Answer: Ornamental plum trees (Prunus) do not usually need much pruning. The exception would be to remove dead or damaged branches and that should be done as soon as you see them. If you need to selectively trim an overly long branch, that would be done in spring right after it blooms. In general, pruning should be kept to the minimum because each cut is an opening for disease and pests. Here is detailed information on pruning.
Good luck with your plum tree!
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