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I have initiated a new experiment for development of bonsai material. When we want to grow a thicker trunk we often hear the advice to “put it in the ground and let it grow.” In my experience collecting material from the clay-heavy soils in my yard, plants growing in the ground will often send their roots ranging far and wide which can result in a tree with too few feeder roots close to the base.

One approach to improving this situation is to use a spade to cut around the plant periodically. Another is to dig a hole in the clay soil and fill it with a looser soil such as sand, effectively using the hole in the ground as a pot. Both are effective ways of, at least partially, containing the roots while allowing the plant to gain girth and vigor. These also lack a certain amount of control that I prefer to have.

My experiment is an attempt at combining these approaches with a grow box which will contain the majority of the root mass. The thing that makes these grow boxes different from one you might use for a newly collected tree is they are not meant to be moved around, but sit directly on the ground allowing some roots to escape the box as the plants get strong.

The bottoms of these boxes are 1/4 inch wire mesh. The site for each box is leveled and a layer of loose soil is put down to support good drainage. The mesh is then placed directly against that soil layer before planting, as seen below.

This approach will give me control over the type of soil in the box, provides a defined space for the root mass and fertilizer application, and creates a barrier between individual plants. It will still allow roots to grow downward into surrounding soil. In theory, the wire mesh might strangle roots that get too large creating a sort of self-pruning, but I fully expect some thick roots to connect the box with the soil below in cases when the tree has been growing for several years.

Part of the experiment we will all have to wait to learn about is to see just how difficult or easy it is to lift a tree grown in the box over several years. If I decide it is necessary, I may elect to cut directly under the box with a straight shovel blade to cut roots at an interim phase.

I have planted a few Amur maples, a hornbeam, and a trident maple in these boxes to start, and each is planted on a tile to encourage a flattened root spread directly below the base of the tree.

I look forward to seeing how this works, and I will be sure to share the results.