A few years ago I came across some tall and slender black pines that made a good starting point for creating literati style bonsai (see examples at “Decandling young bunjin pines“).
The trees were the result of neglect. For several years they’d grown, fifty to a flat, under a bench where they received little light. I wanted to see if I could reproduce the effect so I filled a flat with seedlings last year.
The trees grew well this year, but as the foliage became dense, I wanted to thin the needles to preserve low branches and discourage aphids.

Two year-old black pines

After thinning needles – 14″
I plan to let the trees grow for at least one more year in this container. I’ll consider repotting them next winter depending on how many roots develop over the coming year.
There’s a chance I’ll add some movement to the trees while they’re in the flat, but I’m more likely to wire them when I repot one or two years from now.