Hydrangeas are poised to surpass rhododendrons and azaleas as the most popular garden shrubs and there are good reasons behind this. The most important of these is its long season of colour and the fact that it is in bloom when we’re actually out in the garden, from early summer through to fall. Add to this their usefulness as a long-lasting cut flower and their relative ease of care and you have the makings of a landscape staple. Plant breeders have also picked up on this buying trend and have flooded the market these past few years with dozens of new introductions.
Most of what’s available comes down to six species; Mophead hydrangea (H. macrophylla), PeeGee hydrangea (H. paniculata), Mountain hydrangea (H. serrata), Smooth hydrangea (H. arborescens), Oakleaf hydrangea (H. quercifolia) and Climbing hydrangea (H. anomala subsp. petiolaris). Among these are shrubs that bloom on both old and new wood, those that change flower colour depending on the soil pH and both sun tolerant and shade loving types – so finding the right one for your garden can often prove challenging. To help guide you a little I’m going to give a brief introduction to each species while suggesting some choice cultivars, as well as providing simplified pruning advice and a little insight on changing flower colour.
Hydrangea macrophylla or Hortensia (as they are known as in Europe) are the most common species, with large rounded blooms of blue (‘Nikko Blue’), Red (‘Cardinal Red’), pink (‘Forever Pink’), white (‘Sister Theresa’) and purple (‘Merritt’s Supreme’) – all of which can vary in colour depending on pH, with the exception of whites which are unaffected. Two-tone blooms can also be found on ‘Cityline Mars’ or ‘Pistachio’, which tends to be a lovely pink-green combination, while the variegated foliage of ‘Goldrush’ is equally attractive. There are also lacecap flower forms within this species although they are usually much larger than the similar H. serrata blooms - with ‘Teller Red’, ‘Twist-n-Shout’ and ‘Teller Blue’ all readily available. They range in height from a mere 2-3’ tall (‘Pia’) right up to 5-6’ high and wide, with most of the Cityline series hitting the middle mark with plants maturing at 3’. Mophead hydrangeas prefer a part sun exposure although I have seen them quite happy in full sun with even soil moisture and mulch covering the roots, but windy or hot exposed sites will almost always result in scorched foliage. Older varieties bloom on last year’s wood although newer remontant or repeat-blooming types are becoming the norm with the Endless Summer series or ‘Everlasting Revolution’. Repeat blooming cultivars can be grown in zone 5 while varieties that only flower on old wood should be considered zone 6 hardy.
'Pia', 'Goldrush' & ' Red Teller' Copyright Mike Lascelle
This self-clinging deciduous climbing shrub is often found in the vine section and is commonly available in two incarnations; the green-leaved subspecies (H. anomala subsp. petiolaris) and the gold variegated ‘Miranda’. Both bear lightly fragrant white lacecap blooms from late spring into early summer and have some winter appeal with their reddish-brown exfoliating bark. This is a good choice to cover a chimney, brick wall or any unsightly masonry and while slow to establish, it can reach heights in excess of 30’ at maturity. Both are quite cold hardy at USDA zone 4 and prefer open shade to part sun (morning or late afternoon) exposures.
Hydrangea serrata is native to the mountains of Japan and Korea, hence the common name. The flower is almost always lacecap in form, with the exception being hybrids such as ‘Preziosa’ which is a cross with H. macrophylla. Like the latter, the flower colour of this species is also affected by soil pH, producing pink blooms where alkaline and blue to purple in acidic soils – so one will occasionally stumble across a pink-flowering ‘Bluebird’. Besides the aforementioned cultivar, other worthy varieties include ‘Purple Tiers’, ‘Blue Billow’ and ‘Beni-Gaku’, all of which have a mature height of 4-5’. Most are considered USDA zone 6 hardy and more shade tolerant than mophead types. The exception here is the Proven Winner introduction ‘Tuff Stuff’ which is sun tolerant (with even soil moisture), more compact (3’) and hardier than most Mountain hydrangea at zone 5.
Mountain Hydrangea 'Preziosa', 'BlueBird', ' Tuff Stuff', & 'Bluebird Pink'
Copyright Mike Lascelle
There was a time when your only options within this species were the large-flowering ‘Grandiflora’ and ‘Annabelle’ but that all changed with the recent introductions from Proven Winners. Now you can choose from 30” tall dwarf forms (‘Wee White’, ‘Mini Mauvette’), green-tinted blooms (‘Lime Rickey’, ‘Invincibelle Limetta’) or even pink flowers (‘Invincibelle Spirit’, ‘Incrediball Blush’, ‘Invincibelle Ruby’). All bloom on new wood and are incredibly cold hardy at USDA zone 3. Exposure will depend upon where you live, with full sun being okay on the coast and partial shade in the interior with the hotter summers. The older cultivars had a tendency to splay out in the rain when in bloom, but my wife solves that problem by simply turning our ‘Annabelle’ into cut flower bouquets after they fall out. That said you do have some control over flower size, with hard pruning resulting in larger blooms. Most start flowering from early to midsummer and persist until fall.
Smooth Hydrangea 'Wee White', 'Annabelle', 'Lime Rickey'
Copyright Mike Lascelle
Oakleaf Hydrangeas 'Little Honey' & Alice' Copyright Mike Lascelle
This is another species that has really benefited from Proven Winner introductions, bringing it to the attention of average gardeners. As the common name implies Hydrangea quercifolia has oak-shaped leaves that shift to a vibrant scarlet in the fall. ‘Snow Queen’, ‘Alice’ and ‘Snowflake’ were the standard cultivars, while ‘Ruby Slippers, ‘Pee Wee’ and ‘Little Honey’ represent the more dwarf forms – with the latter having bright chartreuse foliage. The more widely available Proven Winner varieties include ‘Gatsby Star’ (dbl. star-shaped florets), ‘Gatsby Gal’ (compact), ‘Gatsby Pink’ and ‘Gatsby Moon’ (double florets). Oakleaf hydrangea blooms on old wood starting in June with the cone-shaped flowers opening white and fading to pink or a deeper rose colour. While it tolerates full sun in coastal gardens, I find the best specimens are almost always grown in open shade to partial sun. They are a bit more lanky in form than most hydrangea and reach heights of 6-7’ at maturity (3-4’ for compact forms). Hardy to USDA zone 5.
I saved the best for last here because there has been a literal explosion of new Hydrangea paniculata cultivars these past few years and for good reason. They are among the easiest species to grow, tolerate sun rather well, are extremely cold hardy at USDA zone 3 and bloom on new wood, so you are always guaranteed a decent flower display. The common name is derived from the acronym of what was once the most popular cultivar H. paniculata ‘Grandiflora’ or PeeGee. That cold hardiness also makes them good candidates for container culture (particularly the dwarf ‘Bobo’, ‘Bombshell’ and ‘Little Quick Fire’) as well as training to standard tree forms. Green-flowered (‘Limelight’, ‘Lava Lamp Moonrock’ and ‘Little Lime’), two-tone (‘Vanilla Strawberry’, ‘Zinfin Doll’ and ‘Lava Lamp Flare’) and varieties that fade to a deep bronze-red (‘Fire Light’, ‘Pinky Winky’ and ‘Quick Fire’) are all readily available in season. The largest blooms are found on ‘Polar Bear’ and ‘Phantom’, while the lacy flowers of the older cultivars ‘Kyushu’ and ‘Brussels Lace’ are also well worth seeking out. Expect standard-sized cultivars to reach 6-8’ tall and compact varieties to average 3-4’ high. The flowering period lasts from July to September.
PeeGee Hydrangeas 'Pinky Winky' & 'Lava Lamp Moonrock' Copyright Mike Lascelle
You only need to know if your hydrangea blooms on old or new wood to understand your pruning timing. Hydrangea macrophylla, H. serrata, climbing hydrangea and H. quercifolia all bloom on old wood (except for remontant types) and should be pruned immediately after flowering, which is usually mid to late summer. The mophead hydrangeas in particular still look pretty good at this time of year, which is why many people miss the proper pruning time. Hydrangea arborescens and H. paniculata bloom on new wood and can be pruned in early spring, keeping in mind that the harder you prune the larger your flowers will be. Another important consideration here is not to try to hack an ‘old wood’ cultivar that naturally grows to five or six feet tall down to 3’, as you will never see any flowers. Your better choice here is to simply replace it with a more compact variety.
Changing Flower Colour
Smooth, PeeGee, Oakleaf and Climbing hydrangea flowers do not change colour with the soil pH and with the exception of the latter, go through a natural colour fade from green to white and finally bronze-pink. Both Hydrangea macrophylla and serrata do change flower colour depending on soil pH, with alkaline conditions producing pinks to reds and acid conditions blues to purples. The mophead hydrangeas also change colour dramatically when transplanted, but that usually turns out to be a one year wonder. To manipulate flower colour on either one of the aforementioned species add aluminum sulfate (do not exceed application rate as this can kill plants and water before applying) or a soil acidifier for blue tones. For pinks simply apply dolomite lime around the base of your hydrangea. The timing for either application is once in fall and again in early spring, before the flower buds begin to form.
Well that brings us to the end of our crash course in hydrangea selection and care. I’m sure I’ve mentioned at least one or two options which will suit your garden and I look forward to seeing you soon at the nursery.