Better roots

Features - Trees

Look to containers to help curb circling roots

July 13, 2012

Top: Roots of a tree planted using the box cut five years after planting. Bottom: Roots of a tree planted using the standard slicing method five years after planting.

Do you watch black and white TV? Use a rotary dial phone? Listen to your music from a record player? No? Well, that’s probably because all of the technology I just mentioned is from the 1950s. Since then all kinds of new advances have come along. Flat screen TV, cell phones and iPods have replaced what were once old standbys. Unfortunately, the plastic containers most of us use in our nurseries are 1950s-era technology, too.

From container to landscape
Recently a team of Minnesota researchers including Chad Giblin, Gary Johnson and myself harvested trees that had originally been planted five years ago as container stock with circling roots. Four different species were planted (Deborah Norway maple, Sienna Glen Freeman maple, Techny arborvitae and Red Splendor crabapple), so that the level of media at the top of the container at planting was at the level of the soil where the container was planted (as is common with homeowners). Before planting we either sliced the root ball on four sides and across the bottom, as recommended by most extension services; we cut the root ball into the shape of the box; or we did nothing.

When we pulled these root balls out of the ground, we discovered that root balls which had their root system cut into the shape of a box had the fewest roots circling the stem, but even they had a few. Box cutting affected tree growth somewhat the first year (Freeman maples showed particularly small leaves the year after planting), but none of the box-cut trees died, and there was no lasting effect on tree growth.

On a somewhat surprising note, slicing, box cutting and doing nothing all had about the same effect on the number and size of roots emanated from the root balls after five years in the ground. So, at least as our experiment goes, slicing the root ball or box cutting did very little to stimulate root growth away from the original root ball.

But oh, those circling roots — in sliced or untreated trees, if even a little bit of the stem was under the surface of the soil, there were stem-girdling roots.

Problem solvers
The quick cure for this problem is simple. Plant container grown trees at the right depth, with the uppermost roots level with, or even a little bit above, the level of the soil. When this is done, there is no chance for roots to circle the stem, because the stem is above ground. But, let’s face it, homeowners and inexperienced landscapers will make mistakes and plant too deeply — and end up with roots surrounding the stem.

Cutting a tree using the box cut method.

But there is a cure — one that’s a lot simpler than a box cut. To stop roots from spinning, there are containers made today and readily available that can stop roots from spinning around the inside of containers. Products like RootTrappers, Smartpots and Superoots can all play a part in stopping roots from spinning. Yes, they are often somewhat more expensive (usually because of labor — the soft sided ones take more effort to work with).

But then a smartphone costs more than a rotary phone, and we all slowly made the transition, because it is a better product. The same is true with these new containers. Our nursery industry needs to be testing these new containers so that when consumers start to demand them because of the superior product they can offer, we’re ready.


Jeff Gillman is associate professor at the University of Minnesota, He also blogs at The Garden Professors,