AMES, Iowa -- Hydrangeas are one of the most popular summer-blooming shrubs in the home landscape. Their large flower clusters not only look great in the garden, but also make excellent cut and dried flowers. In this article, Iowa State University Extension and Outreach horticulturists answer common questions about growing hydrangeas in your landscape.
What types of hydrangeas grow in Iowa?
Several species of hydrangea are commonly grown in Iowa. One of the most durable and reliable hydrangeas is smooth hydrangea (Hydrangea aborescens). This 3- to 5-foot shrub flowers freely from June to September. The flower clusters are rounded and change from apple green to creamy white during the summer before eventually fading to a tan color. There are several cultivars, with “Annabelle” being one of the most common and popular.
Panicle hydrangea (Hydrangea paniculata) has become one of the most popular landscape shrubs available at the garden centers. Showy pyramidal-shaped flower clusters in a creamy white appear in July. As they fade, blooms turn to a pink or blush, then finally a tan/brown color. Plants range from 3-10 plus feet tall depending on the cultivar. Plants are very cold hardy, doing well in all parts of Iowa.
Oakleaf hydrangea (Hydrangea quercifolia) is a 3- to 5-foot tall shrub with dark green, coarse, lobed leaves resembling oak leaves that turn a wonderful burgundy-red color in the fall. White cone-shaped clusters of flowers appear in June and fade to pink as they age. Hardy to USDA Hardiness Zone 5, this shrub does best in southern Iowa.
The mophead or bigleaf hydrangea (Hydrangea macrophylla) has colorful ball-shaped clusters of flowers that open in summer. Flowers are typically pink due to Iowa’s alkaline soils. The blue color that is more familiar to gardeners only occurs in soils with a more acidic pH. These small-statured shrubs typically get no more than 3 feet tall in Iowa and they don’t reliably bloom due to limited winter hardiness.
Not all hydrangeas are shrubs. Climbing hydrangea (Hydrangea anomala ssp. petiolaris) is a vine that clings to tree bark or other structures and over time will climb to over 50 feet in height. The white flowers appear in flat-topped clusters in early July. Winter hardy in all of Iowa, plants are often slow to establish, taking several years before vigorous growth begins.
What conditions do hydrangeas best grow in?
Most hydrangeas grow best in fertile, moist, well-drained soils. Shrubs grow best in partial sun but will tolerate full sun. Smooth, bigleaf and oakleaf hydrangea benefit from protection from the late-day sun. Bigleaf and oakleaf hydrangea need some protection from harsh winter conditions but smooth and panicle hydrangea are noted for their exceptional winter hardiness. While hydrangeas will not do well in overly wet locations, they are not drought tolerant and require supplemental water during dry periods.
How do I prune hydrangeas?
Hydrangeas are divided into two groups, those that bloom in new or current season’s growth and those that bloom on old or last year’s growth. Smooth and panicle hydrangea bloom on new growth and can be pruned in early spring. On smooth hydrangea, remove all stems down to the ground. On panicle hydrangea, the size can be reduced by selectively removing a few branches in early spring, or large, old shrubs can be pruned by removing several of the largest stems at their base.
Oakleaf and climbing hydrangea bloom on old growth and perform best with minimal pruning. If stems must be trimmed back, do so just after the flowers fade. In Iowa, bigleaf hydrangea often sees a lot of winter dieback. Wait until the shrubs begin to leaf out before pruning. When growth appears, prune out all dead wood.
Why doesn’t my hydrangea bloom?
Smooth, oakleaf, bigleaf and climbing hydrangea all flower well in partial shade, but when in too much shade may not bloom well. Panicle hydrangea requires at least partial shade to set flowers and often blooms best in full sun. Most hydrangeas need to be established before flowering and may not produce blooms for the first two to three years, especially climbing hydrangea. Bigleaf, climbing and oakleaf hydrangeas that are pruned heavily in the spring will likely not set flowers. Heavily pruned panicle hydrangeas usually set flowers later in the growing season than expected.
Many older varieties of bigleaf hydrangea, such as ‘Forever Pink’ and ‘Nikko Blue,’ usually do not bloom in Iowa. The bigleaf hydrangea blooms on the previous season's growth. Unfortunately, in Iowa, the plants often die back to the ground in winter. The plants grow back vigorously in spring but don't bloom since the previous season's growth has been destroyed. Newer bigleaf hydrangea cultivars, such as Endless Summer, Twist-n-Shout, ‘Blushing Bride’ and BloomStruck, bloom much more reliably in Iowa gardens as they set flowers on both old and new growth.
How do I get my hydrangea to produce blue flowers instead of pink?
Of all the hydrangeas that grow in Iowa, only the bigleaf hydrangea can potentially bloom with blue flowers. The flower color of this species depends on soil pH – blue in acidic soils and pink in alkaline soils. In Iowa, most bigleaf hydrangeas are purplish-pink due to our slightly alkaline soils. Soil pH must be at 5.5 or lower to reliably get blue flowers and in Iowa, soil pH tends to be between 6.5 and 7.5.
Changing soil pH is necessary to get blue blooms in bigleaf hydrangea. Planting shrubs with Canadian sphagnum peat moss can help to lower the soil pH in the root zone. Watering plants with an acidifying fertilizer can also help. Elemental sulfur or aluminum sulfate can be applied to the soil to lower soil pH. Conduct a soil test to determine the current pH and apply these amendments at a rate that will lower the pH to 5.5 to 6. Over time, soils will return to their natural state so regular applications of soil amendments will be necessary to maintain the desired pH.
I purchased a potted hydrangea from the florist shop this winter, can I plant it in my garden?
The hydrangeas found in flower shops are bigleaf hydrangeas forced into bloom out of season. These plants are intended to be enjoyed as a temporary potted plant indoors, much like poinsettia, and are not intended for planting in the landscape. They are typically cultivars that are not reliably winter hardy in Iowa. Even if they survive the winter, they will not reliably flower since they only set flowers on stems from last year and all the stems die to the ground over winter.
Shareable photo: Hydrangea arborescens Annabelle flower.