TO TRIM OR NOT: If you don't know what variety hydrangeas you have? Just trim off old flowers this spring? Or deeper to increase new flowers this year? What's the rule, flowers only on new growth for Woody shrubs? Or don't cut?
Scott County Minnesota
There are many varieties of hydrangeas and some bloom on old wood and others on new wood and others on old and new wood. Below is a quick summary of some of the more popular hydrangeas and their pruning requirements.
The Bigleaf Hydrangeas (Hydrangea macrophylla) have showy pink, blue or purple flowers (depending on soil pH - acid soil yields blue flowers; alkaline soil causes a pink color). They bloom in the summer on the old wood (from flower buds formed on the previous season's growth), so they can be pruned back immediately after the summer flowers fade. Don't prune them in the spring or fall, or you will be removing the flower buds.
Oakleaf Hydrangeas (Hydrangea quercifolia) bloom from both old wood (mostly) and new wood, starting to flower in June. The showy white flower clusters may fade to pink and then brown. This type of hydrangea does not need much pruning. If light pruning is necessary, do so as soon as flowers fade (by mid-July), to give plenty of time for re-growth, new flower bud development, and winter acclimation.
Panicle Hydrangea (Hydrangea paniculata) cultivars such as "Grandiflora" (PeeGee) and "Tardiva" bloom during mid-to-late summer with showy white flowers changing to light pink by late summer. These plants bloom on the new growth, so they can be pruned in early March, before growth begins. The flowers will develop from the new stems produced this spring. The same would be true of the earlier blooming "Hills of Snow Hydrangea" (Hydrangea arborescens). Annabelle Hydrangea (another cultivar of Hydrangea arborescens) blooms for at least eight weeks, starting in early June. After cutting this plant to the ground in early March each year, it quickly develops flower buds on the new shoots developing in the spring. By mid-June, it is filled with huge, rounded flower clusters as big as your head. These change from light green to white for several weeks, and then back to light green, and finally brown, persisting on the plant into fall and winter.
There is one exception - "Endless Blooming" Endless Summer variety produces flowers on old and new wood. It blooms in early spring and then blooms later on new wood. This plant can continue to produce flowers all summer. Cut faded flower stems to half their length to encourage new growth and buds. Prune after last bloom in the fall to control shape and height.