Hydrangeas are a well-known and commonly grown plant, but many people will be surprised at the different varieties that exist and are less well known. Although I will certainly mention the tried-and-true mop tops, I will also try to enlighten you about the other fascinating members of the Hydrangeaceae family, and the fact that most of them flower only white! By Bernard Chapman
In Sydney, and other warm climate areas around the world, the most commonly grown hydrangeas are the mop tops (Hydrangea macrophylla), in all their various forms. They are grown for large showy colourful flowers, and there are quite a few interesting variations. I will elaborate on these, but also look at some of the other 75 species that exist. I will also look at hydrangea look-alikes that gardeners might like to grow, and which are best for Sydney, and cooler climates.
Of course, the majority of hydrangeas are shrubs, but there are also some climbers. There are also some genus (i.e. that are in this family but are not botanically hydrangeas), that are worth knowing about and using in our gardens. Some will be known to keen gardeners, but they may not have known that they are of this family.
I will also give you some suggestions on how to prune hydrangeas, and it can be different for different species. Admittedly, pruning can be a bit like a bad haircut, the damage will grow out, but if timing and style of pruning are done more expertly, more flowers and better plant shape will almost certainly result.
Growing mop top hydrangeas
Mop tops can vary in colour from pink to purple, to red, and, of course, white. With the flowers of colour, pH (soil acidity or alkalinity) can make a massive difference to the colour. It is, in fact, possible to change the colour from blue to pink, vice versa and any shade between. White mop tops basically stay white, sometimes perhaps affecting a slight pink or blue tinge.
So there are the normal mop tops, but other major parts of this group are the lace caps, with a cluster of small sterile flowers in the middle and normal size flowers like a constellation around them, and cupped or dimpled hydrangeas. One well known one is Hydrangea macrophylla ‘Ayesha’. The latter group are often only seen in old gardens. They tend to be very tough!
Still within the mop top umbrella there are the more recently introduced Endless Summer range, developed in the early 1980s in Minnesota USA. They are reported to flower twice in the one year. There are four cultivars available, The Original (a chance sport found in the Bailey’s Nursery in Minneapolis-St. Paul), Blushing Bride, a dazzling white, Twist N Shout, a lace cap form, and Bloom Struck, which has dramatic red stems.
Growing climbing hydrangeas
Other significant and commonly grown species include the climbing hydrangea, Hydrangea anomala ssp. petiolaris, Hydrangea arborescens, Hydrangea serrata (grown in Japan to make a sweet tea), Hydrangea paniculata, with delicate white flowers and Hydrangea quercifolia, known as the oak leaf hydrangea because of the shape of its leaves.
Most of these need a cooler climate and, indeed, thrive in it. I know of one climbing hydrangea in Willoughby in Sydney which survives but does not thrive! I am interested to know if other readers in warm climates have had more success.
Of these other species, the only one that I have grown well in the warm climate of Sydney is the oak leaf. It has stunning single or double flowers and pretty autumn leaves (and often flowers) that persist through the winter and drop just before spring and the growth of the new leaves.
When the double oak leaf is flowering in my garden, passers-by often ring my doorbell or stop their cars to ask what the stunning flower is! I find this species surprisingly capable of coping with really hot weather, even over 40 degrees Celsius!
Apart from these other species of Hydrangea there are some significant genus members of this family. Philadelphus, the mock orange, and Deutzia are in this family. The less well known Carpentaria and Dichroa are too. Dichroa actually does grow well in Sydney and therefore warmer, more humid environments.
Dichroa versicolour and Dichroa febrifuga are the only other plants in this family for which the flowers come in colours other than white, blue and pink, and also change colour depending on the soil pH.
Growing hydrangea look-alikes
Of course, many people do not want to try hydrangeas, as they seem tricky to prune, and seem prone to mildew problems. For these people there are hydrangea look-alikes! Look at the Viburnum opulus Sterile, the Snow Ball tree (very like Hydrangea paniculata and Phildadelphus), Eupahtorium megaphyllum, Kalanchoe blossfeldiana (and the species is crucial), Spirea cantoniensis (May Bush), and even Sedum (now classified as Hylotelephium). All of these will give lovely rich florets reminiscent of hydrangea flowers.
Hydrangeas are fairly easy plants to grow, and are one of the few plants which are tolerant of heavy clay soils. Their biggest problem is probably the affect mildew has on them. I have had some success applying fungicides but feel, as they are not registered for use on hydrangeas, I cannot suggest them here.
Do not underestimate the importance of mulching these plants, especially if they are planted where they get a lot of sun. On a hot day, an unmulched garden bed can lose up to 60% if the water applied to it. Bountiful watering is also a good idea in the warmer months (but not on the leaves!).
Probably the only other consideration for developing beautiful, floriferous plants, other than good mulching and watering, is pruning. The basic rules of pruning for all plants still hold for hydrangeas. Generally cut them back after flowering. Always cut out dead, dying, diseased, thin, or crossing over canes/stems. Also, as with all flowering and fruiting plants, feeding is important, so use slow release, smelly pellets, or foliar feeds, or a combination!
If mop top plants have been left for a while, their canes, or stems, may have become thick and woody, and lose vigour. Vigorous plants are much more likely to flower, of course.
If mop tops have been left it might be necessary to cut the plants to ground. If the canes seem healthy and strong, it is necessary to cut back only down a couple of bud pairs (still two year old wood), which should be ideally plump and fabulous. The only mop tops exempted from this treatment is the Endless Summer, pruned after their second flowering.
Hydrangea quercifolia are not pruned the same. Ray Rowell, the father of horticulture in Australia, suggested that the oak leaf hydrangeas were best left unpruned except for the removal of the spent flowers. They can become huge if left, of course!
Hydrangeacea plants can produce generous flowers, in shades from pink to purple, although, in the main, the flowers are white. There is surprising variety apart from the mop tops, which, regardless of where you live, you should consider growing, including the climbing hydrangea and the oak leaf ones.
Also consider easy to grow hydrangea look-alikes, such as Sedum and Viburnum. And also do not rule out the other genus in this family. Hydrangeas are extraordinary plants. Ideally planted in banks, but at least consider trying one or two to give your garden a thrill.