Answer: Some hydrangeas are supposed to be white and will not change color no matter what you do. If you have one that could potentially change color from pink to blue, it would be some type of H. macrophylla. Not all of these will change, either, but many will change. The way to try to make it blue is to check the soil pH and, if needed, adjust it toward the more acid side. A pH range of 5.0 to 5.5 should bring out the blue. Conversely, a pH range of 6.0 or 6.5 should bring out pink instead. Any changes would be made gradually to avoid shocking the plant. First off, do not add lime as this makes the soil less acid. Sometimes using a fertilizer for acid loving plants (such as Mir-Acid or Hollytone; read and use according to the label instructions as you would never want to overfertilize) and/or an acid mulch such as pine needles is enough to make the difference, but that depends on the soil's natural pH and what has been done to it in the meantime. You can check pH using a soil test kit. Your local county extension or professional nursery personnel should be able to help you with the tests, interpreting the results, and determining how to proceed based on the results. Good luck with your hydrangea.
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