I have a Laburnum grown from RHS seed. I wish to bonsai the tree (it is currently about 2″ high and in the UK). I would like to know when the tree should need root pruning, branch pruning (obviously when it is much bigger). What is the best soil mixture and what should the watering regime be. I have looked in my bonsai books, but Laburnum trees are somewhat neglected. Any help would be appreciated.
I consulted the many bonsai titles in our library, and found only one which mentions Laburnum in the index, The Complete Book of Bonsai by Harry Tomlinson (Abbeville Press, 1991). The book offers no specific pruning directions for Laburnum. I will paraphrase what the author says: The bonsai specimen should receive full sun, and daily watering plus feeding every 2 weeks during the growing season. It should be repotted annually in early spring, using basic soil mix. Trim shoots to 2 or 3 buds. Appropriate bonsai styles for this plant are informal upright, slanting, semi-cascade, cascade, twin trunk, clump, and multiple trunk.
The book Bonsai: The Art of Dwarfing Trees by Ann Kimball Pipe (Meredith Press, 1964) offers general directions on maintaining a bonsai specimen. Pruning of roots and branches is probably best done in early spring before new growth begins.
There are also online resources. The website of the American Bonsai Society, used to have a list of bonsai resources which included this list of articles on bonsai trees, written by Brent Walston. Here are links to a few of the articles: