Roses are pink, violets are blue,
hydrangeas can be either one of the two.
That, ladies and gentlemen, is my ode to the hydrangea. As you might have guessed, I get pretty excited about these shade-shifting shrubs. I gush about them with every plant enthusiast in Guelph and dote on the ones in my yard. Apparently, I also write poetry about them.
I mean, why not get excited? It’s not every day that we encounter something as magical as a shrub that changes colour. Try it yourself— you’ll be enchanted.
Not All Hydrangeas Are Chameleons!
Before you get too excited about covering your yard in blue hydrangeas, it’s important to note that only a certain variety of hydrangea has this magic power. Bigleaf hydrangeas, or Hydrangea macrophylla, are the only hydrangeas whose colours can be modified (at least without a pot of dye).
Furthermore, it’s not even all bigleaf hydrangeas that will change colour. Cultivars that bloom white, will forever bloom white and they’re gorgeous as-is. However, the other bigleaf cultivars, which bloom in pink or blue, can be coaxed to change from one to the other. Fortunately, these hydrangeas are very popular in Guelph, and it’s easy enough to track them down. (I could name at least one garden centre that carries them!)
Turning Hydrangeas Pink
If you live here in Guelph, your natural soil conditions are either one of two: the gravel ribbon that runs through town or the clay pan on either side. No matter which your garden has, though, both have soil that is naturally alkaline, meaning our hydrangeas are getting exactly what they need to have picture-perfect pink hues. However, if you really want to get those hydrangeas blushing, you can always amend the soil with a bit of garden lime. You’ll want to follow the instructions on the package to apply lime correctly. It will take some time after application to notice the change, but you’ll gradually notice the blooms getting pinker and pinker. Your soil might be basic, but those rosy blooms certainly won’t be!
Turning Hydrangeas Blue
Now that you know that alkaline soil turns hydrangeas pink, would it be any surprise to learn that acidic soil turns them blue? Before you start experimenting with the colour, know that turning hydrangeas blue is a commitment. Due to our naturally alkaline soil, keeping the soil acidic enough to keep your hydrangeas blue will mean regular applications of whatever acidic matter you’ll be using to transform them.
You’ll start noticing a blue tinge to your hydrangea blooms at a soil pH of 5.5 or under. You might decide that just a hint of blue is the colour you like the best. At a just-under neutral pH, these hydrangeas tend to take on a really lovely lavender-purple tone. Personally, blue is my favourite colour, so I never stop here. For the sake of science, let’s just assume that you’re like me and you’re on a mission to see how blue your hydrangeas can go.
To do that, you’ll want to stock up on some aluminium sulfate, which we carry here at Royal City Nursery. You’ll notice the best colour if you amend your soil in the springtime, after new growth appears but well before the hydrangeas bloom. After watering your hydrangeas, mix a quarter ounce of aluminium sulfate into a gallon of water and apply it to the soil around your hydrangeas. Re-apply four weeks later, then four weeks after that.
You can also achieve that same goal using an acidic fertilizer paired with soil for acid-loving plants, like the ones by Fafard. These products are designed specifically to keep that soil just the way the blue hydrangeas need it, making them an easy pick for that pop of colour.
For even better colour, or to simply tweak the acidity slightly, try adding acidic organic matter to the soil, like peat moss. Coffee grounds are also plenty acidic and very easy to work into the soil, and you can also try adding eggshells or citrus peels. It will take some time for the compost to break down and affect the soil pH, but in a matter of years, your hydrangeas will get bluer and bluer.
Playing with Hydrangea Colours
Once you get the hang of this colourful chemistry experiment, you can get pretty creative with it. You can focus your acidity or alkalinity more in some areas than others and create a multi-tonal hydrangea bush. If you’re a real expert, you can even create a pink-to-blue gradient effect from one side of your garden to the next. Why not have some fun with it?
Keep in mind that, no matter how you want your hydrangeas to turn out, they won’t change colour overnight. The other secret ingredient for turning hydrangeas blue or pink is patience. You won’t do your plant much good by overloading it with lime or aluminum sulphate (in fact, quite the opposite), but stick to the recipe book and you’re sure to see blue-tiful results!