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

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.