then placed outdoors in their decorative pots. Although the pot-bound circling roots in these photos are extreme, the treatment would be the same for trees that are not quite as pot-bound as the tree in the following images. Unfortunately, most trees found in nurseries have some degree of circling roots.
How Circling Roots Develop
Once tree roots reach the side of the container, they begin to circle. The roots will then continue to circle when planted into the landscape. Not too many years ago, we were told that slicing the rootball vertically would help break up the circling roots and cause the cut roots to grow out from the rootball. However, it gradually became clear to researchers and other arborists like us, who regularly work with tree roots, that circling roots will continue to grow in a circle even after being sliced.
Much of the latest research on growing better nursery plant (i.e., growing trees that don’t have circling roots) has been done here in Florida by University Enviironmental Horticulture Professor Dr. Ed Gilman. (Dr.Gilman’s vast and comprehensive website section on tree roots can be found at http://hort.ifas.ufl.edu/woody/roots.shtml ).
How roots grow out from the rootball is very important. Its the shallow, lateral roots that grow straight out from the rootball that anchor the tree and help keep it standing in strong winds. Trees that continue to have circling root growth where few if any roots grow straight out into the landscape are more prone to blowing over as the tree crown becomes larger.
Straight Roots Are the Best Roots
The key word to describe good roots is “straight.” To achieve straight roots on trees that are grown in the nursery, the rootball must be shaved each tme the tree is repotted into a larger pot. If rootball shaving isn’t done, then you can have circling roots within circling roots.
Danny is shown shaving the very root-bound rootball. A shovel can be used or a sharp handsaw. Each side of the rootball should be shaved. On the bottom check to see if a tap root is visible. The tap root is probably curling around at the bottom of the pot. Try not to cut the tap root because that could cause root branching near the lower end of the tap root near where the cut has been made. Instead you want the root branching to occur near the upper part of the rootball. Be sure to soak the rootball well before planting it into the landscape.
A tree with a shaved rootball will require lots of supplemental irrigation. Here in Florida during the hottest months (May through September) that means once a day until new growth begins to emerge from the crown of the plant. In the cooler months (Octoboer through April) regular irrigation means applying water two to three times a week. Be sure to adequately soak the rootball. And don’t use your lawn sprinklers to get the tree established. Lawn sprinkler spray output is usually not very uniform and very little irrigation water may be reaching the rootall. Hand water directly onto the top of the rootball or attach a bubbler type irrigation head on top of the rootball so the water soaks the rootball at each irrigation cycle.