Soil's pH affects color of hydrangea

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Q. I have a 'Nikko Blue' hydrangea that was blue when I bought it, but it 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 flowers?

A. Bigleaf hydrangeas (Hydrangea macrophylla) have pink flowers in alkaline soil and blue flowers in acidic soil. 'Nikko Blue' is included in this group. Soil pH measures soil acidity/alkalinity on a scale of 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 6.0.

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 N. Lexington St., Pittsburgh, PA 15208. Make checks payable to Penn State Extension.

When you receive the kit, take the sample and fill out the paperwork, 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 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.

Q. My weeping cherry tree has a lot of growth that is growing straight up rather than weeping. Can I weigh them down with something to get them to weep?

A. Weeping cherries are grafted onto a cherry rootstock. They often revert to the straight growth of the rootstock, which is what is happening to your tree. The straight growth can outgrow the weeping portion and should be pruned off at its point of origin. Weighing the straight branches down will not train them to weep.

Most weeping and contorted trees such as Harry Lauder's walking stick (Corylus avellana 'Contorta'); weeping larch (Larix decidua 'Pendula'); and weeping mulberry (Morus alba 'Pendula') are grafted. Any reversion to straight growth should be pruned out as soon as you notice it.


Send questions to Sandy Feather by email at or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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