How to Change Hydrangea Colours | From Pink to Blue

Home » How to Change Hydrangea Colours | From Pink to Blue
21.06.2024

I divided my potted hydrangea macrophylla back in February, just as it was starting to show signs of life. I’ve had it three, maybe four years now, and it was very root bound. I put off dividing it for probably a little bit too long because it’s a messy job to perform on a narrow balcony, and it was otherwise healthy. By February though, I knew it wouldn’t thrive through another growing season without more space for the roots.

Rather than give it a bigger pot, I decided to divide it and return half the plant to the original pot with some new compost to fill in the space where the other half’s roots had been, and pot up the other half in a second pot.

This was when I thought it would be fun to experiment with colour. Hydrangea macrophylla – big leaf, mop head varieties – are sensitive to the pH of their soil. If the soil they are growing is acidic then they will have blue flowers, and if it’s alkaline then they will have pink flowers. Somewhere between the two extremes, but more towards the acid side of neutral, you can get purple or lilac coloured flowers.

My hydrangea had been growing in neutral pH coco coir and bark. It has always bloomed with pink flowers, and I love them. There’s nothing wrong with pink flowers, I’m just curious. To acidify soil organically, you can add sulphur and a bit of aluminium. I had a bag sulphur chips to hand already to further increase the acidity of a raised bed containing blueberries and cranberries down at the allotment. Our soil in Birmingham is acidic, but only slightly. I also have a bag of aluminium sulphate powder in my cupboard because I use it to mordant fabric for use with natural dyes.

So, after dividing my plant, I added 2 tbsps aluminium sulphate powder and 2 tbsps sulphur chips to one pot, and left the other unamended. For reference, the pot measures 30cm (1 foot) in diameter and is about the same again in terms of depth.

In late May I started to see the first hints of colour on my hydrangeas. The unamended plant bloomed first, but the amended plant has just started to flower too, and to my delight the flowers are purple. Interestingly, some of the flowers are still pink, so I have pink and purple flowers on the same plant. Next season I might have purple and blue flowers on the same plant, as the sulphur chips are slower to dissolve than aluminium sulphate powder and will continue to further acidify the potting mix over the coming months.

It’s not a highly scientific method. I can’t tell you exactly how much soil amendment to use for hydrangeas that are growing in the ground, but I wanted to share what I had done and the results I have experienced in case it’s of interest or use. I love having two plants with different coloured flowers, and knowing that they came from the same mother plant. Later in the season I look forward to having flowers on my hydrangea arborescens (Annabelle) and my hydrangea paniculata ‘silver dollar’, both of which bloom on first year wood. I have also started to grow a hydrangea serrata (lacecap), the variety is called ‘blue bird’. I have given that some sulphur chips too, to help create the blue flowers it is named after, but don’t expect flowers until at least next year as it blooms on old wood like the macrophylla does.

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