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. 

My post on root aphids is now available!