How to Prune Hydrangea Plants-Expert Advice for the Novice
The easiest way to avoid having to prune is to always plant your hydrangeas where they can grow au naturale and won't need pruning, except to clean out dead stems or to deadhead the blossoms as they fade, both of which can be done at any time of the year.
But, if you have established hydrangeas, the first thing you must do is attempt to identify them. If you already know the type of hydrangea you have, just skip down to the How to Prune Your Hydrangea section. Different types of hydrangeas have different pruning requirements and improper pruning can literally mean a bloomless seasonor worse.
How to Identify Your Hydrangea
There are four common types of hydrangea:
Mopheads & Lacecaps (macrophylla) are considered one group. The leaves on this species are usually heart-shaped or ovoid with serratededges and are about 4 to 6-inches long and 3 to 5-inches wide, though some varieties will be larger. The leaves are somewhat thick and semi-shiny. The leaf stems are the biggest clue to your hydrangea's identity, especially when combined with the type of flower it produces. The leaf stems (petioles) on a mophead or lacecap will be short, meaning that the leaves hug the main stem. On a mophead hydrangea, the blossoms grow in round and oval mounds of tightly clumped individual flowers. On the lacecap varieties, the flower head shape is almost the same, but you will have itty-bitty, lacy-looking flowerbuds in the middle, surrounded by larger, fully developed flowers. The buds are the fertile flowers, while the full blossoms around the edges are infertile. Though considered one group when it comes to their pruning requirements, each of these look quite different when in bloom. It is also interesting to note that mopheads are the ONLY hyrdrangea that has colored blossoms when they first open. All other species will be white, so that may be the first hint that you don't have a mophead, unless you have a white mophead cultivar, which is relatively uncommon. Photo is courtesy of Ginger.
Oakleaf hydrangeas are so-named, just for that reason; they have leaves that are shaped similar to the leaves on a red oak. The size of the leaves can range from 4-inches to 10-inches long and wide, and will often stay on the plant most of the winter. They are not really considered an evergreen though because after several freezes they are not very attractive. They also have cones of flowers, as opposed to mounds or balls of flowers and ALL oakleaf hydrangeas will bloom white before changing color. It should be very easy to identify this species if you have it. Photo is courtesy of RPOP.
Snowball hydrangeas (H. arborescens), the most common of which is â€˜Annabelle', might remind you of lollipops. The flower heads are usually very large, but made up of tiny, individual blossoms. The leaves are usually thinner, though oftentimes heart shaped, somewhat similar to the macrophylla. They also tend to be a bit floppier than the ones on mopheads and are not shiny, instead having a matte finish. The leaf stems (petioles) are also long, holding the leaves further out from the main stem. The one single trait that sets this species apart is that the blossoms will open green, turn white for two or three weeks and then turn green again, which is when you can dry them. These humongous blossoms also tend to fall over in high wind and heavy rain, so you might want to plant them on the side of the house with the least wind, as long as it is not fully shaded.
Finally there is the PG hydrangea (paniculata). The leaves are normally smaller than other hydrangeas; they are also thinner and can either be finely or coarsely toothed. They have a rougher overall texture and are medium-green with a matte finish. The biggest identifying characteristic is that the leaves grow in a threesome from one stem node and are spaced around the node, in a whorl. This type of hydrangea can be pruned to grow in both a tree and shrub form and is also not easily identified by the type of flower heads. They can be cone-shaped or round, full or sparse, stand erect or droop. In fact, the name paniculata is derived from the panicle-type flower head that most of them bear. They, like the oakleaf, will first emerge white, turning pink as they age. These hydrangeas can grow to lofty heights of 8 to 10 feet and sometimes taller, matching their height in breadth. Photo courtesy of Alan Buckingham.
How to Prune Your Hydrangea
Now that you've identified your hydrangea(s), we can talk about how to prune to keep your hydrangeas healthy and beautiful. Please note that pruning and deadheading are two different things. Deadheading is just removing the old blossoms as they fade, while pruning changes the total appearance and form of the plant. There are two methods for pruning:
Method 1This method is for mopheads, lacecaps and for oakleaf hydrangeas. These hydrangeas bloom on old wood which means they develop buds on stems that have been on the plant since the summer before the current season. They develop these buds sometime between August and October the previous year, for the following summer's blossoms. Therefore, if you remove these stems in the late fall, winter, or spring, the flower-producing buds will be removed, meaning NO FLOWERS (or only a very few) this summer. So, prudence and patience is required when pruning mopheads, lacecaps or oakleaf hydrangeas:
- You can remove dead stems at anytime throughout the year and they should be removed every year.
- Once your plant is at least 5 years old, remove about 1/3 of the older, living stems, cutting them down to the ground in late June through early August. Try to choose ones not already blooming or that are starting to look a bit naked. Keep an eye to how this will change the shape of the current plant, stepping back once in a while to see how it's looking. Doing this will revitalize the plant.
- To reduce the size of a plant, it can be cut back in June or July without doing away with the following year's blossoms, but it won't take long for it to return to its original size, which is why planting where it doesn't require pruning is recommended.
Method 2This method is used for the snowball (H. arborescens) and PG (PeeGee or H. paniculata) type of hydrangeas. Both of these hydrangeas bloom on new wood, which means that you can prune them any time of year, except in the spring when they are setting buds, or in the summer when they are either preparing to bloom or are in full bloom. Some people even grow hedges of the snowball type, pruning them back almost to the ground in the fall, so as to present a neater winter appearance; but be aware that this type of drastic pruning can keep the stems from reaching the sturdiest size in order to adequately support the huge flower heads. If you do this, you may have to stake your flowers in the spring and summer, or grow them along a fence and use string across the front to offer support when in full bloom. When pruning PGs, we don't recommend pruning every year, but trimming out criss-crossing branches or those that detract from the overall form. These hydrangeas can be pruned from the bottom into tree-form. The developing trunk and the top branches should not be removed and you should also not attempt to make it look like the tree the first year or two. Patience is key here. Each year just trim a few of the lower branches in order to expose the developing trunk, and then late nature take its natural course. One note: if a tree-pruned paniculata's main trunk is broken close to the ground, it will grow back as a shrub unless the training process is started again from the new shoots.
Though this may seem like a lot of work, it really isn't. Once you have the knowledge, the rest is easy. Butif you'd rather not worry about it there are a small group of mopheads that will bloom regardless of when they are pruned. These are called everbloomers and will bloom on both old and new wood. And if you want to amend your soil to change the color of your mopheads, it is also quite simple. Our Hi-Yield Agricultural Limestone will reduce the pH of your soil relatively quickly, resulting in blue coloration, while our Hi-Yield Aluminum Sulphate will increase the pH, giving you pink. You also might want to invest in one of our inexpensive soil testers to determine where your pH lies right now, especially if you are planting new mophead hydrangeas and want to be sure of a particular flower color.
We hope that this has provided some valuable information, as this is one of the questions that our Master Gardener sees on a regular basis. And if you still have questions after reading this, please contact Karen. Our goal is to help you to be the best gardener you can be! Happy Gardening!