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Mike's Blog - Day 191

A nice Blue Hydrangea for you today next to the woodland garden.

Here it is again in close-up.

It’s nice and blue as the soil in the woodland garden is more acidic. They tend to be more of a dusky pink on neutral to alkaline soils.

It’s not simply the acidity of the soil however. Soil pH effects the availability of certain minerals and elements, and aluminium is more available in acid soils. You can buy hydrangea colourants which normally contain aluminium sulphate. Aluminium is a major constituent of soil but is not required for plant growth. Nitrates, phosphates and potassium (aka NPK) are the main ones needed for growth, plus “micro nutrients” or “trace elements”, and their availability changes with soil pH.

I’ve heard some people put a tea spoon of vinegar in a watering can and also that coffee grounds are good to keep hydrangeas blue. So maybe we should mulch ours with the coffee grounds if it starts to go pink…