Soil Test First to Keep Hydrangeas Pink

Q: We bought pink hydrangeas and we want them to stay pink. I have been advised to keep the soil alkaline. Is fast acting lime okay?

A: I have a group of hydrangeas in the front of my house. I added a great deal of garden lime to that area when they were planted fifteen years ago. For the first five years the blooms were pink. They have gradually turned blue as time passed. I noticed yesterday there was only one pink bloom amongst twenty blue blooms.

If you want to do it right, start by having a soil test done on the area where your hydrangeas are planted.

You are shooting for a pH 6.0 to 6.2. At this pH, aluminum is not available to the plant. Aluminum is what enables blue flowers. If you get above 6.4 the hydrangea might have an iron deficiency.

A soil pH below 7.0 is not technically alkaline soil but this is still the recommended pH for pink flowers.

Your soil test report will give you a recommendation of how much lime to apply for raising the pH to 6.0. You can apply a little bit more than the recommendation to bump it up another 0.10 or so.

Fast-acting lime is ground very fine so it dissolves faster and works faster. But you will still apply at the same amount recommended in the soil test report. If the recommendation specifies 40 pounds per thousand square feet, you can split it into 4 applications per year of 10 pounds per thousand square feet. (maybe 12 pounds each time)

As my experience illustrates, lime’s action in the soil gradually declines. After one year of liming, do a second soil test to see what effect the lime has had.

It is probable you’ll have to apply garden lime every year to keep the pH in the range of 6.0 to 6.2.

See How hydrangeas change color

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