Roses Hydrangeas are red dilly dilly
lavender’s Hydrangea’s blue
When I am rich dilly dilly,
… I will have tee shirts printed for every adult in the world to explain why pink hydrangeas go blue so that no one ever has to ask me again!
I suppose I should be more tolerant. After all, there are things that I always need to double check because I can’t quite get them embedded into my head.
Its and it’s for example, which goes against the possessive rule and should make me more tolerant of the sins of greengrocers who sell satsuma’s and onion’s and the imbeciles at the traffic department who insist that certain roads are unsuitable for HGV’s. Grammar is there for a reason – clarity.
The reason we capitalise abbreviations is so we don’t need apostrophes for clarity – hgvs and cds, or hgv’s and cd’s is confusing but HGVs and CDs are clear as crystal, without any apostrophes sprinkled across the page.
But back to hydrangeas. Having once again been asked about the colour of hydrangeas I feel it necessary to mention it again, and for the last time here – ever. Like everything in gardening, there are exceptions to the rule and I will try to cover my own back but basically it is very simple.
The most common hydrangeas in gardens are Hydrangea macrophylla. These are divided into two types based on the shape of their flower heads. Each flower head has fertile flowers (usually in the centre) that are small and almost insignificant. There are also sterile ‘flowers’ that have four (or five) large bracts around the flowers. If there is a ring of large, sterile flowers around a centre of fertile flowers these are called ‘Lacecaps’. If all (or most) of the flowers are sterile these are called mop heads. They tend to be more showy but are otherwise the same.
This flowering pattern also applies to other types of hydrangea such as H. paniculata and H. arborescens. We tend to grow those with lots of sterile flowers. These all tend to have flowers that are green when immature, white when mature and age to pink. See at the end for their pruning.
Hydrangea macrophylla also has pink flowers and white flowers but they can also be blue. In general, white hydrangeas of this type are always white, though they can age to pink or have pink spots both as they age or if they are in strong sun. *
But most of the pinks can also be blue. It is possible that a pink hydrangea that you planted last year will have blue, or mauve flowers this year. And vice versa.
It is indirectly down to soil acidity and directly down to how much aluminium the plant absorbs. In alkaline soils (chalky – rich in calcium) certain trace elements such as aluminium, iron and magnesium get locked up in the soil chemistry and are not available for plants to absorb. In acid soils (where rhododendrons thrive) these elements are freely available. Aluminium is necessary for a hydrangea to make the blue pigment that colours the flowers. If the plant cannot absorb aluminium from the soil then it cannot be blue.
So how can you make the flowers pink?
Add lime, to prevent availability of aluminium.
How can you make the flowers blue?
You can acidify the soil – by adding sulphur chips, or lots of organic matter.
You can add colourant to the soil. This is aluminium sulphate. It is watered into the soil.
You can try the old method of burying iron in the soil which may help.
You can grow them in a pot and use lime-free compost and use an acid plant food.
Remember that once the flowers are pink (or blue) they will stay that way until they fade so you need to treat the plant from early summer. Do not add too much lime because it will cause leaf yellowing.
Some hydrangeas are sold as blue or pink because they do that colour particularly well. In other cases two seemingly different plants are the same plant with different growing media!
We live in a time when hydrangeas are being actively bred and big developments are happening. Many are very small and I suspect that some are better as pot plants than as garden plants. There is also breeding to produce tall plants for cut flowers. We get to buy these, often without a real explanation of what we are getting. For this reason I am still a little skeptical of these and have a hankering for some of the older kinds that I know perform well in gardens.
And now a word (or several hundred) on pruning.
I have mentioned breeding and recently the situation has changed slightly but I will start with the basic rules. Hydrangeas produce long stems that will, if the season is long enough, produce flowers at the tips. But this generally does not happen and our short summers result in these shoots not blooming. But the following year these stems will produce sideshoots and these produce flowers. If you regularly and relentlessly cut back hydrangeas every year you get lots of lovely leaves and no flowers. So you should not cut back these long shoots apart from removing the tips that may have been damaged in winter. You do this in March. Likewise, any stems that have dead flowers on should be lightly pruned, back to the uppermost pair of fat buds, in March. We do it then because the old flowers will have given some protection to the buds below from frost.
After a few years the shrub gets congested and the flowerheads will be smaller so we need to thin out the branches. In March, cut out about a quarter of the old stems, those that are the most twiggy, with spindly growth, out at the base, pulling out the branches so the shrub is thinned. Do this first then tidy up the remaining stems.
You cannot make a large hydrangea smaller by hacking it off at the base – the new growths will be very vigorous and soon be as big as when you started – but without flowers.
Now I mentioned that things are changing and there are now hydrangeas that flower on young main shoots as well as sideshoots. The first of these was ‘Endless Summer’ but there are now more. They flower almost continuously through summer rather than with one big flush.
Without wishing to confuse you, if you live in a mild, coastal area with very late autumn frosts, or none at all, you will get away with hard pruning the ordinary kinds too. And those strong shoots will make massive flower heads.
When big houses had gardeners and conservatories, hydrangeas were grown for display indoors and cuttings were rooted and grown as single stems, protected from frost and produced massive heads, one per plant, as potplants for the big house. But I digress, unless you have a team of gardeners you want to challenge!
And then there is ‘Runaway Bride’ Chelsea Flower Show Plant of the Year last year – a white lacecap. This has clusters of flowers along the (spreading) branches and not just at the tips. It seems to resemble a Viburnum plicatum in overall appearance. But this still needs regular pruning. When I have saved up I will get one and see how it does.
Just for completeness I should emphasise that all this is about Hydrangea macrophylla types, what the Americans call ‘big leaf’ hydrangeas (a literal translation of macrophylla). There are lots of other hydrangeas and these need different treatment. The most common in our gardens are H. paniculata and H. arborescens. These flower at the ends of the current season’s growth and that means they are hard pruned in March and will have flowers at the ends of the new shoots in late summer. Think of them as being the same and a buddleia (B. davidii of course) and the harder you prune the bigger the flower heads. Bigger flower heads means more weight of course but that is a whole other story.
I mentioned that white hydrangeas won’t go pink or blue but in fact the tiny fertile flowers and the actual flowers in the centre of those large bracts WILL change to pale pink or pale blue according to availability of magnesium – just that they are so small that we don’t usually bother to mention them.