Question of the week: How do you make hydrangeas blue?

Hydrangea ‘Ningbo Blue’

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.

Hydrangea ‘Curly Sparkle Red’

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.

Hydrangea ‘Benxi’

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.

Hydrangea ‘Invincibelle (R) Spirit’ – the first pink form of H. arborescens – it won’t go blue in acid soil

 

, , ,

Trackbacks/Pingbacks

  1. alifesgayventure - September 1, 2017

    […] i have learnt something new about hydrangea from this post today. it seems it is blue or pink depanding on the acidity of the soil. and that is because more […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Sweetgum and Pines

gardening in the North Carolina piedmont

Ravenscourt Gardens

Learning life's lessons in the garden!

RMW: the blog

Roslyn's photography, art, cats, exploring, writing, life

IGPS Blog

The Irish Garden Plant Society - Lovers of Irish plants and gardens

AltroVerde

un altro blog sul giardinaggio...

vegetablurb

four decades of organic vegetable gardening and barely a clue

The Long Garden Path

A walk round the Estate!

The Tropical Flowering Zone

Photographic Journals from the Tropics

Flowery Prose

Growing words about writing, gardening, and outdoors pursuits in Alberta, Canada.

ontheedgegardening

Gardening on the edge of a cliff

Uprooted Magnolia

I am a freelance Photographer born and raised in the Southeast. I have uprooted my life in Macon Georgia for a new life as an unlikely cowgirl in love with a handsome cowboy in Wyoming. I hope you enjoy my photo journal on life, love, and the spirit of Wyoming.

Interesting Literature

A Library of Literary Interestingness

Garden Variety

A Gardening, Outdoor Lifestyle and Organic Food & Drink Blog

For the Love of Iris

Articles, Tips and Notes from Schreiner's Iris Gardens

One Bean Row

Words and pictures from an Irish garden by Jane Powers

Plant Heritage

The world's leading garden plant conservation charity

HERITAGE IRISES

An English experience of gardening in Ireland - and back in the UK

%d bloggers like this: