Hydrangeas are among the most widely-grown shrubs, and no wonder. Their lavish blooms are long-lasting, and with some shade tolerance these are relatively easy, reliable and tough plants to grow. It is the size of the flower heads, up to a foot across, that makes them so decorative, but today we have a whole new breakthrough in them, with the recent development of double-flowers, raising their beauty to a whole new level. So far these plants are relatively rare, and not widely seen, but once you see one it’s easy to predict that they are the future of hydrangeas.
After all, almost all roses are double flowers, and few people grow the ‘original’ rose, which has just 5 petals. When you think about it, double flowers are common in many of our best garden shrubs, since they offer so much more color and impact. From roses to camellias, and from peonies to carnations, so many of our garden flowers are double that we can easily take it for granted. Yet hydrangeas have stubbornly resisted being doubled. Sure, there are many beautiful plants out there, but they are mainly color variations, with that dome of blooms being pretty uniform across them all.
If those flowers could have more petals, the result would be a richer, fuller flower-head, with much more impact. This has been the dream of breeders, and now we have it, thanks to the Japanese breeder Ryoji Iroe from Kyoto in Japan. We don’t know much about him, but he is young, dynamic breeder eager to break boundaries. We know his wife is called Saori, because he created a beautiful pink and white hydrangea and named it after her – ‘Miss Saori’ – when they were engaged, talk about romantic!
Imagine a typical mop-head hydrangea, with its blooms in a large head. Now imagine those individual flowers, usually flat with 4 or 5 petals, turned into remarkable blooms with centers of many petals, placed inside each other as they become smaller and smaller – now you have a double-flowered hydrangea. Sometimes the inner petals are flat or cupped, making a flower a little like an African violet. Other times they are in clearer rows, almost like a miniature compressed Christmas tree.
Since Ryoji created his first double, back early this century, other breeders have developed their own, so today we have a wide range of these plants in all the classic hydrangea colors – pink, blue and white – plus bicolor flowers and other variations. All are vigorous and strong, and just as easy to grow as regular hydrangeas, but with a big bonus. For this introduction, let’s focus on Ryoji Iroe’s plants.
The YOUME™ series
These plants are sometimes wrongly called ‘You & Me’, with an expanding range of beauties in this group, released by Plant Haven International, they were all created by Ryoji Iroe.
Wow Time™ Hydrangea (‘H21-3’ ): You might see this plant also marketed as Princess Diana, but whatever you call it, this brilliant, neon-pink beauty really has the ‘wow’ factor. With up to 20 petals in each flower, they are arranged to create a miniature Christmas tree. It’s also a good repeat bloomer, giving flowers in early summer and again in late summer and early fall.
Passion Hydrangea (‘Rie 4’): A beautiful soft pink, with flowers that change shade as they mature, this is also a good re-bloomer, and turns blue in acid soil, or with the right treatment in a pot.
Romance Hydrangea (‘Rie 9’): Another beautiful pink, that like falling in love has flowers that begin small and expand before your eyes as the head develops. Turns dark blue in acidic soil.
‘Perfection’ Hydrangea: A recent addition to the YOUME™ collection, this one is well-named. A reliable rebloomer, likely to give you flowers from May to October, it has 15 petals in each large flower, on enormous ball-shaped heads. Don’t worry, the extra-strong stems hold them perfectly upright.
Now this phenomenon of doubling in flowers is pretty fascinating, and complex, but let’s take a more detailed look at how it all works.
Hydrangea Flowers – More Complex Than They Look
Hydrangeas have a complicated floral arrangement, which makes them pretty special. As most people know, a typical flower has both male parts (stamens that produce pollen) and female parts (a stigma and ovary where the seeds develop). The purpose of the petals, with their bright colors, is to attract pollinators, but it takes work to make them. Flowers are often arranged in heads of many, to increase their showiness. Hydrangeas do this, but they also evolved a clever trick to minimize the work of attracting those insects. Their heads have a ring of flowers around the outside that are just petals – no sexual parts – and are sterile florets. In the center the flowers have no petals, but they do have stamens and ovaries – they are fertile florets. Look at this picture, where you can see this clearly.
These classic ‘lace-cap’ hydrangeas are wonderful garden shrubs too, and shouldn’t be overlooked when choosing shrubs for your garden. However most people go for the ‘mophead’ type, which has almost all the flowers sterile, with just a few fertile ones hidden away between them. This was the original hydrangea brought from Japan, and it created a sensation. Only later was it realized it wasn’t how the plant grew in the wild. The wild type just has a circle of sterile flowers around a center of fertile one. The few larger ones do the attracting, and once there the bees visit the fertile ones for pollen. It’s a clever trick to economize on effort while maximizing the draw of your blooms.
Why Do Some Flowers Have So Many Petals?
Double flowers have been known for a very long time. The Greek philosopher and naturalist Theophrastus described a double rose over 2,000 years ago in his book on plants, and they have been discovered time and time again. Yet it is only recently that we have found the mechanism that creates them.
Flowers are a continuation of the leafy stem that carries them, with the flower parts – sepals, petals, stamens and ovaries – simply modified leaves. The important difference is that stems keep growing endlessly longer, while flowers represent an ending of the growth of that stem. Botanist say a stem has ‘indeterminate’ growth, while a flower is ‘determinate’.
Recent research* has shown that a simple feedback mechanism helps shoots preserve the undifferentiated stem cells they need to keep growing, while that feedback is turned off in flowers. A mutation in the gene controlling that feedback leaves the flower growing indeterminately, making many more petals that it normally does, and giving us the beauty of double flowers. Other researchers** have identified the actual gene mutation that is involved – just one gene turns this seemingly complex process on and off. Instead of forming stamens, which are abundant in most flowers, a petal is formed. This means that double flowers tend to be sterile, but many are not, still forming some functional flower parts in their center, hidden by all those whirling petals.
Making all those petals takes a lot of energy, and few, if any, seeds are produced. This means that if double flowers develop in wild settings, the chances are high the plants will fail, leaving no offspring. In a garden, though, with care and protection from competition, they can be made to thrive – as we can see with the wonderful plant creations of Ryoji Iroe.
- * “Unraveling the Mystery of Double Flowers”, Adrienne H.K. Roeder, Martin F. Yanofsky. https://www.sciencedirect.com/science/article/pii/S1534580701000132#aep-abstract-id5
- ** “Mapping a double flower phenotype-associated gene DcAP2L in Dianthus chinensis”, Qijian Wang, Xiaoni Zhang, et. al. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7242084/