Horticulture Magazine

How To Keep Your Hydrangea Blue

dark blue hydrangea blooms

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 have to be a soil which 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.

blue hydrangea flowers in summer
Blue hydrangeas

Why Common Solutions to Keep Hydrangea Blue are Not a Good Idea

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. 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 within the garden.

Applying aluminium sulphate can also lead to excessive and potentially toxic levels of aluminium in soil, which not only makes it difficult to garden successfully, but which also damages the precious ecosystem of the soil which, as organic gardeners, we should be doing our very best to protect.

You should also avoid synthetic fertilizers – 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. The ammonia industry, from which synthetic nitrogen fertilizers are derived, is one of the four most problematic and polluting industries, and one of the main contributors to global warming along with concrete, steel and ethylene (plastic).

Ferrous sulphate, which is commonly available at garden centres, can also interfere with phosphorus levels in your soil. So using this to acidify soil is also not a sustainable choice.

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. Contributing to their destruction through your gardening practices  is never a sustainable choice.

Is a Blue Hydrangea Really Right For You?

flowering pink hydrangea with green foliage
Pink hydrangeas can look good too

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. 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 decide 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 which are already ideally suited to the natural soil conditions where you live.

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 Hydrangeas in Containers

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 in the natural rainfall in your area, so this is another import factor to take into account.

Fertilizers which are low in phosphorous and high in potassium can also be helpful in producing good blue colouration. A good fertilizer ratio to aim for is 25/5/30 NPK, though remember that you should always seek out organic fertilisers (or make your own).

Alternative Shrubs With Blue Flowers to Consider

clusters of beautiful blue Ceanothus flowers
Ceanothus could be a better alternative for some gardens

If you are struggling to keep your hydrangea blue, then you should ask yourself whether you might be better simply choosing an alternative shrub with blue flowers. Blue flowers can be beautiful in a garden – but it is important to recognise that hydrangeas certainly aren’t the only option.

Some other shrubs with blue flowers to consider include:

  • Buddleia (e.g. ‘Blue chip’)
  • Ceanothus (Californian lilac)
  • Caryopteris x clandonensis (Blue mist bush)
  • Hibiscus syriacus (Blue Rose of Sharon)
  • Lavenders
  • Syringa vulgaris (Lilacs, e.g. ‘Nadezhda’)
  • 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 drain soil. And will not require an intensive regime to maintain its blue blooms over time. It’s blooms are even more impressive than those on a hydrangea, and there are varieties which 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. 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.

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