Hydrangeas in the Granite StateJuly, 2019
By Emma Erler UNH
Extension Education Center Coordinator
Although I’m not usually one to pay too much attention to gardening trends, it’s been hard not to notice the buzz around hydrangeas over the past few years. Every time I turn around it seems there is another new cultivar (cultivated variety) to try.
With so many different plants to choose from, it’s easy to see why many gardener’s heads are spinning. After all, there are at least six species of hydrangeas which can be grown in New Hampshire, all with more differences than similarities. When the right hydrangea is planted in the right place, it should be easy to care for and provide consistent bloom.
To begin, it’s essential to learn the main differences between the common hydrangea species. All of which have very different growth habits, cultural needs and pruning requirements. While some plants bloom on new growth, others primarily set flower buds on old wood.
Use the following guide to decide which hydrangeas will grow best in your landscape and reward you with blooms season after season.
Bigleaf hydrangea (Hydrangea macrophylla)
Bigleaf hydrangeas are the quintessential blue hydrangeas with which most gardeners are familiar. Also called mopheads or lacecaps, bigleaf — one word is the proper spelling — hydrangeas are characterized by shiny, dark green foliage and large colorful blooms.
They tend to stay relatively compact, with an ultimate height and spread of 3-4 feet. They generally bloom on old wood and can be difficult to get to flower in New Hampshire. Cold winter temperatures and drying winds often kill flower buds, leaving plants with lush foliage but no signs of blooms.
In their native habitat in Japan, bigleaf hydrangeas are often found growing along seaside cliffs, making it a salt-tolerant plant that prefers ample moisture and moderate temperatures. In our climate, this translates to planting this shrub in protected areas out of prevailing winds, with partial shade and plenty of soil moisture.
The color of their flowers is determined by the soil. Although pH is often listed as the factor that determines flower color, it is really only the start of the process. Blooms are blue or pink depending on how much aluminum in the soil can be absorbed by the plant’s roots.
When soils are acidic, aluminum is in a soluble form and available for uptake. When soils are more alkaline, aluminum is bound in insoluble forms and cannot be readily absorbed. Thus, when soil is acidic, bigleaf hydrangea flowers are blue, and when it is not, they are pink or mauve.
Aluminum sulfate is often sold at garden centers for bluing hydrangeas, but it usually isn’t necessary in New Hampshire where our native soils are acidic and aluminum is naturally available.
Because bigleaf hydrangeas are temperamental when it comes to flowering, you’ll likely have more success if you plant a “remontant,” or reblooming, type. Remontant bigleaf hydrangeas form flower buds on both old and new growth, so you have a good chance of getting blooms even if your shrub is killed to the ground over the winter.
Fortunately, through extensive plant breeding efforts, there are now many cultivars to choose from such as Endless Summer® BloomStruck™ and Endless Summer® Twist-n-Shout®. The staff at your local garden center should be able to point you in the right direction.
Smooth Hydrangea (Hydrangea arborescens)
Smooth hydrangeas are native to eastern North America and are well-suited to most New Hampshire gardens. They are much more cold-hardy than bigleaf and are reliable bloomers no matter what winter throws at them.
Smooth hydrangeas have a mounded growth habit and white to pale pink flowers that bloom mid-summer. Plants rarely exceed 4 feet in height, but usually gradually spread outwards to form larger clumps.
Smooth hydrangeas can be grown in either full sun or partial shade but are usually happier in the latter unless the soil is kept consistently moist throughout the summer. On hot, sunny days, even when the soil is damp, this shrub will often display droopy leaves that perk up once the sun goes down or the temperature cools.
In its native habitat, smooth hydrangea grows as an understory plant in deciduous forests with well-drained, organic soils. It’s most often at home at the back of a partially shaded perennial border or cottage garden.
Only one pest, the hydrangea leaftier (Olethreutes ferriferana), has the potential to cause aesthetic damage to smooth hydrangeas. Hydrangea leaftier is a caterpillar (moth) species that is common in the Northeast and feeds on the flower buds of hydrangeas.
Damage appears as three or four cupped leaves tied together with silk at the end of a branch. The caterpillar within this leafy enclosure feeds on the flower buds and leaves. Fortunately, this pest does not cause any significant damage to the plant and is easy to control by either hand-removing and squishing the caterpillars, or pruning away infested leaves.
For many gardeners, the name “Annabelle” is synonymous with smooth hydrangea. Although Annabelle is a long-time classic, it is hardly the only cultivar on the market.
Plant breeders have introduced a suite of new plants that have stronger stems that don’t bend from dew or rain (as Annabelle is prone to do) and come in shades of pink, white and lime-green. A couple to look for are Incrediball® and Invincibelle® Spirit II.
Panicle Hydrangea (Hydrangea paniculata)
Undoubtedly the most common hydrangea grown in New Hampshire is panicle hydrangea, an incredibly cold-hardy shrub that flowers reliably from year to year. It’s the least temperamental of hydrangea species.
Although it prefers evenly moist soil, it is also very drought-tolerant (once established) and will grow without complaint in both sandy loam and clay. It grows as an upright, low-branched small tree or large shrub, whose branches tend to arch under the weight of numerous, large conical flowers.
Panicle hydrangeas bloom slightly later than bigleaf or smooth hydrangeas, typically reaching their peak in August. As the flowers age, they change from white to pink and last throughout the fall unless clipped for dried floral arrangements.
The size of blooms is impacted by pruning. Flowers on untrimmed plants are typically around 6 inches long, while those that are heavily pruned can produce huge flowers that are upwards of 12-18 inches long.
Most gardeners find that a happy medium between these extremes suits their tastes. Panicle hydrangeas can be grown in either the sun or partial shade, but will flower more heavily in full sun.
In choosing which panicle hydrangea to plant, the most important thing to keep in mind is mature size. Although they can withstand heavy pruning, there is no sense in planting a cultivar that will quickly outgrow its space and pose a constant maintenance problem.
Old-fashioned cultivars such as “Grandiflora,” commonly called peegee, can grow upwards of 20 feet tall. Fortunately, there are several more compact choices, such as Bobo® or Little Quick Fire® that are suitable for smaller gardens.
Having success with hydrangeas mainly comes down to choosing the right plants for the conditions in your landscape. Those who are looking for reliable blooms no matter what should stick to smooth and panicle hydrangeas, of which there are many types to choose.
More adventurous gardeners who are willing to forego flowers at Mother Nature’s whim might try a reblooming bigleaf hydrangea for more of a challenge.