Q: I want to be sure my pink hydrangeas stay pink and my blue hydrangeas stay blue. I have been adding lime to make the flowers pink and aluminum sulfate to turn them blue. What I want to know is – what time of year is best to amend the soil?
A: Good question, I was not sure myself so I did a little investigation. Generally, most gardeners suggest adding the amendments just before the leaves or flower buds begin to develop or flush out – possibly late February or early March. Altering the color of the flowers only applies to the mop head hydrangeas which are often called big leafed. We will not be able to change the color of the oakleaf hydrangeas – only those species – H. macrophylla or serrata.
I would also caution you about using too much of any amendment because over applying can create problems. Lime can take up to 3 months to increase the soil pH, so just because you do not see an immediate reaction does not mean it is not working. Be patient. It is possible to cause other nutrient deficiencies, such as iron chlorosis, when adding lime to ornamental beds and lawns. Remember, adding lime raises or increases the pH making the soil more alkaline. If you wish the flowers to be blue, then add aluminum sulfate. Lastly, there are some genetic boundaries which even adding amendments cannot alter.