Prune Siberian pea trees after flowering. The plant flowers in the spring, so blossoms develop during the growing season and overwinter in buds. Cutting them at the beginning of the season prevents the plant from flowering.
Identify a central leader. Because Siberian pea trees grow into a bush if not pruned, you must choose the strongest upright branch to act as a trunk.
Remove branches that compete with the central leader. Cut them off where they start growing. Also target branches and limbs that are crowded and rubbing against each other. These prevent optimum air circulation through the tree's canopy.
Eliminate broken, diseased and dead branches whenever you notice them. Cut them off where they meet healthy branches that you plan to keep. Make the cuts flush; stubs invite infestation and decay.