The 'Dirt' on Hydrangeas

The 'Dirt' on Hydrangeas

By Jim Hole

Fact or Fiction?
You can change the colour of your hydrangea flowers by making the soil acidic.’

Hydrangeas are some of our most spectacular flowering shrubs. And while there are over a dozen great varieties that grow beautifully on the prairies, there are those among us who just can’t resist the challenge of changing a pink flowered hydrangea to one that flowers blue or vice versa. Today there are over a dozen varieties that we can grow here successfully.

But can one really change hydrangea flower colour? The answer is yes…well, sort of.

Changing the colour of hydrangea flowers starts with understanding a bit about soil chemistry and then choosing the right varieties. In our greenhouses, I was the guy who was in charge of adding the correct ingredients, in the right proportions to the soil half of the hydrangeas would flower blue while other half would flower pink. But I’ll be the first to admit that I didn’t always get it right. More often than I care to admit, I ended up with what are known as ‘blurple’ hydrangeas – mostly blue but with enough red blended in to give the hydrangea flowers a purpley tone. Now, I thought the blurples were rather attractive but, apparently, that sentiment wasn’t shared by everyone!

So how does one get a red, blue or even a blurple hydrangea for that matter? It all begins with choosing hydrangeas that have the capacity to change colour. The vast majority of the hydrangeas that we grow here are incapable of changing colour regardless of what you do. For example, white hydrangeas will remain white regardless of what treatments you provide.

If you have responsive hydrangeas then the next step is to raise or lower the soil pH above a threshold level depending on whether you want blue flowers or pink flowers. If you want a pink hydrangea, the soil must be fairly alkaline (higher pH) but if you want a blue hydrangea the soil must be rather acidic (lower pH).

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 aluminum 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 blurple colour.

Keep in mind that only a select few hydrangeas are responsive to manipulation of soil pH. In you want to experiment, a variety called Bloomstruck is one variety to have some fun with.

Remember too that once an existing flower is already pink or blue, it won’t change colour. Tweaking soil acidity will only affect the coloration of flowers that have yet to develop.

Also, in the garden, pH manipulation can be very difficult particularly if you have a clay-loam soil with lots of lime in it. Acidifying this type of soil is nearly impossible so just be satisfied with growing a healthy, floriferous hydrangea and enjoy whatever colour you get!

The florist type hydrangeas (hydrangea macrophylla) are the best hydrangeas for having some fun with flower colour transformation. They are only marginally hardy outside but are great for playing around with outside in containers during the summer. They love morning sun but hate intense afternoon sun so place them in a spot where they won’t suffer from sunburn.

Remember that even if you change soil pH beyond certain threshold levels, you won’t get a rainbow colours from your hydrangea flowers. Pink and blue are your only two choices…and blurple, if you weren’t paying attention in your soil chemistry class.

Some great Hole’s Hydrangeas to grow:

  • Annabelle, Bloomstruck, Bobo, Incrediball, Limelight