Biology Question #9423

Jenny Davison, a 29 year old female from Nova Scotia asks on August 3, 2017,

Will burying rusty nails around a hydrangea plant turn its blossoms from pink to blue? Why or why not? There is conflicting information online about this, and I haven't come across anything from overly reputable sources. In general, gardeners seem to be debating whether or not the iron in nails is in a form available to plants, and whether the rusting nails can contribute to upping the soil acidity, which hydrangeas like. There is also discrepancy on whether increased soil acidity will change the flower color from pink to blue, or just deepen an existing blue color.

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The answer

Barry Shell answered on August 3, 2017

Rusty nails are probably not the easiest or best way to achieve the desired result which is to turn the blossoms from pink to blue. In Vancouver (home of the soil is generally acidic because of high annual rainfall in the Pacific Northwest, so our hydrangeas are almost always blue. However on one bush this summer there are blue AND pink blossoms, so how does one explain that?

The advice provided by Stephanie Rose on her garden therapy website seems correct. She advises using an acidifying additive for the soil such as aluminum sulfate or ammonium sulfate. Acidifying agents typically have sulfur in them. It takes a year before you see any effects though. Do not expect any colour changes to happen overnight. 
The rusty nail thing is probably a myth as stated on the Laid Back Gardener website. Also, once the surface of a nail rusts, it protects the rest of the iron from rusting for many  years, so it’s not going to be the best source for acidifying soil. On the other hand according to the Wikipedia entry on Ferric iron, "A salt of ferric iron hydrolyzes water and produces iron(III) oxide-hydroxides while contributing hydrogen ions to the solution, lowering the pH." Translation: iron oxide (rust) could create acidic soils. But iron oxide is not the best acidifying agent. Aluminum or ammonium sulfate is better. 
The question of whether the acid soil deepens the existing blue colour or if it changes the colour probably depends on the botanical variety of the particular hydrangea plant. Hydrangeas can be bred to flower in many colours from white through red, blue, and many shades of mauve and purple, so if you have a variety that is bred to be red, it may be very difficult to make it turn blue with acid soils. 

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