To prune or not to prune during late summer? Not in September, landscape gardeners. …not unless you have no choice or the pruning will be very light. We are refering to woody plants, of course.
As a generalization, the best time to prune your woody plants is in early spring……with the exceptions: 1) certain deciduous plants whose blooms you value and want to show off during the coming season, and 2) certain evergreen conifers such as pine, fir, and spruce. Plants in both of these categories should not be pruned in late summer or fall.
Pruning out broken limbs, die back, or very light corrective pruning usually can be done at any time of the year.
Some deciduous shrubs can be pruned to the very ground in late fall and produce beautifully the next spring. The smokebushes, (Cotinus) hardy usually in the Twin City area is an example. Black Lace Elderberry is another.
Most arborvitae, juniper, hemlock and yew can be trimmed or gently reshaped in late summer or autumn. Radical pruning is not recommended.
Those flowering deciduous shrubs whose blossoms you cherish, should be pruned immediately after their peak blooming season. French lilacs, then, would be pruned sometime around June 15th, a bit later for the late blooming varieties such as Donald Wyman or Minuet. VanHouttei Spiraea, often called northern bridal wreath spiraea, blooms around Memorial Day in the Twin City area, and should be pruned at the same time as the French lilacs. Most of the rest of the spiraeas develop their flower buds during the same time as their new spring foliage, called “new wood”. French lilacs developed their flowering buds the previous late summer and fall along with last years foliage, which wood would be called “old wood”.