Why root washing is important – an illustrated cautionary tale

I’ve promoted root washing of containerized and B&B trees and shrubs for a few decades now. The experimental science is slowly coming along – it can take several years to determine if the practice is more successful in terms of plant survival than leaving the rootball intact. But we know how soils function in terms of water, air and root movement, and we understand woody plant physiology. So it’s pretty easy to predict what will happen when trees, whose roots are held captive in layers of stuff, are then planted, intact, into the landscape.

Maple newly purchased from nursery.

Early in spring 2021 I purchased a couple of Japanese maples to frame our garage. As always, I root washed these specimens. Here’s a play by play of what we did, and what we found.

Container removed, exposing fine roots. Some of the media has fallen away and is at the bottom of the wheelbarrow.
Since we can’t see the root flare, we mark the point at which the trunk and soil meet.
As we remove the container media, we find burlap and twine. And under that, a clay root ball. There is a root crown somewhere…
Some beating on the clay rootball helps create some cracks where water can then help with the process.
Into a nice soaky bath to loosen up that clay. The longer it sits, the more clay will slough off.
We speed the process along with a directional spray of water.
Jim gets his fingers into the wet clay to pry it away from the roots. Still no root crown, but you can see the Sharpie line on the trunk a couple inches above the clay.
Eureka! A root flare several inches below the original media level.

After more cleaning and untangling, we have a root system ready for planting. Well, almost.

We have roots, but we still have some problems.
It’s got some pretty crappy roots (from not being potted up properly at the nursery), and the remanent of a stake next to the trunk (about 4 o’clock). But there is a nice structural root to the left, with healthy fibrous branches.
“Knee roots” have to go (I call them “knee roots” because they are at 90 degree angles). They have poor structure and will only continue that downward growth pattern, rather than growing outwards. The easiest thing to do is sever them when they turn downward at 90 degrees – don’t worry about removing them if they are too tightly entwined. New root growth at the cut will be directed outwards.
We neglected to get “beauty shots” of our maples through the summer, but you can see one of them to the left of the New Zealand flax plant in the pot. Both maples established their root systems quickly and grew vigorously throughout the summer.
Now in late October, the maples are turning color. Note the distance between the trees and the garage – this ensures that we will have little branch/building conflict as the trees grow in height and spread.
Here’s one of our beauties getting ready to shut down for the winter. They thrived throughout the summer, even when we reached record high temperatures. We look forward to their continued success in years to come.

If you are still wondering why this is a cautionary tale, consider what would have happened if the rootball was planted intact:

  • The root flare would have been buried below grade.
  • There would be multiple layers of stuff between the roots and the native soil (i.e., clay, burlap, and media).
  • The twine circled around the trunk would girdle it eventually.
  • The poor structural roots would not create a stable support system.

Now, one can argue all they like that there isn’t a robust body of scientific literature to recommend this practice – and there isn’t, yet. But leaving rootballs intact creates textural discontinuities between the roots and the native soil, and poorly structured woody roots are not going to correct themselves. So why not embrace a practice that removes both the soil and root problems?

Caveat emptor!

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

27 thoughts on “Why root washing is important – an illustrated cautionary tale”

      1. Prof Linda,

        What is the benefit of planting the root flare at ground level, and what is the negative effect of not planting at ground level?

  1. Thanks for this article. Unfortunately, most trees are still planted too deep, mainly because they come that way from the nursery. Your method allows the correct depth to be easily determined. I will follow your lead in the future.

  2. It’s clear why root washing would be promoted for containerized trees. Leaving that root ball as it comes out of the pot is pretty much always a disaster. What I don’t understand is why you are proposing the same treatment for field-grown B&B trees? I always go to the nursery and tag the trees myself so I can make a good selection, locating first order roots, staying away from trees that have been headed back, etc. I also note if, as a result of nursery practices, the trunk flare is buried. Any and all such problems are then addressed in a specification that I provide to the nursery such as the requirement that all excess soil above the trunk flare be removed prior to digging so that soil is at the correct grade in relation to the flare. That way I have a tree that is properly situated in relation to the resulting ball and the tree gets the maximum amount of roots the tree spade can provide and according to ANSI Standards. I don’t think root washing should be recommended if proper procedures are in place. In addition, we know from bare root research that not every tree can be produced using bare root protocols which might also have relevance to root washing practices.

    1. “Field grown” doesn’t mean the tree was grown from seed in the field. All of the trees I’ve purchased as “field grown” were obviously planted from containers and then finished in the field. Furthermore, if you don’t take off the clay, or find the root flare, you will have problems as outlined in this post as well as several others I’ve posted in this blog over the years.

      Regardless of what ANSI standards say, they cannot change the science of soil functionality. And there is no theoretical or practical science to support the notion that root washing would work for some species but not others. What would be the physiological reason for this?

  3. Thanks for the illustrations! It seems that all the “standard” advice about planting containerized trees is bound to fail these days because it assumes the trees are planted corrected in their pots. “Don’t break up the root ball, plant at the same level, dig a huge hole and amend it heavily”….well, good luck with that. Your methods are working for me; it’s essentially creating a bare root plant out of a containerized one. The few trees or shrubs that I have just planted pretty much “straight from the bucket” have done very poorly or even died, whereas anything bare-root is growing nicely. Planting to the root flare, not amending the soil and “mudding in” have been very valuable techniques. As is not staking the trees.

    Growers pull a lot of tricks and if you don’t investigate the root ball, you’ll never know. I’ve seen where they like to put up a very small tree in a bigger pot — no roots at all in most of the soil. They bury the root flare under five inches of mulch, dirt and fertilizer. They water with drip irrigation in one spot on the side of the pot, and that’s where all the roots grow, including roots that went to the bottom of the pot, turned around and grew back up to the top! It’s stressful to buy an expensive potted tree, pull it out of the pot and then have to cut away half the roots.

    But thanks again for your practical advice. The trees want to grow, we just need to get out of their way….

  4. I’m so thrilled to read this! I have planted dozens of trees this way over the last several decades. It works! I struggled with doing my planting this way because I knew that I was going against the “standard advice” of the time, but I always have good results with bare rooting the potted specimens before planting them. Occasionally I have root washed a tree from the nursery and realized it was not worth the investment of planting that particular specimen.
    Thank you so much for the work you do and communicating it so clearly!

  5. So, if you previously planted a tree (Spring 2020 and 2021), without root washing, when should I dig them up to redo? Both are apple trees and leaves are just coming off. One was severely damaged by hail this summer. Manitoba Canada.

  6. Coincidentally, I’ve been watching your series available on Amazon Prime. I’ve done a lot of fall planting and have practiced this method now. I also dug up some trees planted earlier this year and made sure the roots were treated correctly. Previously, I would simply scour the root ball minimally and plant but never removed all the organic matter that comes from nurseries.

  7. Hi!

    I have established junipers and other leafy shrubs, that I would like to dig up and move to another location in my yard. Would you also apply root washing in those situations too?

    1. Mike, it depends on if they are doing well. If you dig them up and they have a vigorous root system, you don’t need to do anything other than move them. If they come up easily and don’t seem well established, I would wash, correct, and replant.

      1. Thanks for the response. They are well establishes, but would it harm them if I still went ahead with the root washing in order to reduce the moving weight?

  8. I have done the same thing for years after losing many trees and shrubs due to a root bound and hardened “root ball” that would not allow moisture and nutrients to penetrate it once planted. I have consistent success with this method! Thanks for publishing the method and logic behind it.

  9. Extraordinairyily helpful! Sending it out to both my Garden clubs. Not too sure if I understand ‘Knee Roots’ though; cut where they go 90 deg. down,, but in the photo it looks as if they angle suddenly to the left ??

  10. I came across another good reason for root washing as I finally got around to planting the nursery plants I’d accumulated over the summer. I started with what looked like a multi-stemmed camellia planted too deep, and found three distinct plants! Same thing happened with an Oregon grape (four plants, one was not even the same kind of Oregon grape) and an evergreen huckleberry (two plants). Apparently nurseries may not be bothering to cull their seedlings.

    I was a bit stumped as I bare-rooted a Western azalea because it looked like several plants growing tightly together. I ended up leaving them all together. How can a gardener tell whether we’re dealing with one multi-stemmed or clumping plant, versus multiple plants that need to be separated?

    1. You can’t tell until you get it unpotted. It’s best to separate them, as otherwise they compete with each other and nothing does well.

      Look at it as a bonus.

      (See a related post on this blog – five little lavenders.)

    2. Hello ! We have to remember that horticulture is big business . And big business is out to make big money. The growers rarely care or are knowledgeable about absolute appropriate care for optimum growth. They care about fast production, to the market, and “nice looking plant “for the gardeners, who in turn can investigate and perform all the steps we’ve seen here -if at all informed , or interested–and incidentally, I do this to tropicals oh so often! And do any of us suspect for a minute, that landscapers take the time , or even care about this ? I was a contractor for almost 30 years , and am well trained in horticulture and landscaping prior, and KNOW these things. But never could I afford the processes involved to do this to every plant we installed. That is not to say I am always watching and aware of potential problems. So , these procedures are for the individuals totally interested , and having so much time to do this. So for correct procedures to REALLY have an effect on plant growth , it as to start from the very beginning . A very good place to start !! Cheers ! Allan , Bsc., O.D.H.

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