Early spring is the best time to prune a mature tree peony. The window to do this in the zone 6 ande below is after the buds have begun to swell, and before new growth has commenced. In warmer areas, zone 7 and above where there is little risk of winter injury, pruning can be done in fall or winter.
Prune tree peonies once the buds have begun to swell a bit. This way there is no question if a bud is alive or not. In New England, this typically occurs in late March.
Recommended tools are a pair of bypass pruners and a small pruning saw. Make sure they’re sharp!
Look at the plant as a whole and identify dead wood.
Cut the entire dead side branch down to the main branch.
When pruning a branch which still has some viable buds, cut down to the nearest live bud. The cut should be positioned just above the first live bud and angled down, sloping away form the bud. This allows water to more easy drain off and results in better healing.
A good pruning cut. We do not coat the wood with any substance.
A poor pruning cut. The stub of dead wood above the bud is too long. This does not promote good healing.
After you have pruned up the upper branches, examine the base of the plant. Spindly interior growth often dies and this dead wood should be cut to the ground.
Saw large diameter stems or branches.
Finally, after all the dead wood is removed, examine the plant for small and spindly growth. If the plant is vigorous and growing well, remove these very small side branches. They are usually found in the interior of the plant. They will not flower and will only inhibit air circulation.
Lastly, remove all pruning from garden, they could potentially harbor fungal spores.