This is a question that could be answered at any time during the growing season and is probably the question I am asked most frequently. When working at Bloom this year I wish I had had a T shirt printed with the answer (along with how to prune hydrangeas on the back) – I was almost hoarse at answering so many times! What was especially ironic was that, after five days of the show, on the last day, getting back to town at 10pm and starving, I popped into the local chicken and chips shop and, seeing I was wearing my ‘garden uniform’ the lady behind the counter asked where I had been and asked if she could ask me a question about her mother’s hydrangea – yep you guessed it! And working on Sundays at the local garden centre, Springmount, where hydrangeas are sold, the question gets asked with great regularity. So here goes…
Hydrangea flower colour is influenced by soil pH (acidity and alkalinity). The soil pH affects the uptake of aluminium salts from the soil. Aluminium is needed for the plants to make blue pigments. In acid soils (pH below 7) aluminium salts are readily available and absorbed so hydrangea flowers will usually be blue. In alkaline soils (pH above 7) aluminium cannot be absorbed so flowers are pink. This applies primarily to the mophead and lacecap forms of Hydrangea macrophylla, the hortensias (or, if you are American, bigleaf hydrangeas). These hydrangeas grow best in neutral or acid conditions. In very alkaline (limey or chalky) soils, the flowers will be pink but the leaves can suffer from lime-induced chlorosis and become yellow with green veins.
These hydrangeas are very popular container plants and then they should be potted in John Innes No 3 compost (if you want pink flowers) or lime-free John Innes (if you want blue) or ericaceous compost (for blue).
If your garden is perfect for camellias and azaleas your hydrangeas will be blue.
There is the old story that burying iron under the plant will turn them blue but that is because iron oxide (rust) is acidic so increases the aluminium absorption by the plant.
A more direct method is to add aluminium sulfate to the soil which can be bought as ‘hydrangea colourant’. This should be added to the plant as the crystals in spring and then watered on every month after that – how much you add affects whether the flowers go from pink to mauve or to blue. You need to apply this every year. You can have fun with this and make your hydrangeas a different colour every year if you desire. But don’t add to much lime (to make them pink) or you will do the plants harm. If you are a tea drinker you could empty the tea pot around your hydrangeas too – tea is rich in aluminium and generally acid.
Intensity of colour is dependent on the variety: a pale pink hydrangea will become pale blue but never deep blue.
It is vital to add this BEFORE the flowers start to develop – once they are open it is too late to make much change. Of course, the true flowers are those tiny things in the centre of the large sepals.
White hydrangeas should be white every year but the soil pH will affect the tiny true flowers. But to keep them pure white they are best grown in some shade – in full sun they will turn pink through scorching.
And all this applies only to these types of hydrangeas – Hydrangea arborescens is usually greenish as it opens and ages to cream (and then brown) and many forms of H. paniculata open white and age to pink or red.