I live in zone 5B, and last year cut an overgrown purple smoke bush down to a few inches from the ground to rejuvenate it. It came back very well, but while most of the branches grew upright in a nice spreading form, other branches were very lax and lay on the ground. Early this spring before the leaves began, I got in there and cut off all of these trailing branches and lightly pruned the good ones. The top part is doing really well, but the bottom has resprouted. I am wondering if I should go in and leave most of it, just taking out the really lax ones or any too close to the ground? I am scared of exhausting it, but it certainly doesn’t LOOK exhausted! It is growing in mixed sun and shade, clay loam, moderate water supply.
Thank you for contacting the Toronto Master Gardeners with your question on pruning your Purple Smoke Bush.
It is not an uncommon practice to hard cut Smoke Bush to generate more newer growth which is more attractive, although you do sacrifice the production of the feathery flower heads. It is a thoughtful question you pose, and a good consideration.
One additional technique you can use in doing your pruning is to look for little black buds near the bottom of the stems that you are cutting. By cutting immediately above a bud that will determine the direction of the new growth in the spring. This can help you in better structuring the new growth as it emerges.
To answer you specific question, as long as you leave enough bulk or mass in the crown of the plant to regenerate enough energy and reserve for the following winter, that is the measure. You can safely remove wayward branches that are emerging while they are smaller, not allowing to much energy be spent and thrown away with a cut. This should promote more vigor in the branches you are leaving, but ensure there is enough to grow throughout the summer, and provide a store for the following season.
So it really boils down to the proportion of new emerging growth you are culling, compared to how many newer vigorous canes you are letting develop.
Please read a good article pruning smoke bush;