You Can Keep Your Hydrangea Blue With An Acidifying Routine, But Is It Really Worth It?
Elizabeth is a Permaculture Garden Designer, Sustainability Consultant and Professional Writer, working as an advocate for positive change. She graduated from the University of St. Andrews with an MA in English and Philosophy and obtained a Diploma in Applied Permaculture Design from the Permaculture Association.
Reviewed By COLIN SKELLY
Colin is a Horticulturist and Horticultural Consultant with experience in a range of practical and managerial roles across heritage, commercial and public horticulture. He holds the Royal Horticultural Society’s Master of Horticulture award and has a particular interest in horticultural ecology and naturalistic planting for habitat and climate resilience.
IN THIS GUIDE
Some hydrangeas will be pink or blue depending on the pH of the soil in a garden and its mineral composition.
For blue flowers on your hydrangea the soil needs to have an acidic pH level of between 5.2 and 5.5.
It will also need soil that has a high level of aluminium in its chemical, mineral composition.
It is possible to keep a blue hydrangea blue, even when the soil will not naturally allow it, but you’ll need to keep up an acidifying routine over time.
And it will not be the most eco-friendly or sustainable choice.
Why Common Solutions Are Not A Good Idea
1) Blueing Agents / Aluminium Sulphate
Do not be tempted to buy ‘blueing agents’ like aluminium sulphate!
Effects are fast – and these will certainly keep blue hydrangeas blue over time.
However, a lot of this substance will reduce pH more than you wish, and it can also potentially affect phosphorus levels in soil.
Since phosphorus is one of three key nutrients required for plant growth, this can have detrimental effects on the garden.1Cho, R. (2013, April 1). Phosphorus: Essential to Life—Are We Running Out? State of the Planet. Retrieved March 20, 2023, from https://news.climate.columbia.edu/2013/04/01/phosphorus-essential-to-life-are-we-running-out/
Applying aluminium sulphate can also lead to excessive and potentially toxic levels of aluminium in soil.2Panhwar, Q. A., Naher, U. A., Radziah, O., Shamshuddin, J., & Razi, I. M. (2015). Eliminating Aluminum Toxicity in an Acid Sulfate Soil for Rice Cultivation Using Plant Growth Promoting Bacteria. Molecules, 20(3), 3628–3646. https://doi.org/10.3390/molecules20033628
This not only makes it difficult to garden successfully but also damages the precious ecosystem of the soil which – as organic gardeners – we should be doing our very best to protect.
2) Synthetic Fertilisers
You should also avoid synthetic fertilisers – those which contain ammonium sulfate or ammonium nitrate for example.
These may be somewhat effective in making soil more acidic, but the costs are too great to countenance.
Avoid these harmful products at all costs to ensure that you are not contributing to our climate crisis or environmental destruction.3Buckler, L. (2018, April 1). The Hidden Dangers of Chemical Fertilizers. Occupational Health & Safety. Retrieved March 20, 2023, from https://ohsonline.com/articles/2017/12/07/the-hidden-dangers-of-chemical-fertilizers.aspx
The ammonia industry, from which synthetic nitrogen fertilisers are derived, is one of the most problematic and polluting industries, and one of the main contributors to global warming.4Ammonia emissions from agriculture continue to pose problems for Europe. (2019, June 28). European Environment Agency. Retrieved March 20, 2023, from https://www.eea.europa.eu/highlights/ammonia-emissions-from-agriculture-continue
3) Peat
Don’t use sphagnum peat moss / peat to add acidity either.
As an eco-friendly gardener, it is important to remember that peat bogs are an important carbon sink.5Peatlands store twice as much carbon as all the world’s forests. (2019, February 1). United Nations Environment Programme. Retrieved March 20, 2023, from https://www.unep.org/news-and-stories/story/peatlands-store-twice-much-carbon-all-worlds-forests
Contributing to their destruction through your gardening practices is never a sustainable choice.
Is Blue Hydrangea Really Right For You?
Trying to keep blue hydrangeas blue in an area where they are not naturally this shade is a lot of work.
And really, you need to ask yourself whether it is really worth the bother.
Pink Hydrangeas Can Look Good Too
You may prefer the blue to the pink blooms that would naturally occur on a hydrangea in the garden.
But in that case, you might be better simply to select a different plant.
There is nothing wrong with pink hydrangeas either.
These can be attractive in a garden too if this is the natural hue for these plants where you live.
So you are better to embrace the pink, or, if you really dislike the pink flowers, to remove the plant or plants and opt for a different planting scheme instead.
If you do decide to grow a blue hydrangea in a garden with more neutral or alkaline soil, think carefully before you decide to bother with this.
Honestly, it is far, far better to choose plants that are already ideally suited to the natural soil conditions where you live.
“If you’re interested in the science behind Hydrangea colours,” says Horticulturist Colin Skelly, “check out this article from American Scientist.”
“However, the principle of right plant, right place is [always] the best option – don’t fight the soil conditions, work with them.”
If you really are dead set on having a blue-flowered hydrangea, it is best to grow it in a container, so you can more easily manage the intense ongoing routine and make sure you can maintain the right acidic and aluminium-rich conditions which are required without causing as much damage in the surrounding ecosystem.
How To Keep Soil Acidic For Potted Hydrangeas
To make sure that you maintain the right, acidic conditions for a blue hydrangea grown in a pot or container, you will need to use an ericaceous compost, and replenish it regularly.
Adding sulphur, and acidic organic materials may also help to a degree, though these will only acidify soil very slowly, and will not always do so to the degree required to keep your hydrangea blooming blue.
If you do consider adding sulphur to the soil, or to a growing medium you are using, remember that its efficacy will vary depending on the composition of the soil or potting mix.
The acidity of the mix will also depend on the water you use to water them, and on the natural rainfall in your area, so this is another important factor to take into account.
Fertilisers which are low in phosphorous and high in potassium can also be helpful in producing good blue colouration.
A good fertiliser ratio to aim for is 25/5/30 NPK, though remember that you should always seek out organic fertilisers (or make your own).
You will need to monitor and maintain the pH of your soil – usually through the use of soil testing kits.
It might also be worth checking out other companion plants that also prefer acidic soil conditions.
Alternative Shrubs With Blue Flowers
If you are struggling to keep your hydrangea blue, then you may be better simply choosing an alternative shrub with blue flowers.
Blue flowers can be beautiful in a garden – and hydrangeas certainly aren’t the only option.
Some other shrubs with blue flowers to consider include:
1) Buddleja (e.g. ‘Empire Blue’)
2) Ceanothus (Californian lilac)
3) Caryopteris x clandonensis (Blue mist bush)
4) Hibiscus syriacus (Blue Rose of Sharon)
5) Lavenders
6) Syringa vulgaris (Lilacs, e.g. ‘Nadezhda’)
7) Rosemary
Of all of these, Ceanothus is perhaps the best alternative shrub for blue flowers for areas where there is alkaline or neutral soil.
It will thrive in many gardens as long as it is in a sunny spot, with moist but well-drained or well-drained soil, and will not require an intensive regime to maintain its blue blooms over time.
Its blooms are even more impressive than those on a hydrangea, and there are varieties that can be chosen for true blue blooms throughout much of the year.
There are also plenty of other blue flowering perennials to consider for the space vacated by a hydrangea whose blooms are somewhat variable in a garden.
Don’t Be Blue
If you choose the right plants for the location then you can enjoy blue blooms in the garden over a long period of time, without having to do a lot of maintenance work.
Remember, it is always better to choose plants suited to the garden, rather than trying to amend conditions to suit specific plants.
Choose the right plants for the right places and you can create a far more low-maintenance and eco-friendly garden.