Yamaki Pine at the Museum (Photo courtesy of USDA Agricultural Research Service, US National Arboretum)

National Bonsai & Penjing Museum and U.S. National Arboretum staff have repotted the Yamaki pine, one of the world’s most eminent and symbolically powerful bonsai.

Almost 400 years old, the Japanese white pine has survived generations of travel and travesty, most famously the 1945 bombing of Hiroshima where the tree sat in late bonsai master Masaru Yamaki’s garden. Yamaki donated the pine in 1976 as part of Japan’s bicentennial gift to the American people – the gift that created the Museum.

Museum Curator Michael James alongside former First Curator’s Apprentice Andy Bello worked with Patrick Lynch and Bradley Evans, two Arboretum staff members, to repot the magnificent tree on March 26. James spoke to NBF about the logistics of the repotting.

Last repotted in 2015, the tree’s soil had compacted so much that water had begun to run off the surface of the soil and barely soak in. Sometimes the water might take up to 15 minutes to infiltrate the pine’s root ball, which is a sign that repotting is needed, James said.

The particles of Akadama, or the granular, clay-like type of soil used to plant the Yamaki pine, had compacted from their usual ball-shaped structure to very small, clay-sized fines, blocking water and air from traveling throughout the root ball.

“When the soil gets that dry, it becomes almost like a brick, and then there’s no oxygen in the soil for the roots to breathe and they rot,” James said.

Finding dead roots and compacted soil could derail the repotting plan, raising issues that have to be dealt with on the spot instead of the intended process.

“It’s hard to do everything you want to do in one repot,” James said. “This one went really well and pretty much as expected. We found healthy roots all around and a really strong mycorrhizal network of beneficial fungal surrounding the roots.”

Yamaki Pine’s exposed roots (Photo courtesy of USDA Agricultural Research Service, US National Arboretum)

The process

In preparation to remove the pine from its pot, James, Bello and the staff members pushed the tree off of its pedestal and onto a hydraulic lift cart.

The tree and container weighed somewhere between 200 and 300 pounds, requiring the group to use straps – cushioned with a towel to distribute the pressure point and weight of the pine – to secure the tree to a beam on the roof of the Japanese Pavilion.

The group then carefully cut between the rim of the pot and the edge of the soil ball a few times to release the ball from the pot. Because the Yamaki pine is so large, the group couldn’t simply lift the pine from the pot. They had to lower the hydraulic cart from under the tree and carefully catch the pot as it fell away, freeing the tree, James said.

“The tree looked like it was floating, especially when you’re not looking up to see the strap and the root ball is suspended from the bottom of its trunk,” he said. “It’s a very surreal image because trees don’t float this way and they’re never positioned like this.”

Tools used during the repotting (Photo courtesy of USDA Agricultural Research Service, US National Arboretum)

James said that, during the winter, the pot had been enclosed in a mulch box, which buffered the pine’s roots from cold temperatures over the past few years, so the bottom and sides of the soil were in good shape. The surface soil, which the mulch did not cover, contained the least healthy roots, so the team used awls, small hooks and brushes to slowly break up that compact area.

“It's a little bit like art restoration and a little bit like archeology,” James said. “It’s slow, a little tedious and you have to be really careful that you are removing soil and not roots.”

One challenge the group faced was clearing away old roots that had died without removing healthy new roots or disrupting “essential” symbiotic relationships the tree had formed with other organisms.

James said mycorrhizal fungi, a white cottony material found among tree roots (see last photo below which shows the white mycorrhizae), feeds off of sugars the pine’s roots created. The pine then benefits from moisture and nutrients the fungus pulls into the roots, so the team had to ensure that some of the fungus remained.

“When you break up that soil, you destroy or hamper that relationship, and it has to regrow,” James said. “The relationship between the roots and the fungus has to remain intact for the pine to stay healthy.”

He added that no specific date is set for the next repotting, but the process could happen again between the next three to seven years, depending on how the pine recovers from this repotting, how well water drains through the soil and the tree’s overall health.

“You want to do these repots few and far between on an old tree like this,” James said.

Andy Bello with the Yamaki Pine during the repotting (@bellcraze).