Hydrangeas are always at the top of my favourites list for flowering shrubs.
The reason I love them is simple. Few other shrubs can match the ‘flower power' of hydrangeas. There are over a dozen Prairie-hardy varieties ranging from the big, bold ‘Anabelle' to the pretty and petite ‘Bobo' and each one is a show stealer.
Yet, despite the wide range of great varieties with their stunning flowers, there those among us who still aren't satisfied. Thoughts then turn to trying to change hydrangea flower colours by tweaking the acidity of the soil around their roots.
But will altering of the pH of garden soil really cause a hydrangea to change its flower colour?
The answer is yes…well, sort of.
A Bit of Science
Changing the colour of hydrangea flowers is a well-known and proven phenomenon but first you need to understand a bit about the relationship between hydrangea varieties and soil chemistry.
In our greenhouses, I was the guy in charge of adding the correct ingredients to the potting soil, at the correct rates, so that half of the hydrangeas would flower blue while the other half would flower pink.
But truth be told, I didn't always get it right. More often than I care to admit, I ended up with what are known as ‘blurple' hydrangeas – flowers with a blend of pink and blue. No matter how hard I tried to convince my family that these hydrangeas were beautiful, I knew that blurple was, in essence, just a euphemism for failure!
So how does one get a pink, blue or even a blurple hydrangea for that matter? First, it's critical to choose only those hydrangeas varieties have the capacity to change colour. The vast majority of the hydrangeas that we can grow here will not change colour change no matter what you do – short of spray-painting the flowers.
But if you want to try your hand at messing around with hydrangea flower colours, there are only about three hydrangea varieties that are commonly available here that are receptive to change. They include tender florist-type hydrangeas, and cold hardy varieties like ‘Endless Summer' and ‘Bloomstruck' .
As far as outdoor varieties are concerned Bloomstruck is a tougher than Endless Summer, so given the choice of hardy hydrangeas, I would suggest Bloomstruck as your first choice.
Once you have your responsive hydrangeas in hand, the next step is to raise or lower the soil pH above a specific threshold level. If you want a pink hydrangea, the soil must be fairly alkaline (pH 6.5 and higher) but if you want a blue hydrangea the soil must be rather acidic (pH 5.5 and lower). Lime will drive soil pH levels higher whereas various sulphur containing compounds will push pH levels lower.
Diving into soil chemistry just a bit deeper, acidic soils make aluminum (a naturally occurring soil element) more soluble and more readily absorbed by plant roots whereas alkaline soils make aluminum less soluble and thus more difficult for roots to absorb. At the cellular level the quantity of aluminum absorbed by hydrangea roots alters the pigments in the hydrangea blooms and, voila, the flower colours change. But the caveat here is that if you don't tweak the soil correctly, you'll end up with my ‘beautiful' blurple hydrangea.
What should you do?
Remember that your hydrangea choices are limited when it comes to flower colour manipulation. Also, once a flower is either pink or blue, it won't change colour no matter how much you mess around with the soil chemistry. Tweaking soil acidity will only affect the coloration of flowers that have yet to develop. Also, some garden soils are notoriously resistant to pH change. Clay-loam soils that contain lots of lime will fight your acidification efforts tooth and nail, so just be satisfied with growing a healthy, floriferous hydrangea regardless of what colour you get!
And keep in mind that changing your soil pH won't provide you with a rainbow of colours from your hydrangea flowers. Pink and blue are your only two choices.
OK, of course for those of us with more refined taste, there is also blurple.
Jim Hole is a Certified Professional Horticulturalist with the American Society for Horticultural Science, and is co-owner of Hole's Greenhouses in St. Albert.