Louise Tomlin tries to get to the bottom of the unique ability of the Hydrangea to change colour

There was a point a few years ago when I noticed that a hydrangea bush I was looking at had two different coloured flowers on it, pink and also blue. I was genuinely puzzled once I’d checked that it really was one bush. It made me wonder, what sorcery was at work here? I asked one of my knowledgeable gardening friends how this could happen. I’d seen pink ones, blue ones and white ones, and presumed they were different varieties like white, red or yellow roses.

Well imagine my surprise when I was told, it wasn’t some dark and mysterious magic, in fact most experienced gardeners were well aware the colour of the blooms depended on the type of soil they were growing in. If the soil was acid the flowers are blue, and high alkaline soil levels produce pink. So that would explain why a single bush could have different coloured flowers, the roots must be picking up traces in the soil. Not magic but chemistry. Job done… but maybe not?

Back at school one of the chemistry lessons I still remember well was the one where you used litmus paper to check the acidity or alkalinity of a substance. The litmus paper was made from extracts of different lichens and was used as a water soluble pH indicator – if it was acid, the paper turned deep red, if it was alkaline it was dark blue with varying colours in between,
the mid point being green, that was pH neutral. Well surely that’s the opposite of what the hydrangeas are showing? If the soil is acid the flowers turn blue and likewise, if it’s alkaline they turn red. It seems a bit contrary.

Further research is definitely required here to check if there’s an explanation I can easily understand. Here’s what I found; the key is aluminium, which is common in most soils. If the soil is acidic it allows a receptor in the hydrangea to unlock so it can absorb the aluminium, which moves it from the roots to the flowers making them blue. When the soil is more alkaline the receptor closes down, stopping the absorption so the flower reverts to pink when no aluminium is getting through.

Ok, that’s probably enough chemistry for one day, back to plants, where I feel much more comfy. Apparently white hydrangeas stay white and never ever change colour because of the soil they are growing in, although some of them do turn delicate shades of pink as the blooms reach the end of the season. I must admit I do prefer the simplicity of the white ones, particularly this pretty one I have photographed with cone shaped flowers, I think it’s called Gatsby Gal. These varieties are much less common than the mophead and lacecap culitvars of the bigleaf Hydrangea macrophylla that you see in many gardens. You’ll see from the main photo the mopheads are just like their name with large round blooms full of large petals. They’re a bit too big and blousy for me, but that’s just personal taste. Once again I prefer something more delicate in colour and form, favouring the lacecap in a subtle powdery pink, they have larger petals around the outer edges of a flat or dish shaped flower with smaller flowers towards the centre.

Compared to very strong pink ones, which as we now know must be growing in very alkaline soil, they sort of hurt my eyes a bit. Maybe I’m getting over sensitive as I get older.

Another type worthy of mention is Hydrangea petiorlaris, which is a climber. It produces aerial roots that will attach it to walls and fences, so it’s well worth considering if you fancy a hydrangea but haven’t got space for the bushy type.

Most of these lovely plants will bloom from June to October and will thrive in well-drained soil, in full sun or partial shade, but will appreciate a sheltered position away from harsh winds. Prune them back to about a third of their original size in the spring to help them produce lots of healthy new growth, but remember to enjoy their pleasing architectural presence as the flower heads fadeand dry out through the autumn frosts and the winter months, reminding us of their colourful presence in the summer.

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