When to prune your hydrangeas is a question that has been asked by gardeners year after year. Before you take out those secateurs knowing which type of hydrangea you have will make pruning a lot easier.
There are many species of Hydrangeas but they can be divided into two large groups, those that bloom on old wood and those that bloom on new wood.
The following is a list of the more common species and whether they bloom on old or new wood:
H. macrophylla earns the nickname 'Mophead,' due to the recognizable shape of their blooms. Blue, pink, purple and white are the most common colours. Mophead hydrangeas do not have to be pruned back unless they are very old. Removing dead stems and blooms is the only pruning that should be done for the health of the plant, and these can be removed at any time.
These hydrangeas bloom on old wood. (Stems that have been on the hydrangea since the summer before the current season.) They produce flower buds in August, September or October for the following summer's blooms. If those stems are pruned in the fall, winter, or spring, the bloom buds will be removed, and there may be little or no bloom the following summer (usually June/July for the northern hemisphere).
When pruning these ‘old wood’ hydrangeas, you must hold off cutting until you see their little buds called “broccoli.” It’s only when those buds emerge that you will know which stems to discard. However, if you need to cut your plant back because it has outgrown its allotted space, then have at it knowing the potential consequences.
Unfortunately, H. macrophylla is the variety most susceptible to winter bud injury in Southern Ontario. Our winters can be severe, so you might need to offer it some winter protection from severe winds to protect the flower buds. Tying the branches together and wrapping them with burlap isn’t pretty, but it could mean winter survival. Remove the burlap when the buds begin to swell.
Recently new H. macrophylla cultivars have been developed that bloom on old and new wood, like the Endless Summer Series and Let’s Dance Series.
If your flowers are football- or cone-shaped, you have either a panicle or oak leaf hydrangea. Flowers start out cream, white or green, and age to shades of pink. No blue flowers are ever on either of these two varieties.
If the foliage looks like an oak tree, you have an oak leaf hydrangea. You might have ‘Alice’, ‘Snowflake’, or “Snow Queen’ among others. The blooms on Hydrangea quercifolia form on old wood (last summer’s growth). Oak-leaf hydrangeas generally do not need to be pruned except to limit their size or direct the shape. When necessary, branches can be pruned back half-way as the flowers fade.
'Annabelle' is one variety in the species H. arborescens other varieties on the market include ‘Incrediball®’ and ‘Invincibelle® Spirit’. Their blooms may bring lollipops to mind. They are usually very large, and made up of tiny individual blossoms. There is one trait that sets 'Annabelle' apart from most other hydrangeas - the blooms open green, turn white for 2-3 weeks, and gradually turn green again (at which time they can be used in dried arrangements). These huge blooms are notorious for falling over in wind and rain storms.
H. arborescens blooms on new wood (stems that developed on the plant during the current season), so late winter or early spring is the best time to prune. Just look for the first set of fat new buds and prune back the stem to just above this new growth. If you have a lusty plant and want to keep it contained, you can cut back drastically -- almost to the ground -- in late winter before the new growth appears. Or you can leave more of the woody stem in order to give the heavy heads more support. Cut any dead branches to the ground at any time; they'll be brittle, so easy to identify.
Pruning in late winter or early spring when bare branches of the plant are exposed makes it easier to carry out the necessary cuts. Some gardeners prune to rejuvenate an older plant. This can be done every 3-5 years. If you remove several of the oldest stems by 1/3rd, take out dead, broken, diseased and crossed branches it will produce a much healthier plant. Pruning back some of the branches to 18-24" provides a sturdy framework for supporting new blooms.
Panicle/cone shaped hydrangea (‘Limelight,’ ‘Vanilla Strawberry ™,’ ‘Strawberry Sundae®,’ etc.), on the other hand, flowers on the wood it will grow in the coming months, i.e., new wood.
As Limelight hydrangeas only bloom on new wood, it is best to prune them in late winter or early spring. This way you will avoid cutting off any flower buds for the new season. If pruning is done before the new growth appears, you can prune them all the way down to above one bud union on each stem and you can also prune down any main branch that you want to keep small. Strong new growth will appear where these cuts are made. It will produce much larger blooms if pruned hard like this each year. Some gardeners prefer to leave a framework of old growth to reduce flopping. This may result in smaller blooms but on sturdier stems.