All About Hydrangeas

All About Hydrangeas

Hydrangeas are grown in gardens the world over, and it isn't hard to see why! These classic flowering shrubs offer stunning masses of blooms in shades of blue, purple, pink, and white that are held above pleasing mounds of foliage. There are hundreds of varieties to choose from for both the shade garden and the sunny border, and the care for each is a little different, which is why we've put together this guide to help you grow Hydrangeas to perfection here in the Pacific Northwest.

Shade vs. Sun Hydrangeas

You will often hear people refer to "shade" and "sun" hydrangeas, and while this is useful shorthand, it's a bit of an oversimplification. In truth, most hydrangeas benefit from at least some shade — especially in warmer climates like our Portland Zone 8 — and only a few will tolerate the extremes of full sun or full shade. That being said, it's helpful to know that most of the hydrangeas commonly encountered in the nursery trade are derived from one of two species: The Bigleaf Hydrangea (H. macrophylla) and the Panicle Hydrangea (H. paniculata). The latter is what most people have in mind when talking about a "sun hydrangea," and by contrast, the term "shade hydrangea" is applied to most other plants in the genus, but especially cultivars derived from H. macrophylla and the closely-related H. serrata.

Panicle Hydrangeas perform remarkably well in full-sun conditions, and a few others can be pushed into the sun as long as they are provided with sufficient moisture — namely, the Smooth Hydrangea (H. arborescens) and the Oakleaf Hydrangea (H. quercifolia). (Oakleaf Hydrangeas are perhaps the most adaptable in this regard, being able to tolerate a fair amount of either sun or shade.) Even among the so-called "shade hydrangeas," in too much shade, their floral display may be diminished and they may exhibit floppy stems. To ensure good bud set and garden performance, the ideal site for growing shade hydrangeas is one that receives morning sun and afternoon shade. All hydrangeas will benefit from supplemental water during the heat of the summer, especially within the first few years as the plants get established. But after this, Panicle Hydrangeas are more drought-tolerant than most of their counterparts.

Hydrangeas have two kinds of flowers — sterile and fertile — which are arranged into flower heads called corymbs. Many shade hydrangeas have been bred to maximize the number of showy sterile flowers for that classic "mophead" or "hortensia" form. Their full, domed flower heads create a beautiful display, but at the expense of much of their value to pollinators. If you want to supply your local bees and butterflies with nectar and pollen, "lacecap" forms featuring a ring of sterile flowers around a central cluster of small, fertile flowers are a better choice. Many sun-tolerant hydrangeas also feature a good mixture of both sterile and fertile flowers, which tend to be arranged in longer, more conical flower heads. No matter which kind of hydrangea you choose, though, they make beautiful garden subjects with just a little care, and their blooms make lovely cut flower — both fresh and dried.

Hydrangea Pruning Demystified

The most frequent questions we get about hydrangeas relate to pruning. Many people have learned the hard way that cutting them back at the wrong time can result in a plant that never flowers. (While the leaves of hydrangeas are lovely — particularly on Oakleaf Hydrangeas — it's their blooms that are their selling feature!) The good news is that pruning hydrangeas isn't nearly as complicated as many people fear. It all boils down to whether the plant flowers on new wood or old wood.

Plants that flower on old wood set their blooms for the next year during the previous growing season, and these buds overwinter on the plant, allowing them to flower earlier in the spring. In contrast, plants that bloom on new wood set buds on growth from the same year and tend to bloom later in the summer and into the fall. As a general rule, most hydrangeas that prefer a shadier spot bloom on old wood, and hydrangeas that grow in more sun bloom on new wood, but it's a good idea to double-check what variety you have before pulling out the loppers. That's why we've put together the handy chart below for quick reference.

Variety Exposure When to Prune Flowers on
Climbing Hydrangea (H. anomala subsp. petiolaris) Shade to Part Sun After flowering (summer) Old Wood
Mountain Hydrangea (H. serrata) Shade to Part Shade After flowering (summer) Old Wood*
Bigleaf Hydrangea (H. macrophylla) Part Shade to Part Sun After flowering (summer) Old Wood*
Oakleaf Hydrangea (H. quercifolia) Shade to Sun After flowering (summer) Old Wood
Smooth Hydrangea (H. arborescens) Part Shade to Sun Late winter/early spring, before spring growth New Wood
Panicle Hydrangea (H. paniculata) Part Sun to Sun Late winter/early spring, before spring growth New Wood

*Some newer cultivars of bigleaf and mountain hydrangeas are remontant or reblooming, meaning they set blooms on both old and new growth, providing a longer season of bloom and helping to avoid a total loss of bloom in the event that the previous year's buds are damaged by frost. Look for varieties in the Endless Summer, Seaside Serenade, Let's Dance, and Tuff Stuff series.

It's a good idea to keep in mind that a well-sited hydrangea, regardless of whether it blooms on new or old wood, doesn't require much pruning at all. Gardeners most often run into trouble with pruning hydrangeas when they've sited a plant in too small of a place for its ultimate size. These days, there are plenty of hydrangea cultivars that stay more compact, so if you are gardening in a confined space, look for one of these!

Hydrangeas that bloom on old wood should only require dead-heading to keep them looking their best. Spent blooms can be removed once they begin to fade, but your cuts should be sparing — only going down to the next pair of leaves. If you need to make other cuts, do so selectively to shape the plant immediately after it has finished flowering to give your hydrangea as much time as possible to grow and set buds for the next growing season.

Hydrangeas that bloom on new wood respond much better to pruning, and can be cut back anytime between late fall and early spring. (If pruning in the fall, wait until two weeks after the plant has dropped all its leaves to make sure it has entered its dormancy; otherwise, you risk your hydrangea sending out a flush of new growth that will get knocked back by frost.) If you are starting with a very small plant, it's best to keep pruning to minimum until your specimen is larger, but once it has filled out, you can cut back up to a third of the plant's height per year.

Color Shifting

Hydrangea lovers are surely familiar with the unique color-changing ability of some cultivars: Over time, their blooms can morph from blue to pink and back again — and in some cases, the difference can be quite dramatic. This change is based on the availability of aluminum in your soil, which in turn is linked to soil acidity: Alkaline or basic soils result in pinker blooms, whereas acidic soils result in bluer blooms. As such, gardeners can influence their plants in either direction with specific soil amendments, but only if the plants have the genetic capacity to do so.

Unfortunately, no amount of effort on our part as gardeners will cause a white-blooming hydrangea to produce more colorful blooms. And not every pink hydrangea can change color, either. Some modern cultivars of more sun-tolerant species like H. paniculata, H. quercifolia, and H. arborescens feature blooms that are tinged with — or age to — a reddish pink, but the pigments responsible for this coloration are distinct. Only hydrangeas derived from H. macrophylla and H. serrata that naturally bloom blue, pink, or purple can perform this color-shifting trick.

If you happen to have one of these hydrangeas, you can nudge their blooms in either direction along this color spectrum, but keep in mind that the process isn't instantaneous. For pinker blooms, regular applications of agricultural lime can lower the pH of your soil. And for bluer blooms, you can use an organic fertilizer formulated for acid-loving plants or a "blueing formula" like the one we sell at the Farm for just this purpose! Most Portland gardeners enjoy decently acidic soils, so blue-purple blooms are readily achievable.

With their timeless beauty and long-lasting blooms, hydrangeas are a welcome addition to any garden, and with so many types to choose from, there's sure to be a hydrangea to fit your unique growing conditions. We carry many varieties at the Farm, and our team is always happy to help you out with selecting the best one for your space.