Why Has my Hydrangea Gone Black All Over?

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Hydrangeas are well known for producing an abundance of long lasting pink, blue and white blooms during their flowering season. There are lots of different types of hydrangea – I have a big leaved Mophead hydrangea (Hydrangea macrophylla) which is probably the most popular variety found here in the UK.

Autumn in the UK this year has been notably mild and has been followed by a prolonged period of cold – including severe overnight frosts with temperatures dropping below -10.

Although it was wintertime my hydrangea still had plenty of leaves, and during this cold weather I noticed that its leaves had turned a very dark black colour all over. So why was this? Was it normal? And should I be worried? I did some research to find out.

What Can Cause Hydrangea leaves to go Black all over?

There are many diseases that hydrangeas are susceptible to that can affect their leaves in different ways. But very cold weather and frost can potentially cause damage to any leaves or blooms present on the plant.

Exposure to very cold temperatures and frost, whether it be in late autumn, winter or in early spring, can cause hydrangea leaves to take on a dark, blackened appearance.

I suspect exposure to frost to be the root cause of the dark black leaf damage that I have seen on my hydrangea.

Black Hydrangea Leaves – December 2022

What should I do if I suspect my Hydrangea has been damaged by frost?

Frost damage does not necessarily mean the end of your hydrangea. Most plants will in fact recover in the coming year.

The best approach when you suspect that your plant has sustained this damage is to just “wait and see”. Once spring comes around and the frosts are over, check to see if there is any new growth. Hopefully you will find that your hydrangea has survived and you may only need to tidy the plant up and prune a few isolated dead or damaged sections… but only time will tell!

Do Hydrangeas need to be protected over winter?

As the old saying goes, “prevention is better than cure”.

The protective steps that you should take will depend upon the variety of hydrangea that you have. Here, we are looking at the “mophead” variety commonly found in the UK. If you are not sure which variety of hydrangea that you have, check out our related article on getting the most from your hydrangea.

The “big leaved” Mophead hydrangeas form new buds during the autumn on old wood, ready for growth in the following spring. After shedding their leaves, they spend winter in a dormant state – taking on a dead looking, stick like appearance.

Because next year’s buds are on the plant, ready and waiting, all over winter, it is wise to take steps to protect your Mophead hydrangea from sharp, cold winds and frost. There are a few different ways that you can do this.

– Shelter from frost and direct wind

This is most important if your hydrangea is either newly planted this year, or planted in a pot.

If your hydrangea is in an exposed position and is growing in a pot or container that can be easily moved, it is worth relocating it to a more sheltered area over winter.

If it can’t be moved, you can protect your hydrangea by either erecting a protective cage around it or covering it during particularly cold nights. Protective cages can be created by steaking around the plant and wrapping the steaks with chicken wire. The cage you have created can then be filled with leaves which will act as an insulator and protector.

– Protect the roots by mulching

Mulching helps to ensure that the hydrangea has enough nutrients and moisture throughout the winter and into spring. A mulch using leaves, bark or straw is ideal and can help keep the ground from freezing.

– Don’t prune the flower heads until spring

Winter is a bad time for pruning your mophead hydrangea – I always prune during early spring. There are a few reasons for this. Firstly, pruning just before or during winter can stimulate new growth at the wrong time, which will place the plant under unnecessary stress.

Secondly, leaving old flower heads throughout the winter will help offer some protection against the frost and cold for next year’s developing buds. If you don’t prune the flower heads they will usually stay attached to the plant throughout the winter and it is normal for them to turn brown in colour. They will stay like this all through the winter into the spring, when they can then be dead headed once the plant begins to display signs of new growth.

January 2023 – Many of the dark black leaves have dropped by now, giving a stick like appearance. Flower heads have been left in place over winter to protect the new buds below.

I have accepted that Hydrangeas just don’t look their best over winter! The presence of dead, brown coloured flower heads during this season are in fact beneficial to the overall health of the plant. They are most definitely not a problem!

Did my hydrangea survive its frost damage?

I am happy to say that my hydrangea survived. By the end of January 2023, the green shoots of recovery were clearly visible beneath last year’s flower heads.

Buds starting to grow (circled) in Jan 2023

And by early April 2023 there were plenty of new leaves growing:

Update: April 2023

It was now time to prune the hydrangea!

How do I prune my Hydrangea?

So, once spring has arrived, how do you go about pruning your hydrangea?

As we have already mentioned, the type of hydrangea you have will dictate exactly how it should be pruned.

As my hydrangea is a Mophead, it flowers on old wood and only needs a light prune for shaping and to remove any old flowers and any spindly wood. The best time to prune is in spring, as leaving old flower heads throughout the winter will help provide some protection against the frost and cold for next year’s developing buds.

Firstly I removed last year’s dead flower heads. When doing this, to encourage the development of new shoots, I pruned the stems back down to just above a newly formed healthy bud (circled):

Removing spent flowers – I pruned back to just above the first newly formed healthy bud

If you cut too low (ie below the lowest bud) it will stop the hydrangea from flowering in the coming year from that stem. The worst thing to do if you want flowers would be to cut your Hydrangea macrophylla down below the lowest bud on all stems as this would cause the shrub not flower at all until the next year!!

Why not check out the video below for more info on pruning your hydrangea:

If you are happy with the shape and density of the shrub once all of the old flowers have been removed then further pruning may not be needed at all. My hydrangea was quite bushy so, as well as removing any dead wood, I thinned it out by removing some healthy stems to make space for new growth in the coming year:

April 2023: Last year’s flowers all now removed
May 2023: this year’s flowers are starting to form
August 2023 – full recovery

Should I feed my Hydrangea?

Feeding your hydrangea can result in stronger growth and larger blooms. The best time of the year to apply feed is from early spring through to early / mid summer (when hydrangeas are in their most active growing phase). Typically hydrangeas will only need around 2 feeds each year.

Shop for hydrangea feed on Amazon here, available in both powder and liquid forms! (Always follow manufacturer’s usage instructions).

And finally…

If you are looking to add more hydrangeas to your garden, or need to replace any, why not check out the below on Amazon!

Did you know?

The colour of hydrangea flowers are dependent upon the PH value and acidity of the soil – a shrub in alkaline soil produces pink flowers whereas one in acidic soil produces blue. It is possible to change the colour of mophead hydrangea flowers by altering the PH value of the soil – check out the below colourant on Amazon that changes pink hydrangea flowers to blue! (Always follow manufacturer’s usage instructions).