Most older gardening books will say that there are between seventy and ninety species of Hydrangeas. Now, the Royal Botanic gardens, Kew, (as the internationally recognized taxonomical authority) has published a new definitive list of all 16,167 plant genera and has categorized 1,040,426 species names as accepted, synonyms or unresolved.
This great work, the culmination of a 10-year project met its deadline by being published on December 31, 2010, just before midnight! You can check it out on www.theplantlist.org. This new listing has simplified the Hydrangea genus significantly into 49 species.
Hydrangea macrophylla and H. m. ssp. serrata
Hydrangea macrophylla and H. m. ssp. serrata are, by far, the most commonly grown hydrangeas. They epitomize everything that gardeners expect in Hydrangeas – mophead and lacecap flowers, the ability to change colour dependent on the acidity of the soil (more on this later) and easy to grow in the Lower Mainland climate.
There are a great many cultivars of this species and sub-species – in fact, in my opinion, there are far too many. Each year plant breeders introduce new cultivars, most of which are no better than their predecessors – and, who needs yet another mediocre pink hydrangea? So, I choose to grow only award-winning plants. I trust the award schemes of the RHS (Award of Garden merit) and the Great Plant Picks scheme. Both are independent organizations without any commercial interests. Other schemes, such as Proven Winners and All American Selections are simply marketing programmes run by the growers to help promote their new plants so don’t have much credibility. I also seek out heritage hydrangeas that have shown their superiority by being grown over a long period.
Note that it is only Hydrangea macrophylla and H. m. ssp. serrata that have mophead and lacecap flowers and it is only these that have the ability to change colour dependent on the acidity of the soil. To be precise, it is the availability of aluminium ions in the soil that makes the difference and these are only available to plants in acidic soils. As we all know, by adding lime we can increase the alkalinity in the soil to enhance the pink or red colours, or add aluminium sulphate to increase the blues in these species. Whether flowers will change to pink or blue is dependent on the cultivar, not all will change and certainly the white ones will stay white.
Hydrangea paniculata
So called because the flower shape is a panicle. Almost all cultivars have white flowers, but these can adopt pink shading as they age.
Excellent cultivars are ‘D.V.P.Pinky’ (AGM), ‘Phantom’ (AGM), and ‘Unique’ (AGM). ‘D.V.P.Pinky’ is often sold under the trade-marked or patent-protected name of PINKY WINKY. I hate this trend of trademarking and patent-protecting plants; it means nothing and is just a device to inflate the cost.
Hydrangea paniculata plants are more tolerant of full sun than other hydrangea species and can be pruned to a standard and grown as a small tree.
Hydrangea aspera
Examples of the Hydrangea aspera species are quite commonly found growing in the Lower Mainland. The interesting features are their coarse, bristly leaves and hairy (pubescent) stems. H. a. ‘Villosa Group’ is probably the most popular variety but H. a. ‘Sargentiana’, if you can find it, is much more interesting. It has huge purple lacecap flowers, but most unusually, it has a very remarkable white moss-like growth on the stems.
Hydrangea petiolaris (formerly H. anomola ssp. petiolaris)
One of the few climbing hydrangeas. Clings to walls and fences much like ivy, but has a neater growing habit and can easily be controlled. It has white flowers but may be reluctant to flower if it gets too much sun. It is intolerant of sun.
Hydrangea integrifolia
Another climber, but this one is evergreen, very rare, has involucral bracts and is Zone 9. The flower buds are as large as the largest peony buds.
Hydrangea quercifolia
One of two species that are native to N. America (the other one is H. arborescens).
The best H. quercifolia cultivar is probably ‘Snowflake’ with its double flowers. Also available are ‘Snow Queen’ (AGM), ‘Alice” and ‘Pee Wee’.
Hydrangea arborescens
‘Annabelle’ (AGM) is a very popular cultivar. Others available are ‘Incrediball’ and ‘Invincibelle Spirit’.
Of special note is the sub-species radiata. Hydrangea arborescens ssp.radiata ‘Samantha’ is something special. The differentiating feature of the radiata sub-species is the brilliant silvery-white indumentum on the reverse side of the leaves. Very spectacular in a slight breeze.
Hydrangea heteromalla
These are the tree-like hydrangeas. They can grow to up to 5 m, with leaves up to 20 cm and the flowers can be 60 cm across. An interesting feature they share with some other hydrangea species is that the sterile flowers will invert after the central fertile flowers have been pollinated. Since the reverse side is often a different colour from the topside, the flowers appear to change colour!
Plant names
It is essential for all good gardeners to know the names of their plants; and I mean the real botanical name, not the confusing common names.
There are two reasons for this:
1.The cultivation, pruning and propagation requirements vary by species. You need to know the correct botanical name if you want to research, read or learn about your plants.
2. There will be times when you will have to stress out your plants. When taking cuttings, repotting, pruning or simply deadheading, your plants will be under stress. You need to reassure them by talking to them and it would be quite impolite to talk to your plants without knowing their names.
Why hydrangeas?
Why am I forsaking my Cannas in favour of Hydrangeas?
I’m not. I am continuing to grow my cannas but I am branching out and adding hydrangeas to my garden as another collection.
My garden is surrounded by many of those dark green, morbidly depressing, evergreen tree weeds often incorrectly referred to by Master Gardeners as Cedars. To add some brightness and colour to this gloomy environment I am introducing as many hydrangeas as I can.
Also … my wife said to me (needs to be read in the usual nagging voice):“One of these days you are going to collapse on the ground when you are digging up, potting up or planting all those Cannas. I am only 5ft 3in and 92 lbs, so I won’t be able to pick you up and help you into the house. The best I will be able to do will be to drag you onto the compost heap!”
So, now I know – I will end my days on the compost heap. How appropriate.
Author: Barry Roberts, MG, Vancouver Chapter
(Barry is a member of the Vancouver Master Gardeners Speakers Group and is available to speak at garden clubs).
