It's like waiting for water to boil.
Many people ask me how to turn their pink hydrangea blue – a lot more than ask how to turn a blue hydrangea pink (the South loves the blues, I guess). I answer just like every other know-it-all.
The flower color of a French aka bigleaf hydrangea (Hydrangea macrophylla) depends on whether the soil is acid (below pH 7.0) or alkaline (above pH 7.0). Strongly acid soil of 6.5 or below produces blue flowers; alkaline soil produces pink or red flowers; and near-neutral soil of pH 6.5 to 7 gives you purple flowers or a mixture of blue blooms and pink ones.
How do you change the soil pH? To make it more acid, you add sulfur. To make it more alkaline, you add lime. Easy-peasy, right? Not so fast.
Some hydrangeas change color easily. The original ‘Endless Summer’ is one. I bought one that was blooming pink in the pot, planted it into my strongly acid soil, and the next year it bore bright-blue flowers. In a neighbor’s yard with near-neutral soil, half of its flowers were blue and half were pink.
My ‘L.A. Dreamin’ hydrangea (above) is another matter. I was smitten the first time I saw it out in California. Big blooms in colors of blue, purple, and pink smothered it. It also blooms on both old and new growth, so even if winter kills the flower buds or you cut them off by pruning at the wrong time (fall and winter), you still get blooms.
‘L.A’ Dreamin’ performed admirably for me in a big container (hydrangea are great container plants BTW, but that’s a future story). The flowers emerged bright-pink, slowly aged to nearly red, and lasted for months. I ended up drying the last flowers indoors. My only quibble is I wanted a little more blue, a little more purple.
I turned to Espoma Organic Soil Acidifier which contains 30% sulfur. (I’m a big fan of Espoma products and they didn’t pay me to say that. I bought the bag.) The label recommends sprinkling about 1-2 cups around the plant and watering it in. Repeat at 60-day intervals until you get the color you want.
My hydrangea just received its third application and I’m still waiting for the first sign of blue. I think it will happen. I’ll just have to take the same advice I so often dole out to you. Be patient.
WATCH: Easy Ways To Change Hydrangea Colors
One more factor in this people seldom consider is their water quality and its pH. Hard water contains lots of dissolved minerals like calcium and magnesium and is alkaline. Soft water contains few minerals and is acid. So if you’re watering your hydrangea with hard water, changing the flower color from pink to blue is going to be a longer and ongoing process.