Q. I have a ‘Nikko Blue’
hydrangea that was blue when I bought it, but has been pink since I
planted it near my porch. I know the flower color has something to
do with soil nutrients – can you tell me what to do to get blue
A. Bigleaf hydrangeas
(Hydrangea macrophylla) have pink flowers in alkaline soil and blue
flowers in acid soil. ‘Nikko Blue’ is included in this group. Soil
pH measures soil acidity/alkalinity on a scale from 0 – 14, with 7
being neutral. Higher numbers indicate alkaline soil while lower
numbers indicate acid soil. Bigleaf hydrangeas are blue if the pH is
between 5.0 and 5.5. The flowers turn pink when the pH gets around
'Nikko Blue' Hydrangea
The flower color is dependent on
the concentration of aluminum ions (Al+++) in the soil. Aluminum is
more available to the plant when the soil is acidic, less available
when the soil is alkaline. In this case, you should use aluminum
sulfate to lower the pH of your soil and supply additional aluminum.
Have your soil tested to determine the pH so that you know exactly
how much aluminum sulfate to use. It is possible to use too much.
Soil test kits are available from
your local Penn State Cooperative Extension office. In Allegheny
County, soil test kits are $12 for the first kit and $9 for
additional kits ordered at the same time. They come with complete
information for taking representative samples and understanding your
soil test results. You can send a check for the cost of the number
of kits you want to Penn State Extension, Soil Test Kits, 400 North
Lexington Street, Pittsburgh, PA 15208. Make checks payable to Penn
Once you purchase the kit, take the
sample and fill out the paperwork, and then send it to Penn State’s
Agricultural Analytical Laboratory. The fee covers the cost of the
kit and the actual testing. Your only other cost is the postage to
send it to the university. The kit is a self-contained mailer with
the lab’s address pre-printed on it.
Although we tend to have acid soils
in western Pennsylvania, there are situations where the pH can move
into the alkaline range. For example, garden beds located near
cement structures (patios, porches, walls, sidewalks and driveways
or the foundation of your house) often have a slightly alkaline pH.
This is because lime leaches out of concrete. We use limestone to
raise soil pH when the soil is too acid for crops such as
vegetables, flowers or turfgrass. If your porch is made of concrete,
or there are other concrete structures near the hydrangea, it
explains why it is pink now, even if it was blue when you bought it.
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