All About Hydrangeas
Tips from Linn County Master Gardener
By Master Gardener, Lisa Slattery
I love Hydrangeas. They are a garden classic, both elegant and showy. Hydrangea is a genus of around 100 species of flowering plants native to southern and eastern Asia and North and South Americas. They are perfect as a focal planting in landscapes and used in mixed borders. Hydrangeas bloom from early summer to late fall with a wide range of colors and bloom shapes. They are also excellent as cut flowers in a bouquet and dried flower arrangements.
There are many Hydrangea varieties that grow well in Iowa. One f the most popular hydrangeas is the smooth hydrangea or the Arborescens which are cold reliable for Iowa gardeners. This variety grows from 3 to 5 feet in height and flowers start blooming in June and continue into September. The smooth Hydrangea flowers are round and can be up to 11 inches in diameter, ranging in color from green to cream and drying to a tan color as fall approaches. ‘Annabelle’ is one of the most popular of the smooth hydrangeas. Others include ‘Incrediball’, and ‘Haas Halo’ which has a lacecap bloom. The Arborescens prefer morning sun and afternoon shade.
The Paniculata Hydrangeas prefer much more sun and bloom later in the summer than the smooth ones. There’s a lot of choice within the Paniculata species. They are available in both shrub and tree form, with some shrubs reaching heights of 8 to 10 feet. There are also many terrific dwarf varieties that only grow 3 to 4 foot in height and spread. All of the paniculatas have cone-shaped flowers that can be huge (up to 12 inches long) on the larger varieties, ranging in color from cream to pink to limey green. The ‘Limelight’ Hydrangea along with ‘Grandiflora’ commonly known as PeeGee, are two of the largest cultivars with white to green blooms. ‘Firelight’ has dark pink blooms, while ‘Quick Fire’ and ‘Vanilla Strawberry’ and ‘Pink Diamond’ are creamy white to pink.
Some of the smaller paniculatas are ‘Bobo’ (my favorite) which is white, ‘Little Quick Fire’ and ‘Pinky Winky’ which are pink and ‘Little Lime’ which is limey green.
A less popular but most interesting hydrangea is the oak leaf hydrangea or Quercifolia species. The leaves look like oak leaves and can grow up to 8 inches long. The blooms on the oak leaf hydrangeas are white turning a little pinkish as they mature and the foliage turns a deep burgundy in the fall. Several popular varieties include ‘Snow Queen,’ ‘Pee Wee’ and ‘Alice.’
A much less popular hydrangea is the climbing Hydrangea (Hydrangea anomala ssp. petiolaris). This vine will cling to structures and tree bark and can grow up to 50 feet in height, but it can take several years to establish it before major growth begins. The climbing hydrangeas have white flowers, and an interesting shaggy brown bark on the vine itself. This species prefers some shade but will need lots of space when mature.
One question Master Gardeners hear a lot is: Why doesn’t my hydrangea bloom? Probably because it’s a big leaf Macrophylla Hydrangea like ‘Forever Pink’ or ‘Nikko Blue.’ This species of hydrangea don’t perform well in Iowa because they bloom on old growth (or the previous year’s growth) and our winters are too cold. Many times the foliage grows beautifully but no blooms occur. There are two common varieties of Macrophylla that were developed several years back that are supposed to bloom on old and new growth, the ‘Endless Summer’ and ‘Blushing Bride.’ These varieties bloom for some gardeners, and don’t bloom for others.
Another common question about Hydrangeas is when to prune them. Hydrangeas do not require annual pruning but after they are established can benefit from pruning. For the Hydrangea arborescens or smooth hydrangea, prune in late winter or early spring. Cut the stems back to 1 to 2 feet which will encourage vigorous spring growth.
Late winter is also the time to prune hydrangea paniculata. Since these are larger you can either cut them back to 3 feet from the ground, or only remove 1/3 of the oldest branches at the ground to keep the height of the shrub. The dwarf varieties require very little pruning. I like to leave the dried flower heads on for winter interest and then cut them off in late winter or early spring to the first leaf bud.
If you have the big leaf Macrophylla hydrangeas that do bloom for you, they require really no pruning, just a light trimming back after blooming.
Hydrangeas reliable and beautiful shrubs so even if you make a pruning mistake, they will grow back.