(Editor's Note: This article was originally published on December 6, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
Hydrangea are among the most popular shrubs for providing fall colour in the garden. For northern gardeners in zone 5 and colder we have four species to choose from; H. arborescens, H. paniculata and in milder areas, H. macrophylla and H. quercifolia. Confusion exists on how these three differ. Some change flower colour depending on the soil pH. Others change colour as the flowers age. Some do both! Let's see if we can clear up the confusion.
Before we get into particulars in regards to species and cultivars, one aspect of their flowers needs to be addressed. Hydrangea have two types of flowers. The ones often located in the middle of the flower panicle are small and have both male (stamen) and female (stigma) parts. These flowers are the fertile ones which produce the seed. Located on the outer perimeter of the flower cluster are the larger, more conspicuous flowers. These 4 or 5 ‘petals' are actually an enlarged calyx. These sterile flowers do not produce pollen nor seeds, but act as a lure to bring potential pollinators into closer proximity to the fertile flowers. Since the sterile flowers are what we, as gardeners, find most attractive, plant breeders have selected hydrangea which have a higher than normal number of sterile flowers. In fact, some cultivars have nothing but sterile flowers. These consequently have the largest, most attractive flowers.
I'll start with H. arborescens, commonly called the Annabelle Hydrangea. The wild form has a flat-topped head of numerous tiny greenish-white flowers with only scattered sterile flowers; not particularly attractive. However, the cultivar ‘Annabelle' has nearly all sterile flowers. Their flowers are produced in a large rounded head, appearing at first greenish, then later creamy white and finally pink-hued. This twiggy shrub blooms from July to September, depending on your location. The hardiest of the group, this one will survive zone 3. Plants may be severely pruned in spring with little loss of flowers. In fact, an annual shearing to 15 cm can be beneficial. The flower colour of this species is unaffected by soil pH. Morning sun with afternoon shade is ideal but too much shade will reduce the floral display.
Above are pictures of the Annabelle Hydrangea
The largest species is H. paniculata, commonly called the Peegee Hydrangea. In reality, only the cultivar ‘Grandiflora' should be called Peegee but in the trade, it seems all H. paniculata are called Peegee, which is unfortunate as these many ‘Peegee' hydrangea can show considerable variation in their flowers. There are well over 30 cultivars of H. paniculata but ‘Grandiflora' is by far the most popular. This selection produces huge conical flowers heads of mostly sterile flowers in August through October. Flowers open greenish-white, then age through cream to pink. This selection can reach over 3 m so demands space and full sun to perform best. They are rated hardy to zone 4. Full sun is best to maximize flower production. Again, soil pH will not alter the flower colour. ‘Pink Diamond' is a newer cultivar whose large flower-heads are a mix of both sterile and fertile flowers, which lend the blooms are more delicate, lacy effect. Blooms also open greenish and age through white (brighter than ‘Grandiflora') then to a rich pink. Overall, the flowers are open for over 2 months! ‘Limelight' is very much like ‘Grandiflora' but the flowers retain a greenish-white colour until they later turn pink.
Examples of H. paniculata include 'Grandiflora', 'Pink Diamond' and 'Limelight'
It is with some reservations that I include the oakleaf hydrangea, H. quercifolia, in this article. While hardy to zone 5 they are certainly not reliable plants in my cool-summer coastal zone 5b. Mine has survived for many years but it behaves herbaceous and I have yet to see a bloom. I essentially grow it for its excellent fall red to orange colour. However, zone 5 gardeners in longer, warmer summer regions have had some success with this beauty. The foliage is certainly the most attractive of the hardy hydrangea, being very oak-like. The huge panicle of white flowers are a mix of sterile and fertile flowers and their colour is unaffected by soil pH. They are earliest blooming of the four species being discuss in this article. This shrub, which commonly grows from 1.2-2.5 m, prefers part shade and rich, organic soil. Like the others, there are several named selections which vary in the proportion of sterile to fertile flowers or plant height.
Flowers and fall foliage of the oak-leaf hydrangea
This then brings us to the most spectacular and colourful hydrangea, H. macrophylla. This species has flowers that fall into two main groups; the mopheads with large rounded heads of mostly sterile flowers and the lacecaps, with flat-topped flowers whose small inner fertile blossoms are surrounded along the perimeter by large sterile flowers. These hydrangea commonly grow 1-1.5 m tall and bloom from late August through October. They prefer morning sun and afternoon shade but tolerate shade better than the previous two species. Pruning should be confined to deadheading and this is best done in spring. Unfortunately, like the oak-leaf hydrangea, these are relatively tender in the north, however, I have found them to be very reliable in my coastal zone 5b. They have been known to survive into zone 4b if snow cover is guaranteed.
These are the hydrangea whose flower colours vary depending on the soil pH. Under acidic soil conditions the flowers of most cultivars will be bright blue while alkaline soil will result in pink flowers. Neutral soils may result in purple flowers or a combination of blue and pink. If your soil is not naturally acidic, you can add aluminum sulfate to the growing area to acidify the soil. However, do not use this around rhododendrons as it is toxic to them.
There are over 300 named cultivars of mophead hydrangea. ‘Nikko Blue' is perhaps the most popular of these. Most selections have flowers that are affected by soil pH but plant breeders have selected some forms that retain their pink, purple or blue colour regardless of soil pH. An example is ‘Glowing Embers', a purple-flowered selection that remains essentially purple regardless to soil pH. In recent years have come the famous ‘Endless Summer' hydrangea. This one has a longer blooming season than the old standard mopheads. In the south they bloom nearly all summer and well into the autumn. For northern gardeners, we may only get a single blooming in August through October but ‘Endless Summer' seems to be more reliable for blooms in the north than many of the other cultivars.
Above are 'Nikko Blue', 'Glowing Embers' and 'Endless Summer (the latter one being grown under alkaline soil conditions)
The lacecap hydrangea are sometimes classified as being selections of H. serrata while others sell them as H. macrophylla. I'm sure the plant itself could care less! In the garden, I find these hydrangea to be more delicate and less imposing. There are also many named cultivars of these and again, some have been selected to retain a single colour regardless to soil pH. ‘Blue Wave' and ‘Blue Bird' are very popular selections. In acidic soil, sterile flowers are brilliant lilac-blue while the fertile flowers are deeper blue. All age to purplish. Under lime soils, the flowers will be pink. Perhaps one of the most spectacular selections is ‘Beni Gaku'. This one opens white but the sterile flowers then change through pink to red-purple. The inner fertile flowers are purplish-blue. All these blending of colours creates the most spectacular display. Among the lacecaps, as the flowers age, the outer sterile flowers often arch downwards, adding yet another wonderful effect in the garden. And if all this is not enough, many of the lacecaps have spectacular autumn foliage.
Above left is 'Blue Wave', center are the early blooms of 'Beni Gaku' while the right shows the later colours of 'Beni Gaku'
The fall colours of 'Blue Wave' and 'Beni Gaku'.
For late season blooms in northern gardens, hydrangea come highly recommended. Even in some parts of zone 4, with careful attention to winter protection, you may have success with H. macrocephala, which is perhaps the most lovely of all the hydrangea.
I would like to thank the following people for the use of their pictures: Bug_Girl (H. arborescens close-up), hczone6 (H. quercifolia fall foliage), Jeff_Beck (H. quercifolia flower), normal1234 (H. arborescens full plant), scutler (H. paniculata 'Limelight') and tyke (H. macrophylla 'Endless Summer')