Pruning hydrangeas can be very tricky because there are so many varieties and species. I hope in this blog to help you understand how to prune your hydrangeas to keep them healthy, happy, and flowering at their best. I am sharing the advice of Dr. Bir, North Carolina plant expert, extraordinaire.
According the Richard Bir (http://www.ces.ncsu.edu/fletcher/staff/rbir/index.html), “The most straightforward hydrangeas to prune are the mostly white flowered, extremely tough and hardy Hydrangea paniculata and Hydrangea arborescens cultivars.” The H. paniculata cultivars found at Caan’s, include, ‘Great Star’, ‘Pinky Winky’, ‘Tardiva’, ‘Quick Fire’, ‘Pee Gee’, and ‘Bombshell’. H. arborescens cultivars include ‘Annabelle’ and ‘Incrediball’, and ‘Bella Anna’, and ‘Invincible Spirit’. Dr. Bir further states, “All selections of these two species flower on new wood. Therefore, they can be cut back immediately after flowering…or they can be cut back in the winter and still be expected to flower the following year. How far you cut them back is strictly personal preference. Unless you live in a restricted community with pruning covenants, there are no pruning police. You do not HAVE to prune H. arborescens or H. paniculata at all but it makes for a neater plant…”
Again, according to Mr. Bir, “These species, the oak leaf and big leaf hydrangeas, are alleged to flower on buds that were formed the previous season. Therefore, if you prune them in late fall, winter or early spring, you will be cutting off that season’s flowers. If drastic pruning is required, do it immediately after flowering. The best and safest way to prune them is to remove some older stems but leave most stems so that you are removing about one-third of the growth each year. That said, my experience is that these plants have not all read the books. We have cultivars in our H. macrophylla trials that continue to produce new flowers for months on growth that is feet from any buds that could seemingly have been formed the previous season. It seems unlikely that these flower buds opening in September were formed the previous September. Many of these cultivars are also killed nearly to the earth every winter but regrow from buds an inch or two above the earth and flower a few feet above the ground. In nurseries I see quart liners put in five gallon pots that have full five gallon, flowering plants of cultivars like ‘Nikko Blue’ by fall. These flowers all appear to be on current season growth to me.”
Now, let’s talk about the rebloomers like “Endless Summer” and “Forever and Ever” mopheads. In my experience, wait till the new growth greens up nicely—don’t rush, these plants are a little slow off the starting block in spring, then prune off the obviously dead tips of the branches. There should be nice big fat buds visible on the healthy green growth below. After the first run of blooms, remove the flower heads promptly to induce new growth.
For winter interest, I like to leave the last run of flower heads on the plant for winter interest. As with any woody plant, it is not good to prune too late in the season. This exposes the plants to winter injury. So, leaving these late season flowers on is a great way to prevent late season pruning.
One final thought, hydrangeas that aren’t white are unique to the plant world because you can manipulate the color of the blooms. A neutral to alkaline soil will produce pink flowers; an acidic soil, blue ones. The easiest way to turn them from pink to blue is to add Espoma Soil Acidifier along with a bit of peat moss around the base of the plant. Mulch as needed to dress up your garden bed.