Buy a blue hydrangea and plant it next to concrete path and it’ll turn bright pink. The reason is that lime washed out of the concrete turns the soil alkaline. Hydrangeas need aluminium for blue flowers and even if it is present they can only access it if the soil is acid. So how do you turn it back?
Potting up your hydrangea in a tub of ericaceous compost is good start to provide the acid conditions. But it may not be enough. They may still need aluminium. For this you can buy a bluing agent especially for hydrangeas that contains aluminium sulphate. Be very careful though, it can easily scorch the roots. Add less than half a teaspoon to a gallon of water and don’t use it when the compost is dry or when the plants are under stress in very hot weather.
Personally I like white hydrangeas. It avoids a lot of problems.
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