The Q&A Archives: Keeping Hydrangeas Blue

Question: What do you recommend I use to keep my Hydrangeas blue without using Aluminum Sulfate, which I have been told eventually poisons the soil? I'm looking for a safe, natural way to do this. Whatever I use, could it change the color of my pink Hydrangeas which are closeby?

Answer: The changeable color varieties of hydrangea should be blue in a pH range of about 5.0 to 5.5 and turn pink when the pH is a bit higher, say between 6.0 and 6.5 or so. In your area, the soil may naturally be acidic enough (if it has not been limed) for the hydrangeas to stay blue. Using an acidic mulch such as pine needles or oak leaves would possibly be enough to naturally maintain the pH in the range you need for the blue coloration. Lime can take a few years to leach out of the soil, so it may take some time for the effects to be visible in the flower color. Note though that some hydrangeas such as "Forever Pink" or "Pia" are pink regardless of the pH and some varieties will stay closer to blue than others in a higher pH soil so the variety you are growing will be a factor in how much color change you could possibly achieve.

It is true that excessive amounts of aluminum sulphate would lower the pH to the point where it was too acid for plants to grow in it, and there are occasionally soils where aluminum toxicity is a problem, just as there are also soils that are too alkaline for many plants to grow. The problems can occur in the native soil or can be due to applications of soil amendments without regard to the initial pH and soil type.

To avoid any danger of this happening by overapplying any means of adjusting the soil pH, by all means run some basic soil tests first. Then add your amendments according to the results of the tests, and be sure to allow the appropriate amount of time for the material to take effect. Your UMASS extension should be able to help you with the tests and interpreting the results.

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