Pink blooms mean aluminum is not reaching the flowers. That hydrangea pink color tells you more about your dirt than about your shrub, and your plant is doing what the chemistry tells it to do. Aluminum inside the petals gives blue, a trace of it gives violet, and none at all gives pink. The metal is usually sitting in your bed already. The metal just cannot move, so your flowers open pink by default.
Eight seasons in the same alkaline clay, and my own mophead has opened pink in seven of them. The one blue summer came after a spring when I dug sulfur into that bed twice, once in March and again in June. Even then the heads came in a muddy violet instead of a clean blue. By the next year the flowers were back to dusty pink. I checked the meter that fall and the clay had pulled its pH right back to the reading it started at.
The order matters more than the pH number you read. Soil pH does not color a petal. Your soil pH decides whether aluminum can dissolve and ride up into the roots, and the aluminum sets the color once it reaches the bloom. So hydrangea soil pH is the lever you pull, and aluminum is the cause. Acid ground at 5.5 or lower frees the metal. Ground at 7.0 or higher locks the metal away, even when your lab test finds plenty of aluminum in soil from that same bed.
A soil test takes ten minutes and tells you whether pink is a choice or the only color your bed will hand you this year.
Purple tells you that your bed sits between the two bands. Half your heads lean blue and half lean pink, sometimes on one shrub, because the roots on one side pull from slightly different ground than the roots on the other. That patchiness is normal in a bed you amended once and then left alone. Even color needs even pH, and few yards have that without your help.
Mississippi State Extension sets the bands you can aim for. Soil at pH 6.0 to 6.5 turns your blooms pink, 5.0 to 5.5 turns them blue, and the narrow strip at 5.5 to 6.0 can come out purple. Iowa State Extension reports blue needs 5.5 or lower to hold. Typical soils there run 6.5 to 7.5, so gardens open purplish-pink year after year whatever the plant tag promised.
Check your species before you buy anything. Only bigleaf and mountain types swing between the two ends, so bigleaf hydrangea color is the one you can steer. White hydrangeas cannot be changed at all, and neither sulfur nor lime will touch your white blooms. I watch people dose a panicle hydrangea all summer and wonder why nothing happens. Those flowers were never going to respond, and the money went into the ground for nothing.
Start with a soil test, then use the amendment rates in the main guide instead of guessing. Sulfur and aluminum sulfate push your pH down for blue. Garden lime pushes your pH up and holds pink. Which way your bed drifts back depends on the ground you started with, so test again each spring and plan on repeating whatever you did. Soil returns to its own pH over time, so the hydrangea pink color you have this year is a standing chore and not a one-time fix.
Read the full article: 10 Pink Flowers and What Makes Them Pink
