A rainbow of colour hugging the walkway greeted me in the Hydrangea section of the Christchurch Botanic Gardens. On my recent trip to New Zealand, I made a point of wandering through the Gardens. Being Summer rather than Spring, some of the displays lacked flowers , but the Hydrangeas did not disappoint.

Have you ever wondered why Hydrangeas can change flower colour based on soil type, but other flowers can’t?

Its because while other flowers rely purely on genetics to determine flower colour, Hydrangea flowers contain a pigment that is capable of changing colour in the presence of aluminium ions. White Hydrangea don’t have this pigment so remain white.

So let’s get the dirt on this colour changing business.

Acidic soils (pH 6 or less) produce blue flowers because the plant is able to take up aluminium ions from the soil as they are soluble and mobile. Aluminium builds up in the flower turning it blue. Aluminium sulphate can be added to the soil to achieve this also. Coffee grounds mixed in the soil are another way to achieve blue flowers.

In alkaline soil (pH 7 and above) aluminium ions are not available for uptake as they combine with other elements making them unavailable to the plant. As a result, pink and red flowers bloom. Lime can be added to the soil to encourage pink and red flowers.

A neutral soil (between pH6 and pH 7) will produce purple or mauve flowers as only a small amount of aluminium is available.

Now that’s enough science.

The most diverse range of Hydrangeas are native to Asia, mainly China, Korea and Japan. As well as shrubs, there are small trees and climbers.

Hydrangeas like morning sun, dappled light, and definitely protection from afternoon sun. Regular watering is important too.

In my garden, I don’t try to manipulate my soil for specific colours at all. In the garden bed I get blue flowers and in my pots I get pink flowers. At the Christchurch Botanic Gardens, the staff must work diligently to manipulate the pH of the Hydrangea soil as there are different colours blooming alongside each other.

Hydrangeas are a spectacular plant, with their large, showy flowers through Spring and Summer. They are one of my favourites.