Terra Cotta Vs. Colander – A small sample root experiment.

Posted by on Feb 21, 2020 | 9 Comments
Terra Cotta Vs. Colander – A small sample root experiment.

Question: A couple years ago, I asked myself – is growing in colanders/pond baskets superior for Japanese Black Pine destined to be bonsai? Common bonsai knowledge says that the answer is yes, but I’ve seen growers get really good results from other containers. I set out to see what differences I might see between two different types of containers.

Experimental setup: Eight Japanese Black pine from the same batch of seeds, started in February of 2016 were transferred in January 2018 into larger growing containers. Four were placed in #8 Japanese terra cotta containers and four were placed in blue plastic mesh colanders, circular 9″ diameter and 4″ deep. Soil volume for the two types of containers was close, but not identical (didn’t think to measure the soil used beforehand…)

  • Soil: 30% Akadama, 40% Pumice, 30% Scoria
  • Irrigation: Hand watering, to maintain soil moisture at optimal levels. This includes watering in different amounts where some trees dry out more quickly than others.
  • Fertilization: All eight trees were kept together, and the fertilization schedule included various types over a two-year period including Dyna-Gro, Cottonseed Meal, Japanese Pellet fertilizers and American Pellet fertilizers.
  • Sun Exposure: All eight trees have been kept together at all times. Sun exposure was optimized for each tree on a regular basis by repositioning as needed. Trees changed growing location twice during the two year period, but at the same time and as a batch.
  • Pruning: Central leaders were left to grow while low side branches were pruned at decandling time in summer. No other pruning was performed.
  • Repotting: No repotting, root pruning or significant top-scraping was performed during the two year period.

Results: In mid-February 2020 I repotted all eight trees to examine the root structure and determine if the type of container had made any difference.

The measurements show that Terra Cotta on average produced a larger tree, larger girth and more root mass; however, most of the added root mass had to be removed during repotting because it was on the outside and bottom of the containers. Retained root mass after repotting was higher for the colander grown trees. Full Data containing measurements is here.

Discussion: Subjective ranking of the quality of the nebari and roots lead the colander grown pines to rank 6,2,5 and 1, while the terra cotta ranked at 7,4,3 and 8. Note:I knew which came from which and had an expectation.

An unexpected result was that three of the four pond baskets contained significant visible mycorrhiza and none of them contained root aphids. Three of the four terra cotta contained root aphids and no visible mycorrhiza, while the fourth contained mycorrhiza and no root aphids.

Root quality and quantity did not strictly correlate to the size of the trunk and height of the tree. The shortest tree, colander 4, had the best root structure and tied for the largest trunk girth. The tallest tree, terra cotta 7, was the most heavily infested with root aphids and also tied for largest trunk girth. Terra cotta 8, had poor root structure; the lowest root volume; the second shortest height and tied for smallest trunk girth.

Conclusions: I’ve summarized in a graphic above. There is not really much surprise. Terra cotta will work perfectly fine, but it does cause additional time and labor in repotting, and may decrease vigor after transplanting trees due to the more severe root reduction.

The blue round colanders, become fragile after just a couple years in the sun and are less stable in windy conditions than the terra cotta or similar size pond baskets. The colanders have a small foot – a circle of plastic that raises them off the ground about 1/4″. However roots regularly fill this space, so no ancillary benefit of reduced disease transmission seems realistic. I recommend the square black baskets as I mentioned in my last post.

Stay tuned for a post on root aphids coming soon!


  1. Ray
    February 21, 2020

    Great information Eric😎, Thanks very much. I use the terracotta and found the same thing but luckily for me no root aphids

  2. Zack Clayton
    February 21, 2020

    Looking at the roots shown, I would guess that I would want the terra cotta pots to start an exposed root JBP. Does this jibe with your thoughts? The micorrhiza in the colander is impressive.

    • Eric Schrader
      February 21, 2020

      Hi Zack, I think so. For any style where the density of the roots is not an issue, and it’s okay to spend extra time removing circling roots terra cotta may be better. However, keep in mind that the lower branching in the terra cotta may lead to more root die off if you’re cutting back to larger unbranched roots. It’d be interesting to run a sample of just exposed root style and see what happens.

  3. Charles Mosse
    February 21, 2020

    Great info. Nice to see larger sample sizes than just one or two pots. You and Jonas are doing us all a good service. Thanks!!

  4. Barry L Altshule
    February 22, 2020

    Hi Eric,
    Very interesting study. Not sure if this is another variable to consider or if it would make a difference where you conducted the experiment, but terra cotta pots get really hot here in Southern California full sun and tend to make soils dry out faster than in the exposed plastic baskets. Just something to think about.

  5. Antonio
    February 23, 2020

    This is amazing. I’d love to see if you could continue the analysis what the results would be in 8 years. I wonder if after certain time, colander, due to have more roots would be able to absorb more nutrients and produce more growth than colander.

    I guess, in colander every time a root gets exposed it needs to use energy to generate a new one, while with terracota it just continue. Maybe after certain limit, colander height and girth growth is better than terracota.

    what do you think?

    • Eric Schrader
      February 23, 2020

      Interesting question – but I don’t think you could do a valid comparison because leaving a tree undisturbed in terra cotta for 8 years would definitely result in some health problems or at least root die off. The colanders are definitely a labor-saving devise. As to whether or not the colander grown tree would overtake the terra cotta after a few more years… From what I saw, both needed repotting. Left without repotting I think the colander grown trees would be healthier in years 4-6. After that I’m not sure. Worth a test!

  6. ceolaf
    February 27, 2020

    I get this comparison. Makes sense.

    But there are more pots that one might add. For example, what about coir or peat pots? Might they offer a compromise?

    Would coir pots have the advantage of pond baskets, without the plastic?

    Inquiring minds want to know.

    • Eric Schrader
      February 27, 2020

      That’s a great idea! I happen to have a larger batch of JBP I’ll be working with for the next few years, so might give that a try.