Harvesting an Experiment

This has been an exciting week for me.  On Monday we started cleaning off 72 rootballs of various tree species that had been planted 5 years ago for a study.  These trees had been planted in containers and become potbound at the nursery from which we received them.  We treated them in one of three ways.  Either we did nothing (in other words we just dropped the pot bound tree in a hole), we used the standard methods that Universities recommend for slicing potbound roots (Four deep slits down the sides and a deeply cut X across the bottom), or we cut off all of the circling roots by cutting the pot bound root system into a box shape.

A root ball cut into a box shape

The plan was to harvest after 4 years to see what happened – we decided to wait 5 – and boy did we see some interesting stuff.  At this point our results are preliminary – we need to run statistics before we can say anything conclusively – but this is what my eyes tell me.

  1. Trees that had their roots cut into boxes suffered reduced growth the first few years, BUT, their root systems look as good as any root systems that I’ve seen – almost no circling.
  2. We planted our trees with the surface of the soil at the same level as the surface of the media in the containers – which is too deep in most cases.  For trees with circling roots this created a severe problem as the circling roots often surrounded the stem – potentially causing suffocation of the tree later in life.
  3. Root systems that were cut using the 4 slit method didn’t look much different from those that weren’t cut at all.
  4. The number of large roots emanating from all of the treatments appeared to be about the same (we’ll need to run the stats before I commit to this one).  This is particularly interesting because many people expect large roots that are circling to continue circling — but that isn’t what usually happens (unless the hole where the tree is planted has hard sides which can force the roots to circle just like the container did).

This root system was from a control — no root pruning at all, but still plenty of large roots.

No matter what the results/statistics end up saying there will be more questions.  For example, all else being equal, how damaging are circling roots to the health of a tree if the tree is planted properly (no stem tissue under the surface of the soil) and the circling roots are under the surface of the soil?  If the answer is that circling roots under the surface of the soil aren’t very damaging (after all, there’s no stem tissue for them to crush) then why are we bothering to try to root-prune pot bound plants at all – what we should really be concentrating on is planting at the proper depth.

All the above is hypothetical though – I just enjoy thinking about this stuff as the data starts to roll in.  As we get more definitive answers and start to run the statistics I’ll let you know more.

12 thoughts on “Harvesting an Experiment”

  1. Linda, I’m so glad you folks are doing this research. I am always very reluctant to cut roots out of girdling root-phobia as the one thing I know is that reducing roots will slow establishment.

    One thing I would have liked you to try is to not cut, but instead to stretch out circling roots so they are planted into their new soil.

    This is something I do all the time but have never evaluated it comparatively. It just seems
    so logical that getting those roots out of the potting medium and into the actual soil is bound to be helpful. It definitely helps to anchor trees without the need of stakes.

    If you evaluate it scientifically it’s your idea.

  2. So the tree species didn’t make a difference? I take it the soil wasn’t amended either. Were the tree mulched?

  3. I’d say the best thing is not to plant them at all.
    It seems to me that the circling roots are bound to become girdling roots in another 5-10 years.

  4. What a great idea for a research study. Can’t wait to read the findings and learn what follow-up questions … umm … follow.

  5. As a gardener, I am interested in root growth, but don’t forget the big picture; how does the tree above the ground look? It seems to me we may not understand everything about root function, so let’s look at the desired result.

  6. I actually did this sort of experiment in my own backyard, though I didn’t plan on it being an experiment. It’s just that I decided to replace some bushes and trees that I planted a couple of years earlier with better ones. This too gave me an opportunity to see what happens to pot-bound roots that were mostly left unpruned.

    I also noticed that circling roots start growing straight once they’re out of the pot, though the old circular portion of the root will of course retain that shape. I would agree that making sure the root flare is exposed and there are no roots strangling the trunk is much more important.

  7. I replanted a Hicks Yew many years ago that a relative gave me when he had to move. In my ignorance, I didn’t do anything to the already encircling root ball. Years later, when the poor thing was about dead, I dug it up to move it to a better spot. This time I was much better educated in the issue of shrub roots and such and noticed that the poor root system had never left the original planting hole. There was a mass of roots, all entangled, within the hole. That was in some pretty awful clay soil. I guess the native soil might make a difference, but now I always gently spread out the roots of any pot grown plant and chop around the sides of the planting hole so the roots can more easily get started in the right direction. For whatever this story is worth… I’m anxious to hear the rest of your results and your interpretation of them.

  8. Sandy, in your case the roots probably didn’t like the surrounding soil, either it was too dry, the wrong pH, too hard or whatnot. I noticed the same thing about a blueberry bush that I used to have. Blueberries like really acidic soil and I have no such thing in my yard. Therefore the roots never expanded beyond the original root ball.

  9. You’re right, Mike. The soil was relatively hard clay and probably around 5.5 to 6.0 pH, not exactly the conditions a yew prefers. At that time I was among the uneducated homeowner class of gardeners. So now that I’m somewhat better educated in the ways of plants, I try to help others do a better job than I did back then.

  10. I hope you & others will evaluate depth issues more!

    My first Japanese Maple was plant too deep & almost died despite spreading roots out. I dug it up early the next spring, added bags of soil under so it sat atop a mound & it has thrived. That lesson was applied to my next Japanese Maple & it is thriving. Both are in Georgia hard clay amended with black soil, & the 2nd one also had cottonmeal & chicken grit added because of nearby roses. Both contend with Silver Maple roots.

    Recently replaced a River Birch with a Japanese Maple. Amended hard clay soil with some black soil & added slow release fertilizer. It went in a bit too deep & new shoots are already showing signs that it will need to be raised, too.

  11. I wonder about washing the container soil off the root ball prior to planting and un-circling the circling roots, pinning them under some soil to anchor them into an outward pattern. I have a deciduous azalea that was started in a four inch pot (probably), moved to a larger pot, new root growth continued to circle, and then finally into a gallon container. Much of the container soil was very loose, almost like mulch bits, so when I took it out of the container a lot fell off right away. Seeing the root structure close to the stem (trunk) of the shrub so tightly wound already, I knew I needed to do something more than just cut four slices and an X or cut into a square. A couple of the roots needed to be pruned away to unwind the majority. This azalea has been in the ground for two full years, hoping this year for it to bloom.
    Yes, in hindsight I should have walked on by this shrub and not purchased it. Plant lust is hard to ignore.

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