Bonnie Lee Appleton was a Professor of Horticulture for Virginia Tech University. She also authored The New York Mid-Atlantic Gardener’s Book of Lists.

In memoriam: Bonnie Lee Appleton



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Is it OK to prune roots?
by Bonnie Lee Appleton       #Pruning   #Trees


The bulk of a tree or shrub’s absorbing roots are cut off and left behind in the nursery field when the plant is harvested.

When you prune a plant, whether you’re pruning stems or branches aboveground, or roots below ground, you’re wounding the plant. A wounded plant will attempt to seal off or compartmentalize the wounded area to prevent decay. This process forces the plant to use stored reserves (starches, etc.), and thus has a depleting effect. Pruning can also stimulate new growth, but for this new growth to occur, additional stored reserves must be used. Therefore, even though top and root pruning can be, for certain objectives and at certain times of the year, beneficial to a plant, the plant does pay a price – use of stored reserves – in order to benefit from the pruning.

           

One of the most extreme examples of root pruning occurs when shrubs and trees are dug from a grower or nursery’s field. As much as 80 to 90 percent of the plant’s smaller roots that absorb and transport water and dissolved minerals are cut and left behind in the nursery field. What is contained in the harvested soil root ball is the greater portion of the tree or shrub’s root system by weight, but not by absorptive surface. Still, that harvested portion does contain stored reserves, since one of the functions of roots is food storage, so those reserves can be used to produce new roots. These new roots begin to anchor that plant into its landscape site, and to absorb needed water and nutrients.

 


Circling roots have formed at the potting soil surface in the lip of the container. Circling roots closest to the surface have the greatest potential to form undesirable girdling roots later in a plant’s life.

If landscape plants can tolerate this extreme of root pruning, then root pruning you might need to do might not seem as extreme or threatening to your plant’s survival. Two examples of root pruning that should be done more often are removing circling and matted roots on container-grown trees and shrubs at transplant time, and removing girdling roots on trees as they grow in the landscape.

           

When you remove the plastic container from around the root ball of a container-grown tree or shrub, if a heavy mat of fine roots or a large number of circling roots are visible on the outside of the root ball, then root pruning is warranted. Most of the roots in that heavy root mat won’t grow out of the root ball into the landscape soil – they’ll just stay in that matted configuration. If the root ball dries out, how will the plant absorb water if its roots haven’t grown into the landscape soil? They won’t and the plant will die – slowly if you’re watering and have mulched, or quickly if a drought occurs.

 

What to do? First, shake or tease the potting mix from the outer inch or so of the root ball if you can. If that’s not possible, cut away the most matted sections of roots. New roots will begin to grow from just behind the ends of the cut roots.

           


This root is now large and girdling the trunk of this tree. If the tissue of this root hasn’t grown together with the tissue of the tree trunk, it should be carefully pruned away.

If you encounter circling roots when you remove the container, especially on trees that have coarser root systems, prune off any large roots that are located on the top 6 inches or so of the root ball. Those roots won’t change their circling configuration once the trees are transplanted, so they won’t help to anchor the plant into the soil or explore for water and nutrients.

           

Equally important, however, if those roots aren’t removed at transplant as they grow and enlarge in diameter, they may eventually begin to press upon the tree’s stem or trunk. They are then called girdling roots because that’s what they’re doing – they’re girdling or compressing the stem, blocking upward movement of water and nutrients from the roots and downward movement of sugars from the leaves. In essence, the tree is strangling itself to death.

           

If you wait to prune away large girdling roots later in the tree’s life, the tissue of those roots may have begun to grow together with the trunk tissue. At that point trying to prune those roots away may leave a big wound around the base of the tree. Neither that wound nor the girdling roots, is desirable, so it’s better to prevent the situation before it gets started.

           

The nursery industry has some options for minimizing or preventing matting and circling root formation. These include using containers with an interior wall treated with a copper substance that inhibits root formation on the ball surface. There are also containers whose walls are punctuated with holes that allow air to kill the tips of the roots – what’s known as air-root pruning. These newer containers still aren’t widely used, but if you recognize them, you’ll know you’re getting a root system that should be better formed and better able to quickly establish into the landscape.

 

 


A field harvested root ball with soil washed away – most of the absorbing roots were cut off and left behind.

 


This matted configuration should not be left intact at transplant time. If the potting mix can’t be adequately teased away from the outer surface of the root ball, then some of the root mat should be pruned away.

 

 


The nursery industry has many new types of containers, some of which can beneficially modify how the roots grow.

 


A new container with air-root pruning holes to help prevent or minimize the formation of matting and circling roots.

 

(Photography by Bonnie Lee Appleton.  From State-by-State Gardening May 2003)

 

Posted: 02/23/11   RSS | Print

 

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COMMENTS

Rosedale Gardens - 02/26/2011

After I built my house on the site of our old dairy farm, I purchased some 3 gallon hollies to put in front of the house. About 10 years later everyone of the taller varieties died.  When I pulled them up I found out the roots were girdled. Apparently the nursery had left them in one gallon pots too long and transplanted them to the larger pots. I couldn’t see it when I planted them.  I had to replant that side of the house.

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