This variable group is beloved for its delicate clusters of papery flowers, borne between July and September depending on the variety. Some types are shrubby while others are more treelike; all bring a stately, old-fashioned feel to the landscape. Most grow quite quickly under good culture, and are long-lived. Most Hydrangea macrophylla flowers on old wood, and so must carry its flower buds through the winter. Early or late freezes may damage flower buds, and these buds also have a low survival rate in colder climates. For example, in Zone 5, bloom may only succeed 3 years out of 5 but the plant itself is hardy there. Fortunately for gardeners in colder zones, recent introductions will bloom for them, as they flower on new growth as well as year-old stems, so blooms are guaranteed even after a cold winter. Click here to shop for Hydrangeas.
Climbing Hydrangea (H. anomala petiolaris) will grow and flower even in a northern exposure. This large, heavy vine has lateral branches that will grow out as much as 3ft from the supporting structure. It is superb for growing up the trunk of large shade trees, walls, or along a stonewall. The reddish brown, peeling bark is attractive in winter. Although growth is slow the first couple of years after planting, this species is vigorous once established. Blooming usually begins in 3-5 years.
Light/Watering: Most varieties thrive in full sun in the North, but in the South require afternoon shade, and H. serrata 'Blue Billow' prefers full to part shade everywhere. Moist soils that do not dry out are best; do not plant in hot, dry, exposed sites. Mulch to conserve moisture and buffer soil temperatures.
Fertilizer/Soil and pH: Fertilize once in spring with a fertilizer designed to encourage blooms (such as 15-30-15). Soils should be moist but well drained, and rich in organic matter. In some varieties (H. macrophylla and H. serrata), flower color is determined by the pH of the soil; at low pH (acid soils) flowers will be blue and at higher pH, flowers will be pink. Generally, a pH below 5.0 results in deep, vivid blues and as the pH rises the flowers range from blue to lavender to mauve to a vivid deep pink at pH 7.0 (neutral). The pH determines the availability of aluminum in the soil; this element is more readily available in acid soils, and this availability results in the blue flower color. Since phosphorus ties up aluminum in soils, using a fertilizer low in this nutrient will aid in attaining blue flowers. If pink flowers are desired and your soil is acid, simply add lime to raise the pH and use a balanced fertilizer. Aluminum sulfate will lower pH if blue flowers are desired.
Pests/Diseases: None serious. Occasionally powdery mildew will infect the foliage, especially in humid areas with poor air circulation. Treat with an appropriate fungicide if the problem is serious, and be sure to rake up and destroy all fallen foliage in the autumn.
Companions: Old-fashioned tawny Daylilies are a classic combination with the PeeGee or Tree Hydrangea (H. paniculata 'Grandiflora'). Astilbes and Oriental Lilies in shades of rose, pink, and white are lovely with the shrubby Hydrangeas.
Pruning: The pruning of Hydrangea shrubs varies by variety, as some flower on old wood and some on new, and others on both.
Hydrangea anomala petiolaris (Climbing Hydrangea): Remove any dead wood in early spring. Prune as needed after flowering. Blooms on old wood.
Hydrangea arborescens (Smooth Hydrangea): Can be shortened, or pruned back to the ground, either in fall or early spring. This includes H. a. 'Annabelle', H. a. Bella Anna™, H. a. Incrediball®, and H. a. Invincibelle® Spirit. These varieties bloom on new wood.
Hydrangea macrophylla (Mophead or Bigleaf Hydrangea): By the end of August, cut back stems by about half if growing too tall, and remove some of the oldest stems at ground level to thin out the shrub if needed. In spring, only prune out dead wood. This includes H. m. 'Big Daddy', H. m. Cityline® Mars, H. m. Cityline® Rio, H. m. Cityline® Venice, H. m. Color Fantasy®, H. m. Everlasting™ Amethyst, H. m. 'Lady in Red', H. m. Light-O-Day®, and H. m. 'Nikko Blue'. These varieties bloom on old wood.
For mophead varieties blooming on both old and new wood, by the end of August cut back stems by about half if plants are growing too tall. Remove some of the oldest stems at ground level to thin out the shrub as needed. In spring, prune out any dead wood. If no fall pruning was done, stems can be cut back or removed at ground level now -- but this will sacrifice the bloom on the old wood and shrubs will not flower until late summer. This includes H. m. 'Blushing Bride', H. m. Endless Summer®, H. m. Endless Summer® Bloomstruck™, H. m. Everlasting® Revolution, H. m. Let's Dance® Big Easy, H. m. Let's Dance® Moonlight, H. m. Let's Dance® Starlight, H. m. Mystical® Opal, H. m. Nantucket Blue™, H. m. Pistachio, and H. m. Twist-n-Shout™.
Hydrangea paniculata (Panicled Hydrangea): Little pruning is needed beyond removing any dead wood whenever seen. If desired, plants can be cut back as needed in early spring.This includes H. p. Bobo®, H. p. 'Bombshell', H. p. Fire and Ice, H. p. 'Grandiflora' (PeeGee), H. p. Great Star®, H. p. 'Limelight', H. p. 'Little Lamb', H. p. Little Lime™, H. p. Little Quick Fire&trade, H. p. Mystical® Flame, H. p. Quick Fire™, H. p. Pinky Winky™, H. p. 'Tardiva', and H. p. Vanilla Strawberry™, H. p. White Diamonds®. These varieties bloom on new wood.
Tree form Hydrangea paniculata: Prune in early spring, removing lower suckers and up to half the older top growth.
Hydrangea quercifolia (Oak Leaf Hydrangea): Remove dead wood at any time. Little pruning is needed, but should be done if necessary right after bloom. This includes H. q. 'Alice', 'H. q. 'Little Honey', H. q. 'Ruby Slippers', and H. q. 'Snow Queen'. These varieties bloom on old wood.
Hydrangea serrata (Mountain Hydrangea): By the end of August, cut back stems by about half if growing too tall, and remove some of the oldest stems at ground level to thin out the shrub if needed. In spring, only prune out dead wood. This includes H. s. 'Blue Billow', H. s. 'Fuji Waterfall', H. s. 'Miranda', H. s. 'Pretty Woman', H. 'Preziosa', H. s. Tuff Stuff™. These varieties bloom on old wood.
Reflowering: Regularly deadheading the blooms of H. macrophylla that bloom on both old and new wood helps encourage repeat bloom on the current year's growth. You may cut the first flowering stems of H. arborescens 'Annabelle' and hang to dry for arrangements; rebloom may then occur in August or September.
Transplanting: Young plants may be transplanted when dormant in early spring; larger tree-form varieties are difficult to move once established, but it can be done. Prune top growth after transplanting to reduce water loss.
End of Season Care: Rake up and destroy any fallen foliage that was infected by powdery mildew or other fungi. You may wrap H. macrophylla varieties with burlap or other protective covering to help preserve flower buds through a cold winter.
Calendar of Care
Early Spring: Prune Hydrangea varieties as indicated above, according to their species. Prune out any deadwood from all varieties. Check soil pH and adjust up or down if needed for desired flower color of H. macrophylla and H. serrata. Feed plants with a fertilizer high in phosphorus (such as 15-30-15) to encourage blooms. Complete any transplanting before leaves unfurl.
Mid-Spring: Mulch plants after soil has warmed to conserve moisture and buffer soil temperatures. Watch for powdery mildew and treat as needed.
Summer: As soon as blooms fade, remove old flowering stems from H. macrophylla, H. quercifolia, and H. serrata varieties. Cut flowering stems from H. arborescens 'Annabelle' and hang to dry if desired.
Fall: Remove and destroy any fallen foliage that was infected by powdery mildew. If desired, wrap H. macrophylla varieties with burlap or other material to help flower buds overwinter in colder climates.