Root Pruning For Long-Term Health

Although not official until March 20, for all practical purposes, it’s spring. And we’re here for it. NMSU Extension Agents from all over the state are reaching out with great questions about pear tree pests (Grant County); hugelkultur and allelopathy (Bernalillo County); dying pine trees and selecting fruit trees (Curry County); tree saplings damaged by mystery nibblers (Socorro County); codling moth control, piñon irrigation, and dormant tree transpiration rates (Valencia County); and elderberry propagation (Bernalillo, Chavez, and Doña Ana Counties). My inbox is bursting at the seams with questions about early ripening figs, hydrophobic compost, peony problems, and so on.

It’s not just New Mexico. I noticed that even “Mr. Smarty Plants,” who answers online questions for the Lady Bird Johnson Wildflower Center in Austin, has posted a statement about being overwhelmed by a flood of incoming messages. I’m not complaining. These questions give me hope that, collectively, we’re paying attention to our surroundings and we care about outcomes. It might take me a while to respond, but please keep the questions coming!

Here are the last lines of the poem I wrote about work for a column in 2019:

    I’m looking for plant problems much like a vulture.
    There’s lots to be learned in the world of horticulture.

    No question is stupid. I’m not a snob.
    I’ll take the easy and tough ones ‘cause I love my plant job.

    If YOU’RE interested in it, you’ve got my attention.
    That’s what we do here in Cooperative Extension.

After several years of slow decline, this 11-year-old Arizona cypress was finally removed. The culprit: A severely knotted rootball that could have been avoided if circling (also called spiraling) roots had been properly cut at the time of planting. Photo courtesy of Dr. Curtis Smith

Question:

Will cutting the circling roots at planting encourage healthy root growth?

- Remy G., Sandoval County

Answer:

Yes. When they are root-bound, this applies to both annual plants (like zinnias and tomatoes) and perennials (like trees and shrubs). That first part is important. Before you start loosening, trimming, or cutting any roots, remove the container and look at the root ball. If the roots are sparse or barely visible, it might be better to plant as is. But if the root ball is a thick mass of mostly roots and little soil that firmly holds the exact shape of the container it was growing in, cutting or itching the roots to loosen the shape can be very helpful and may even save lives (plant lives, of course).

Spiraling root ball of a dead Osage orange tree. Photo courtesy of Dr. John Mexal

This dead tree’s roots were so badly knotted due to being overgrown and root-bound while still in the container and then planted without any correction of those circling roots. Photo courtesy of Dr. John Mexal

The natural plant hormone auxin is famous for regulating shoot and root growth, among other important processes. Auxins produced in terminal plant tissues, like shoot and root tips, inhibit lateral growth. Removing those tips therefore stimulates lateral growth. Removing shoot tips, often called “pinching,” can help plants like basil and tomatoes grow bushier, with more lateral stems. Similarly, when root tips are severed, lateral root development increases.

For perennial plants that keep growing year after year, this is an even bigger issue because circling roots can turn into silent killers. Once planted, circling roots are impossible to see. As the tree, shrub, or vine grows, those circling roots continue to get fatter and fatter, like normal roots. But circling roots can become girdling roots that literally strangle other big roots or the main trunk. This doesn’t happen all the time, but circling roots cannot untie themselves or self-correct underground. Newer recommendations emphasize dealing with circling roots on perennials by cutting them with pruning shears, slicing down through the root ball, or shaving off the outer edge of the entire root ball at the time of planting. If this sounds extreme, that’s because it is. But these methods wouldn’t be recommended if they weren’t proven over time to be crucial for long-term health. For how-to videos, visit the blog version of this column (linked below).

Growing in containers too long isn’t the only reason circling roots develop. Plants with restricted root zones or that are watered only at the base of the trunk can also develop circling—and eventually girdling—roots. In those conditions, roots have trouble growing out laterally, the tree suffers, and it can reach the point where no amount of extra water will help.

Do you suspect that girdling roots could be killing your tree? Telltale signs to watch out for include surface-level roots that are growing horizontally across the trunk base instead of reaching out radially like good roots should, and roots that are visibly squashing other roots. A flat side of the trunk at the base might indicate that roots underneath are restricting normal growth. And there’s a simple technique you can do with young trees to see if their roots are restricted even though they have had enough time to establish healthy lateral roots. I’m calling it the wobble test. Stay tuned. I’ll explain it all in the coming weeks.

Other planting recommendations, like digging the hole no deeper than the root ball and resisting the urge to amend the soil with anything but the same soil you dug out, can also make the difference between life and death for our perennial plants. For more details, check out the column titled Planting Trees: The Hole Story on the Desert Blooms website (and to find the first 16 stanzas of my poem, enter “snob” in the search prompt).

Exposed circling root still in the container. Photo courtesy of Dr. John Mexal

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!