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…