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Anne K Moore
Photographs Anne K Moore

Ample rain has brought us a garden full of wonderful hydrangeas. The blossoms are big, beautiful, and numerous. In my yard, they are also many-colored.

The big mopheads, Hydrangea macrophylla, are very sensitive to soil pH. It occurs to me that all I need to do to guess the soil’s acidity or alkalinity is to look at my hydrangea blossoms. They are planted in many beds in the front and back yards.

The photos illustrate my hypothesis. Pink hydrangeas indicate that the soil is very alkaline (base) with a pH around 6.0




Light blue blossoms indicate soil that is borderline acid, around 5.5 or so.



Deep, dark blue to purple indicates a very acid soil, around 4.5 - 5, which is good for growing blueberries as well as deeply hued hydrangeas.

I wish I could tell you the names of my hydrangeas, but the tickets have long been pulled by the resident squirrel population, either to play with or maybe to reinforce a nest.

For a definitive answer on whether or not your soil is acid or alkaline, you still should have a soil test. If you ask most master gardeners or Extension Service agents, they will explain that you should dig samples with a trowel from all areas of your yard, mix them together, and add the mixture to the box and send it for analysis. I am a master gardener, this is what I was taught, but this never made sense to me.

Luckily, I found another master gardener, Duane Campbell, who holds the same opinion, which I read in his book, “Best of Green Space: 30 Years of Composted Columns.” Duane believes that this is a holdover from agents who mostly dealt with farmers and huge acreage. Those of us with backyards rather than acreage should really get samples from our problem areas and have them tested separately. (It is always nice to get affirmation of one’s own opinion.) In my own yard, it is easy to see that mixing soils where the hydrangeas are pink with the site of the deep bluish-purple hydrangeas would not give me a clear picture of what I need to do at either spot.

In next month’s “In The Dirt” newsletter, Stan Griep (the Roseman) will have a chart and recommendations for raising or lowering the soil’s pH. (If you don’t already receive the GardenSmart E-letter “In The Dirt,” sign up now here at the website.) Even though this is for a rose bed, you can use the information for any garden bed. For specific information on hydrangeas, this website has good information:

Changing the pH involves adding lime to acid soils to raise the pH if you want light blue or pink flowers. If you want blue, then you use either aluminum sulfate or sulfur to lower it. Since these additives take awhile to change the soil, you should add them several times throughout the growing season, so they have time to interact with the soil. In addition, they work best if you can mix these amendments into the soil without injuring the hydrangea roots.

Posted June 21, 2013

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