Baytree Garden Centre’s Mark Cox on the role of soil acidity in determining the colour of a Hydrangea plant

This week Baytree Garden Centre’s Mark Cox writes on Hydrangea plants – and the importance of soil for their colour...

During my weekly telephone call to my parents my Mum said that she was really disappointed in me for the plant advice I’d given her a few months ago. To cut a very long story short, when my parents last made the trip from the south west to Lincolnshire my mum had commented on how beautiful the blue Hydrangea that I had growing in the garden was.

As my mother was so taken by the Hydrangea and because I see them so infrequently I decided to buy my mum a gardening gift voucher that she could use in her local garden centre back in Plymouth. This made more sense to me as I didn’t really want them to carry a plant across central London via the underground as they had caught the train to visit me.

My mother was very puzzled as she had bought a beautiful Hydrangea Macrophylla that I had instructed her to buy. Well the issue was my plant was blue and my mother’s was a lovely pink colour even if she couldn’t see it.

Why the difference in colour when they are the same plant? Had they sold my mother the wrong Hydrangea? In essence, no, it was the same plant. Hydrangeas are wonderful deciduous hardy shrubs with big mop headed showy flowers, usually pink, blue and sometimes white.

Now this is the incredible thing about Hydrangeas... please if you can try to remember back to your secondary school chemistry days and in particular the experiments that involved testing various chemicals and substances with litmus paper to test the PH value. The PH value is a measure of how acidic or alkaline the substance you are testing is.

The PH value of the soil they are planted into determines the colour of the flower heads. Acidic soils, meaning soils which have a PH value below six, produce blue flowers and alkaline soils, which have a PH value above eight, produce pink flower heads. To test this theory I asked my mum to visit her local garden centre and buy a PH testing kit and to call me back the next day with the results. As expected she called back the following evening just after Emmerdale and confirmed that her soil was in fact alkaline with a PH value of nine.

The solution was quite simple, if my mum wanted her Hydrangea to look like mine then she would need to alter the PH value of the soil and move it towards the acidic end of the scale. By feeding her plant with a low phosphorus but high in potassium fertiliser such as Miracle Gro Sulphate of Potash my mother could change the colour of the flowers from pink to blue.

It won’t happen overnight but it will steadily change as the plant begins to take up the new nutrients through its roots.

So with mother now happy again that number one son knows what he’s talking about, it’s time to head into the garden for a spot of dead heading to keep my basket and bedding plants producing more flowers. I’ll wait for it to cool down a bit before I start watering my basket plants.