We’ve covered hydrangeas a few times here at GardenRant, like this post and this one too, but we’re now in what has slowly become my personal favourite part of the hydrangea season.
White flowers turning pink in a glorious display
They are way past their peak now; those nice clear colours we admired in summer are changing as the flowers decay.
It’s All Chemistry
Flower colours are pigments that the plant makes. In a lot of cases flowers drop their petals and hold onto the parts that will produce seed. Hydrangea flowers, the little true flowers hidden within the decorative flowerhead we enjoy, make these same changes.
This hydrangea would be pink if it wasn’t for aluminium in the soil; the pink here faded old flowers
What we generally refer to as ‘flowers’ in the hydrangeas are in fact sterile florets. These persist for many months after the true flowers have finished, and as they age the pigments break down and bring us a very different display.
Different Forms
I’m sure we’re familiar with the different forms of hydrangeas: mopheads, lacecaps, panicles.
The conical panicles of Hydrangea paniculata change colour very quickly, and indeed more often than not it’s these changes that are the very reason we grow them. Hydrangea paniculata flowers are all white at their peak. They’re supposed to be white; the shades of pink are the process of the flowerhead ageing, generally regarded as being much more interesting than the flowers at what should be regarded as their perfection.
Hydrangea paniculata ‘Le Vasterival’ (sold as ‘Great Star’) back in summer
The mophead and lacecap hydrangeas are grown for their colours at their peak; white, pink, reddish-pink and blue.
A Bit More Chemistry
The colouring of mophead and lacecaps hydrangeas is also down to chemistry. Indeed that pigmentation is caused by the presence of absence of one key ingredient: aluminium.
Where amounts of soil-based soluble aluminium are high, hydrangeas sequester unwanted aluminium into their sterile florets. Where there is enough aluminium the ‘flowers’ will be blue, where there is too little the ‘flowers’ will be pink. White mopheads and lacecaps stay white whatever.
“Ah, but it’s down to acidity of the soil.”
Well it is and it isn’t; aluminium is more likely to be available in a form that the plant can take up in acidic soils. In alkaline soils aluminium is often to be found but not in a form that is absorbed by plant roots.
Hydrangea paniculata ‘Vanille Fraise’; the pink flush comes quickly as the florets age, nothing to do with soil
Pedantry? No!
If you pot a lacecap or mophead hydrangea into an ericaceous (lime-free) compost it’s unlikely to turn blue. This is because compost is made of plant material and is aluminium-free. Plants grown on nurseries are treated with aluminium sulphate to make them flower blue, but of course if your garden soil doesn’t have high enough levels of aluminium then that exact same plant that you bought with blue flowers will flower pink in future seasons.
You can give your plants doses of aluminium sulphate to keep them blue, but be warned that this is an ongoing process. However various attempts to acidify soils, using tea, rusty iron nails, pine needles or anything else people seem to suggest, simply don’t work. How likely could it ever be that a few rusty nails could alter soil chemistry that’s the result of millennia of climatic and geological processes?
Autumn
We grow a lot of hydrangeas in my region. They luxuriate in the warm and wet summers, and the mild and wet winters.
Gardens are filled with these glorious fading flowerheads. I love it. They’re suffused with inky blues and dusky pinks and are absolutely captivating. I could preserve these colours for the house by cutting the heads with a stem the length of a pencil and hanging them upside down somewhere cool and letting them dry slowly.
Brooding purple florets of what had been a beetroot-red hydrangea a couple of months ago
As the weeks and months pass their glory fades until they’re left as brown papery skeletal remains of what once was.
The gentle rustle of the hydrangeas in winter, as a breeze passes over their dried up old heads, is music to my ears.
Winter Is Coming
Many gardeners will tell you that you must leave your hydrangea flowerheads on through winter to protect the buds below. Like so many things in gardening that we supposedly must do, it’s not actually true.
How are papery old flowerheads really going to insulate against severe frosts?
Pretty, but no insulating properties whatsoever
The origin of this misunderstanding is likely to be from the fact that Hydrangea macrophylla is not quite as cold hardy as the other hydrangeas we’re likely to be growing. If you’re in a very cold area then Hydrangea macrophylla is one to avoid. Cold winters really will damage buds and knock the plant; at best your plant will die and act as a warning, at worst the plant will linger on but never flower and thrive.
Rather unhelpfully hydrangea breeders don’t seem to like to tell gardeners the parentage of their new hydrangea. This is partly protectionism on the part of the breeder, partly the arrogance that says gardeners aren’t interested in such things and we must be protected from anything even remotely challenging. In the colder regions it’s really worth making friends with a specialist hydrangea nursery, asking them to help you find Hydrangea serrata and other hardier hydrangeas. If it’s not obvious on their website or labels then ask for help; any nursery worth your patronage wants you to have success with their plants.
Bright new shoots among last year’s flowerheads, left on “to protect against frost” I was told
Where spent hydrangea heads look good for winter they are a fantastic extra feature for the garden. They look incredible picked out by frost or with a light dusting of snow. You certainly don’t need me to tell you to cut them off if they look ugly and weather-beaten, but if you’re in the habit of automatically removing the flowerheads in the name of tidiness then it might be worth leaving a few to see if you, like me, find beauty in the hydrangea’s decay.