Why is my 'Midnight Blue' Hydrangea red?

Hydrangea macrophylla 'Midnight Blue' - with red bracts

One of my earliest gardening memories is going on holiday to the English Lake District.  I was about 13 or 14 (so this was many, many years ago).  We went to Lake Windermere and there, on the shore, were masses of sky blue mophead macrophylla hydrangeas.  They looked absolutely fabulous to my untutored eye.  I knew they were hydrangeas.  We had them in my parent's and many other local gardens, in shades of white, pink or red.  But never blue.  It was fascinating.  Dad explained that we couldn't get them to flower in blue shades in Grimsby because of the soil, heavy clay mixed with chalk.  I knew enough about chemistry by this stage of my education to realise that he gardened on alkaline soil while the soil by Windermere's shore was acid.  A check in the school and town libraries confirmed that Hydrangea macrophylla was blue on acid soil, red or pink on alkaline soil.  I was used to using Litmus paper in class experiments so, it seemed pretty obvious that the pigments in the bracts that make up the colourful part of the mophead were, just like litmus paper, directly sensitive to the pH of the soil.

I probably went through the next 15 years thinking exactly that.  And then I started gardening for myself - and growing Hydrangeas.  Of course, as soon as you start to grow a plant you see it in all the local gardens.  I was on acid soil, the whole area was on acid soil, and the local hydrangeas were in every shade from brick red, through shades of pink into pale and deeper blue - even within the space of a few metres.  That's when you start questioning your earlier assumptions - and realise that it's a little more complicated than simple acidity or alkalinity of the soil.

Fortunately Hydrangea macrophylla - in both it's mophead and lacecap varieties - has long been a commercial florist's crop.  The colour change has long been known - and it's mechanism.  Without going into the biochemistry of the process (those days are long in my past) it is a reaction between aluminium salts and the anthocyanin pigment in the colourful parts of the flower head.  High aluminium gives a blue colour, low aluminium the more natural red.  Very acid soils - below pH 5.5 - allow aluminium salts to be freely available to the plant and hydrangeas are likely to be reliably blue.  Less acid to alkaline soils progressively lock up the aluminium salts.  Depending on the acidity of the soil and amount of aluminium natively present in the local soil the hydrangeas will respond in every colour from blue through to red.  Hence the variation even within a small area.

Where your soil is acid but the hydrangeas aren't colouring blue you can boost the availability of aluminium by watering with a diluted aluminium sulphate solution to boost the soil content.  This is available in garden centres.  Don't be tempted to exceed the recommended dose - aluminium sulphate is toxic to plants in higher concentrations.

Which still doesn't explain why my 'Midnight Blue' is red.  Well, I bought it (cheaply) in a distressed plants sale earlier this year.  I resuscitated it in it's pot before planting it out a month or so ago.  It must have been growing in alkaline or low aluminium content compost.  So, this year, the flowers are red.  Over the next couple of years they should change and I should, eventually, get a good blue.

Hydrangea macrophylla is not unique in this colour change - but it's not common.  The smaller Hydrangea serrata has the same properties and so does a rather rarer relative, Dichroa febrifuga.

Dichroa febrifuga

I grew this for a number of years in a large container but, alas, it succumbed to two bad winters. 

Just as an aside, I've seen many assertions that if you choose white mophead or lacecap hydrangeas you won't see the same colour changes.  In fact you do - just on a far smaller scale.  This is the centre of my repeat blooming white mophead 'Madame Emile Mouillere'.

Central flower of Hydrangea 'Madame Emile Mouillere'

A lovely blue.  Which bodes well for 'Midnight Blue'  when it eventually assumes its true colouring.