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Extension > Yard and Garden News > December 2011

Friday, December 2, 2011

What to Do about Pot-Bound Plants

By Jeff Gillman, Associate Professor - Department of Horticultural Science

So you've just bought a pot-bound plant and you don't know what to do? Gary Johnson, Chad Giblin and I have been testing various techniques to get trees out of their pot-bound states for the last 8 years or so, and here are some of the things that we've found.

The number one problem with planting a pot-bound tree is that they are usually planted too deeply. Trees in containers often have three inches between their uppermost roots and the soil line -- or even more! When a tree like this is transplanted into a landscape without having its planting height adjusted, the roots circling near the top of the container will eventually press up against the stem of the tree and strangle it to death (photo 1).



Jeff Gillman

Photo 1: Pot bound plant's root system 5 years after planting; tree was planted too deep based on media level in pot


When planting a container grown tree make sure that the first large root connecting to the stem (usually about ¼ inch in diameter) is visible after you fill in the planting hole. This will ensure that as the tree gets older and roots and stems expand there will be no compression of the stem.

Roots do not continue to grow in a circle after a tree is planted, even in severely pot-bound plants. Roots which were already circling in a pot-bound plant when the plant was transplanted will not straighten out after planting, but as the roots grow they will grow outwards, not in a circle (photo 1). Right now, with the research we currently have, it is not clear how circling roots will affect a tree if they are only present below the stem. Yes, they look ugly, but looking ugly doesn't mean they're not doing their job.

Most of the common techniques that the extension service recommends for pot-bound trees will not really do that much. Scoring the sides of pot-bound root balls with a razor knife or butterflying the root ball with a shovel just doesn't work that well. If you're really serious about not having any circling roots then you need to use a technique called a box cut. A box cut is performed by cutting the root ball into the shape of a box by using a pruning saw (photo 2).



Jeff Gillman

Photo 2: Box cut on potted arborvitae


Root balls treated using the box cut method generally had good looking root systems after 5 years in the ground (Photo 3). Circling roots were drastically reduced, but not entirely eliminated.



Jeff Gillman

Photo 3: Box cut plant 5 years after planting


In terms of what we're actually recommending -- Right now we are recommending that you check the planting depth of all container grown plants before planting. Only in rare cases do we think that you'll find them planted at the proper depth. After removing the media from the top of the root ball to correct planting depth look for circling roots. If you don't see any circling roots thicker than a pencil then it probably isn't worth your time to do anything besides planting the tree, being careful to make sure that the uppermost root is planted at the soil surface. However, if you see any circling roots with a diameter greater than a pencil, we recommend using a box cut on the root ball.

On a final note, none of the trees we purchase ever need to be pot-bound. There are many different containers out there that will all but eliminate circling roots, such as Smart Pots, Superoots, and Root Trappers (photo 4). If we demand that companies provide trees planted in these containers then, someday soon, we may never need to worry about circling roots again.



Jeff Gillman

Photo 4: Root Trapper Pot

Thursday, December 1, 2011

Contents: December 1, 2011

In this issue of the Yard and Garden News:

What is the true cost of planting a tree too deep?

Karl Foord, UMN Extension Educator


Karl Foord

Photo 1: Encircling root has significantly stunted this tree.

If you purchased a tree and planted it at the soil line as it was in the pot, it is likely that this tree was planted too deep - with drastic consequences. Research conducted by Gary Johnson, Jeff Gillman, and Chad Giblin has shown that trees planted too deeply tend to generate roots that can strangle the plant. Dr. Jeff Gillman explains more of the science in the following article; however in this article I want to address what I believe is the true cost of making such an error.


Karl Foord

I have two 'Autumn Blaze' maple trees that were planted approximately 10 years ago. Several years ago I checked the planting depth of the trees and discovered that one had been planted too deep (tree 1). Tree 1 had several encircling roots that severely impacted its growth (Photo 1). Tree 2 had a few encircling roots that I caught before much damage was done (Photo 2). What is the result? The trunk diameter of tree 1 is 4" and the trunk diameter of tree 2 is 8". Tree 1 is @ 25' tall and tree 2 is @ 35' tall. Tree 1's leaves colored and dropped early. Tree 1 looks anemic next to tree 2, and I have concerns as to whether it will survive.

What is the true cost of this error? TIME! If this tree dies and needs to be replaced, it will be some 11 or 12 years behind the other trees. Even if it lives, it is essentially half the size of a tree planted at the same time. All for having planted the tree in the ground at the soil level as it was in the pot; a fairly reasonable assumption all things considered. I can buy another tree but I cannot gain back the 12 years. Plant your trees at the correct depth as noted in Dr. Gillman's article.

Bur Oak Blight

Michelle Grabowski, UMN Extension Educator



T. Harrington, ISU

Photo 1: Leaves killed by Bur Oak Blight clinging to the tree after fall leaf drop

A healthy bur oak will drop all of it's leaves in the fall. Leaves that are infected with the fungal pathogen (Tubakia sp.) that causes Bur Oak Blight (BOB) remain attached to the tree into the winter. As a result, now is a good time to examine landscape bur oaks for possible infection with BOB.

Bur Oak Blight causes leaves of bur oak trees to develop brown wedge shaped lesions in July and August. This fungal disease often starts in the lower canopy and progresses up the tree in following years. Some bur oak trees are highly susceptible to BOB. After several years of infection, the entire canopy can appear brown and scorched. These severely infected trees are weakened and often fall prey to secondary pests like two lined chestnut borer and Armillaria root rot. It is possible for bur oaks to be killed by this combination of fungal and insect attackers.

Bur Oak Blight was first identified in Minnesota in 2010. Since then BOB has been found in 20 Minnesota counties including Mille Lacs, Sherburne, Hennepin, Ramsey, Washington, Anoka, Wright, Dakota, Carver, Pennington, Beltrami, Pope, Lac Qui Parle, Ottertail, Stearns, Polk, Marshall, Mower, McLeod, and Morrison.

If you suspect your bur oak tree is infected with BOB, contact the University of Minnesota Plant Disease Clinic about how to submit a sample for diagnosis. For more information about BOB, read the USDA Forest Service Pest Alert about BOB.
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