Hydrangeas: Showiest of Flowering Shrubs

Hydrangeas are among the showiest of flowering shrubs with long lasting-flowerheads that bloom mid- summer to autumn when few other shrubs are blooming. These beautiful shrubs are easy to grow, untroubled by pests and produce an abundance of large flowers for fresh bouquets and dried arrangements.

In general, hydrangeas grow best in moist soil that is rich in organic matter. The name, “hydrangea” derived from the Greek word for “water” is appropriately named for these moisture-loving plants need water, especially when blooming or during dry spells.

Five hydrangea species are widely cultivated in the U.S., but only three are hardy in our northern climate. Not to worry, though, for these three provide more than enough diversity in terms of size, form, and light requirements to find a good fit for your landscape.

Smooth Hydrangea (Hydrangea arborescens), sometimes called snowball hydrangea, is easy-growing with clusters of pure white flowers from midsummer into autumn; flowers typically fade to green before they turn brown and dry in the fall. As an added bonus, dried blossoms can be cut to make a beautiful bouquet in the house over the winter.

Grow Smooth Hydrangeas in part shade – morning sun and afternoon shade is ideal. They bloom on new growth, so prune in winter or early spring. Cut this shrub back to 6 or 8 inches tall every year to keep it dense and compact.

Two excellent varieties are:

• Hydrangea arborescens ‘Annabelle,’ one of the most popular hydrangeas in northern climates, is a stunning white hydrangea, often producing flower heads over 10″ in diameter. It grows 5 feet tall and wide and is hardy in Zones 3-9.
• Hydrangea arborescens ‘White Dome’ produces fluffy clusters of creamy-white flowers. It grows 6 feet tall and is hardy in Zones 4-9.

Panicle Hydrangea (Hydrangea paniculata) whose species name “paniculata” comes from the fact that blooms are cone-shaped (panicle) rather than ball-shaped like other hydrangea species. Large clusters of white flowers appear in summer and fade to shades of pink or red in the fall before drying to beige. The dried blossoms on the shrub stay looking good through most of the winter – a decorative effect in a snow-covered garden.

This hydrangea grows in full sun and, like all hydrangeas, prefers moist, well-drained soil rich in organic matter. Amend soil with compost, peat moss, or other similar materials before planting. Water during dry spells and keep evenly moist. Prune panicle hydrangea in winter or early spring as they bloom on current year’s growth. Prune to keep their shape and size. These hydrangeas are hardy in Zones 3 – 8.

Growers are coming out with new varieties every year; below are some popular ones:

• Hydrangea paniculata ‘Grandiflora,’ commonly known as PeeGee, bears large, sometimes giant white flower heads reaching 6 to 18 inches long, which turn pinkish with age. ‘Grandiflora’ is a fast- growing shrub that can reach a height from 15 ft. to 30 ft. and spread from 10 ft. to 15 ft. These may be grown as a multi-stemmed shrub, used for hedging, or grown as a single-stemmed tree specimen. Flowers are good for cutting and drying.
• Hydrangea paniculata ‘Unique’ bears 8-inch-long white flower heads that fade to pinkish white. It is similar to, but more vigorous than PeeGee. Height 6 ft. to 10 ft. and spread 6 ft. to 10 ft.
• Hydrangea paniculata ‘Tardiva’ is a late-flowering (early to late autumn) cultivar with loosely- packed, sharply pointed white flower heads that turn purplish-pink with age. It is a vigorous, fast growing deciduous shrub that reaches 6 ft. to 10 ft. with a similar spread of 6 ft. to 10 ft.

Bigleaf Hydrangea (Hydrangea macrophylla), commonly known as French, garden, or florists hydrangea, are covered with clusters of blue, pink or purple flowers most often found in florists’ shops and southern landscapes. Until recently we have not been able to grow these beautiful hydrangeas in Minnesota because they were not hardy in our climate.

In 2004, Bailey Nurseries introduced a Bigleaf hydrangea that is hardy here, Endless Summer®. Since that time, they have added to the Endless Summer® Collection and now offer four re-blooming Bigleaf Hydrangea varieties suitable for Zone 4. Even these, however, are best planted in a protected location and given winter protection.

Choosing the best shrub for your landscape requires knowing a bit about the species. Bigleaf Hydrangea is divided into two main groups: hortensias (mopheads) and lacecaps. Mopheads have large snowball-like flower clusters, while lacecaps have somewhat flat-top flowers, with non-showy flowers in the center and more showy ones on the outside. Both mophead and lacecap hydrangeas can be used as specimen plants, in mixed borders or in mass plantings.

Like Smooth Hydrangea, this hydrangea grows best in locations that get morning sun and afternoon shade; like all hydrangea, they prefer moist, well-drained soil that has a lot of organic matter in it and may need watering during dry spells.

Two challenges with growing recurrent blooming Bigleaf hydrangeas are keeping the color you want and keeping the shrubs blooming. Color (whether blue, purple, or pink) depends on the presence or absence of aluminum in the flowers. Soil pH indirectly affects flower color by affecting the availability of aluminum. With acidic soil, aluminum is more available to the roots and flowers tend to be blue. When the soil is neutral or alkaline the availability of aluminum is decreased and flowers tend toward pink.

Additives can be found in your garden center to adjust pH levels of your soil.

Often the culprit when Bigleaf Hydrangeas don’t bloom is the spring freeze-thaw cycle combined with pruning practices. These hydrangeas bloom on old and new wood, so you need to protect buds that form on both. Don’t prune in the fall and protect shrubs from the winter/spring freeze/thaw cycle as you would a marginally hardy rose so you don’t lose the buds that formed on last summer’s wood.

Prune only dead wood in the spring, so the shrub will have flowers in late summer and autumn from buds that form on this season’s growth.

Endless Summer® Varieties to Try

• Hydrangea macrophylla Endless Summer, ‘The Original,’ bears mophead clusters of pink or blue flowers. 3 to 4 ft. height and 4 to 5 ft. spread. Zone 4 – 9.
• Hydrangea macrophylla Endless Summer ‘Twist-n-Shout’ bears blue or pink lacecap-type flowers. 3 to 4 ft height and 4 to 5 ft. spread. Zone 4 – 9.