In the Garden | Hydrangeas are a versatile bouquet | Gardening
The smooth hydrangea (Hydrangea arborescens) is one of the only two species native to Illinois. This small, often overlooked native shrub is much more common in southern Illinois, although it does occasionally occur in our part of the state as well. Its delicate lacy flowers attract a wide range of pollinators and its foliage serves as a food source for the caterpillars of two native butterflies, lending ecological value to this attractive, shade-loving native.
The floral structures of hydrangeas are the subject of much attention, both from an ornamental point of view and for pollinating insects. Flower heads composed of smooth hydrangeas reach full bloom in mid-summer, containing hundreds of individual flowers in a flat to dome-shaped structure called a panicle.
Tiny fertile flowers sit in the center of the panicle in a massive raceme, each flower head containing over 800 individual flowers, offering pollen and nectar to visiting insects. Around the perimeter of most panicles are more showy sterile flowers that offer no support to pollinators but are visually appealing to humans. This type of arrangement is often referred to as a “lace cap” since the largest sterile blooms occur along the outer edge of the structure, creating a lacy appearance.
In contrast, many hydrangeas have flower structures called “mopheads.” These are mainly the sterile outer flowers of the lacecaps, creating a noticeably showier domed panicle that nevertheless offers no pollen or nectar.
In the early 20th century, the smooth hydrangea sparked interest in the ornamental horticultural trade with the release of H. arborescens, ‘Hills of Snow’. This was developed from an individual plant with a natural mutation for mop flowers. ‘Hills of Snow’, also known as ‘Gradiflora’, was the most popular cultivar for decades until the introduction of the ‘Annabelle’ hydrangea. It comes from another natural mutation for the mop flowers found near Anna, as the name suggests.
Today, plant breeders have gone beyond these natural mutations to create new cultivars with improved characteristics. Although flowers have been the focus, these newer cultivars have also been developed with other characteristics in mind, such as more compact habits and sturdier stems.
Smooth mop-flowered hydrangea cultivars have been known to collapse in mid-summer due to the weight of the large blooms. Individual canes are often sturdier with larger flowers in their first year of flowering, while later years have more flower heads as the canes branch out, compounding the problem.
As a result, many gardeners cut smooth hydrangeas in late winter to concentrate growth on younger stems with less branch structure to support. The practice creates shrub canopies that are stronger and less likely to collapse.
Many newer cultivars have also sought to preserve pollinator value by focusing on lace-like structures with added ornamental appeal, such as more abundant indoor flowers (sometimes up to 2,000 per head) that create larger flower heads or color variations to enhance human visual interest. Breeders have worked with individual plants with promising mutations and have also produced hybrid selections by crossing smooth hydrangeas with other hydrangeas native to North America, such as dusty hydrangea (H. cinereal) and hydrangea. silverleaf hydrangea (H. radiata).
In a recent five-year trial, the Mt. Cuba Center in Delaware evaluated new hydrangea arborescens cultivars on a variety of criteria, including pollinator use, ornamental appeal and landscape performance. Their assessment included 29 hydrangea taxa with 13 lace-headed and 16 mop-headed flower types. Lacecap cultivars received an average of 143 pollinator visits per plant per year, while Mophead cultivars received an average of only 42 visits.
Notable results include the top-rated performer H. arborescens “Hass Halo,” which is a lacecap variety that averaged 192 pollinator visits per year. It turned out to have the biggest overall blooms on the trail and earned the top rating due to its “flawless performance” in shade settings.
H. arborescens “Mary Nell” was another high quality cultivar whose single lacecap flower has a greater abundance of showier sterile flowers while attracting 154 pollinators per year. Surprisingly, one cultivar (H. arborescens ‘Dardom’) received more pollinator visits per year than the pure species. It was the best performer in the landscape and has a smaller size than H. arborescens and larger flowers.
It is truly great to see victories like this in the horticultural industry, where ecological value and ornamental appeal can be combined to create valuable landscape plants. The full report, titled “Wild Hydrangea for the Mid-Atlantic Region”, is available at mtcubacenter.org.
Ryan Pankau is a Horticulture Educator with UI Extension, serving Champaign, Ford, Iroquois, and Vermilion counties. This column also appears on his “Garden Scoop” blog at go.illinois.edu/gardenscoopblog.