Root-Pruning A Field Maple Bonsai On A Wooden Board

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Roots Maple bonsai

The Field Maple bonsai in 2005 after growing in the ground for 5 years to fatten up the trunk.

In March 2018 I had the pleasure of working an old tree of mine that I started as a sapling in 2000, 18 years previously. Now living with a new owner, the tree comes back to me for work every 6-12 months.
An Acer campestre/Field Maple bonsai that maybe familiar to some readers of Bonsai4me.com, a lot of major work has been carried out over the years to build the nebari/surface roots, including 8-9 root grafts. An article covering some of the work previously carried out on this bonsai can be seen here Grafting a Better Nebari onto a Acer campestre/Field Maple

Roots Maple bonsai

March 2018: the Maple bonsai before repotting started.

In March 2016, two years prior to this repotting, I had planted the tree on top of a piece of wood that was screwed into the base of the tree to raise all of the lateral roots to exactly the same height (the base of the nebari has had most of the roots removed over the years to promote lateral growth). The tree was then planted back into its pot from Victor Harris of Erin Pottery.
This technique is useful for most deciduous bonsai species, and in particular Acer/Maples species of all kinds, as they typically have very strong and vigorous rootgrowth.

Roots Maple bonsai

With the tree removed from the pot, I began to carefully remove the soil from the surface of the rootball to reveal all of the nebari (surface roots).

Roots Maple bonsai

Once the majority of the soil had been removed by hand, the surface roots were given a light rinse with water to enable me to see exactly what was going on.

Roots Maple bonsai

The flat piece of wood underneath the roots can be seen in this image.

We were able to look at the results of the wooden-plate and it had done its job, creating a uniform height for the nebari. As there were a good number of 1-2 year old surface roots growing across the wood that we wanted to remain at the same height, we decided to leave the plate in place for another 1-2 years.

Roots Maple bonsai

After pruning the roots to balance their vigour, the tree (and wooden plate) were placed back into the pot.

I balanced the surface roots by pruning thicker roots hard (to make them comparitively weaker) and left thinner roots alone (to encourage strength and thickening). The ideal was to encourage the roots to eventually end up a similar size as eachother, rather than a mixture of fine roots and a small number of vigorous thick roots.

Roots Maple bonsai

The untangled and radially spread roots were pinned into position using wooden skewers.

The roots were then gently teased out radially before being pinned into place with wooden skewers. As they thicken over the coming years they should create a good even radial nebari.

Roots Maple bonsai

Roots Maple bonsai

The Acer campestre/Field Maple bonsai after watering-in.

The surface roots were covered with a layer of water retentive soil with a fine crumb; lightly crushed akadama (including all fines) and biochar. The idea of using this soil is that it will hold on to moisture in this relatively shallow area of the rootball, encouraging stronger root growth.

In another 2 years, on the 20th anniversary of the beginnings of this bonsai, I will be able to see the results of this work!