Changing the Color of Hydrangeas

The long-lasting blooms of hydrangeas look fabulous in a mixed border. Even when the heads have faded they still look good and are great for flower arranging. Hydrangeas make an excellent flowering hedge. I admit they are a bit uninspiring in winter, but I’m prepared to put up with that because their flamboyant flowers look so gorgeous. I especially love Hydrangea arborescens ‘Annabelle’ with its huge creamy white heads. I’m a big fan of Hydrangea paniculata – some of the newer forms like ‘Pinky Winky’ and ‘Limelight’ are very interesting.

I’m not keen on pink flowered hydrangeas, if they must be a colour other than white or white with a hint of something else, I prefer them to be blue. How many times have you planted a supposedly blue hydrangea only to have it come out pink? Exactly how annoying is this? The good news is that you can do something about it because the colour of hydrangea flowers is dictated by the type of soil you plant them in. If you have acid soil you will get blue blooms, if your soil is alkaline your hydrangeas will be pink.

Not all hydrangea varieties can change color, those that are naturally white will remain white no matter what type of soil they’re grown in. Only those that are naturally blue or pink can be made to change colour. So, if you want your pink hydrangea to have blue blooms you must increase the acidity of the soil and vice versa.

It is the aluminium in acid soils that makes the hydrangeas blue. Garden centres supply several different products that make hydrangea blooms blue and they all contain aluminium sulphate. Water the plant well, dilute the product as per packet instructions, and then apply the product around the plant. You’ll need to do this regularly throughout the growing season. Mulching the plants with grass clippings or used coffee grounds will also help to reduce the pH level. Don’t use fertilizers containing bonemeal or phosphates. Also be aware that concrete leaches out lime into the soil which raises the pH level, so don’t plant your hydrangeas near concrete paths or foundations.

Lime in the soil stops the plants from absorbing the aluminium. So, adding powdered limestone and wood ash will help make the soil more alkaline and you’ll get pink blooms. Ask at your local garden centre for a fertilizer high in phosphorus as this will also prevent the plant from taking up aluminum.

Having said all this, changing the pH of the soil is not easy and you must continue applying the product around the plants. If you don’t keep on treating the soil the plants will revert to their chosen colour.