The root pruning scissors, root pruning shears or Hasami, are used to cut thicker or heavier roots. The broad, sharp blades lets you easily cut through the entire width of a thick root and the handle has a very broad grip for easy handling. The root pruning scissors can be considered as one of the “heavier” tools for bonsai cultivation (in the same category as the concave cutter). Usually, smaller general purpose scissors or fine twig shears are used to prune a large part of the (delicate) root ball. Some trainers use the root pruning shears to cut branches as well, although this is not advised. Other tools are more suitable for branch pruning.
When to use it?
Use it to cut the thicker or bigger roots of your bonsai tree. Needless to say, you will only use this tool when you repot your tree.
When not to use it?
Do not use this “heavy” tool when you only have some thin, fragile roots to prune. You could use general purpose shears for that.
For a comprehensive overview of all bonsai tools and their application, check out the Daily Bonsai Tools Wiki page.