soil pH

It’s hydrangea time again. This year the pink-flowered one is right next to the blue-flowered one, because it grows in a pot, and I moved the pot this spring.

This hydrangea (and it’s a single species (Hydrangea macrophylla) despite the colour difference), is of the type known as “lacecaps,” as distinguished from “mopheads.” They are the same species, but the number of fertile and sterile flowers differs: the tiny nubbles in the middle of the flat lacecap flower cluster are the fertile flowers; mopheads have spherical clusters of mainly sterile flowers.

The thing about these two plants that pleases me more than it should is that the pink one is a clone of the blue one. Several years ago, when I had only one plant, I feared that it was going into a decline, so I rooted a cutting from it, intending to plant it in the ever-elusive Better Spot. (Every garden has a Better Spot; it varies depending on what plant is intended for it, but it’s always elusive.)

As it turned out, the hydrangea recovered, because I took the trouble to remove the lilac suckers that were invading its space and furnished it with its own soaker hose. So the cutting remained a potted plant, although the pot it occupies now is a lot bigger than the original one. But the soil in said pot has added lime (left over from a batch prepared for tomato plants), which is why the plant’s flowers are pink. Soil pH determines whether aluminum ions are taken up as the flowers form. I don’t know of any other plant whose flower colour can be changed by fiddling with soil acidity, but it certainly works for hydrangeas.

Blue and pink lacecap hydrangeas July 2024

I must remind myself not to trot out this story again next year when these two plants bloom.

These are blooms of Hydrangea macrophylla normalis, otherwise known as lacecap hydrangea.

Pink lacecap hydrangea, grown from cutting in pot July 2021

The pink one is a clone of the blue one, grown from a cutting. The difference is that the plant with blue flowers is growing in the natural soil in my garden (supplemented with compost, fertilizer, and lots of water), while the pink one lives in a pot. The soil in the pot is a blend of natural soil, compost, various supplements, and lime. It may have been left over from the mix I put together for tomato plants the year I potted up the hydrangea cutting. The key difference is lime. I add extra lime to tomato soil to avoid so-called blossom end rot in the tomatoes. It’s caused by calcium deficiency, hence the need for lime.

According to Wikipedia, “An acidic soil (pH below 7) will usually produce flower color closer to blue, whereas an alkaline soil (pH above 7) will produce flowers more pink. This is caused by a color change of the flower pigments in the presence of aluminum ions which can be taken up into hyperaccumulating plants.”

Either way, hydrangeas perk up the garden, which starts to look tired by July. The flowers last for weeks, and even retain “interest” into the winter (meaning they hang on in a discoloured state, which may be somewhat interesting). I admit I prefer the blue colour, which is why I go out of my way to supply water to the plant starting in June, because it would bloom poorly or not at all otherwise. But seeing the pink flowers on the potted cutting-grown plant (which bloomed for the first time this year) has been a nice demonstration of nature and nurture.