Quick answer: How to grow blue hydrangeas in your garden comes down to choosing the right species and maintaining acidic soil with a pH of 5.5 or lower.
Blue hydrangeas bring unique charm and serenity to gardens that few other plants can match. From deep cobalt to soft powder-blue petals, these blooms add instant elegance to beds, borders and containers. While some hydrangea varieties naturally display blue colouring, others require carefully controlled growing conditions for those sought-after shades.
In the UK, where gardens range from chalky and alkaline to rich and acidic, understanding how to influence hydrangea colour is essential. This guide breaks down everything you need to know, from soil pH to feeding, planting, troubleshooting and maintaining colour throughout the season. Whether you’ve purchased a blue hydrangea or you’re hoping to turn a pink one blue, these methods give you the best chance of success.
A blue hydrangea isn’t a specific variety; it’s the result of a chemical reaction between the plant, the soil, and aluminium availability. Blue blooms typically come from Hydrangea macrophylla and Hydrangea serrata, both of which belong to the mophead or lacecap hydrangea group. These are the only hydrangea species capable of changing colour in response to soil conditions.
Their blue colouring is caused by the presence of aluminium ions. When the soil is sufficiently acidic (pH 5.5 or below), aluminium becomes available to the plant. As the hydrangea absorbs it, the flower pigment shifts toward blue tones. In alkaline or neutral soils, the aluminium is less available, resulting in pink or purple blooms.
Not all hydrangeas can turn blue, and this is one of the most common misconceptions among new gardeners. Only two species allow colour manipulation:
Hydrangea macrophylla (bigleaf hydrangea)
Hydrangea serrata (mountain hydrangea)
These species have pigments that respond to soil conditions. They range from deep purples and pinks to vivid blues, depending on pH.
On the other hand, the following varieties cannot change colour:
Hydrangea paniculata, usually white, cream or soft pink
Hydrangea arborescens, known for its consistent white blooms
Regardless of soil type, paniculata and arborescens hydrangeas retain their colours. If blue flowers are your goal, select a macrophylla or serrata variety. For best results, choose a hydrangea that already leans toward blue, often indicated by its nickname such as ‘Blue Ball’ or ‘Little Blue’.
Soil pH is the determining factor in whether hydrangeas turn blue, pink or purple. It measures the acidity or alkalinity of your soil on a scale from 0 (most acidic) to 14 (most alkaline). Every UK garden varies, and many regions, particularly southern England, have naturally alkaline, chalky soils.
Here’s how pH affects hydrangea colour:
pH 5.5 or lower → Blue blooms (aluminium available)
pH 5.5–6.5 → Purple or lavender blooms
pH 6.5 or higher → Pink blooms (aluminium unavailable)
Testing your soil is essential before planting or making amendments. You can buy an inexpensive pH testing kit or digital meter from any UK garden centre. Test a few areas of your garden, especially if you plan to plant your hydrangeas in the ground, as pH can vary even within one border.
Below is a helpful table summarising how soil pH influences colour:
|
Soil pH level |
Aluminium availability |
Likely hydrangea colour |
Recommended action |
|
4.5–5.0 |
High |
Deep blue |
Maintain acidity and feed regularly |
|
5.0–5.5 |
Moderate |
Light to mid blue |
Add acidifying materials as needed |
|
5.5–6.0 |
Low |
Purple |
Increase aluminium and lower pH |
|
6.0–7.0 |
Very low |
Pink |
Apply aluminium sulphate or grow in containers |
Understanding basic chemistry is key to successfully growing blue hydrangeas.
To grow blue hydrangeas, or turn existing blooms from pink to blue, you must create acidic soil conditions. The process takes time, and consistency is key, but there are plenty of reliable UK-friendly methods.
Aluminium sulphate provides aluminium and lowers pH simultaneously. When applied correctly, it is one of the fastest methods to encourage blue blooms.
Apply monthly during the growing season (spring to early autumn)
Always follow UK product guidelines; concentrations vary
Mix the solution well into the soil around the plant, avoiding direct contact with the stems
Elemental sulphur is a slower, more gradual option ideal for long-term pH management.
Takes several months to show the full effect
Best applied in autumn or early spring
Helps maintain stable acidity over time
These provide gentle and natural acidity, making them suitable for eco-conscious gardeners.
Examples include:
Pine needles
Shredded pine bark
Coffee grounds
Well-rotted leaf litter
Organic matter improves soil structure while gradually lowering pH.
Many areas of the UK have varying water qualities and characteristics. For example, alkaline tap water can work against you if you’re trying to turn hydrangeas blue.
Collect rainwater in a water butt
Use it to water hydrangeas consistently
Reduce reliance on hard tap water, which can raise pH over time
Garden lime (calcium carbonate) might sound acidic, but it actually raises pH and should never be used on hydrangeas intended to turn blue.
If your garden soil is naturally alkaline, growing hydrangeas in containers may be your best option. You have full control over pH, allowing you to maintain blue colouring without constantly battling chalky earth.
Planting hydrangeas correctly sets the foundation for vibrant blue blooms.
Partial shade is ideal; hydrangeas prefer morning sun and afternoon shade.
Avoid areas with harsh midday sun, which can scorch blooms.
Ensure the soil drains well; hydrangeas dislike waterlogging.
Before planting:
Work in ericaceous compost to increase acidity
Add organic materials if planting in borders
Test the soil again after amendments
Dig a hole twice the width of the rootball
Place the plant at the same depth it was in its pot
Backfill with garden soil and ericaceous compost
Water thoroughly with rainwater
Mulch with bark, pine needles or compost to retain moisture and maintain acidity
If using pots:
Choose a container with excellent drainage
Use 100% ericaceous compost
Feed using a hydrangea-specific fertiliser
Place in a sheltered, lightly shaded area
Keeping hydrangeas blue is easy, but maintenance still matters.
Monitor soil pH every 4–6 weeks
Apply aluminium sulphate or sulphur as needed
Water with rainwater consistently
Use low-phosphorus fertiliser, as high phosphorus binds aluminium
Look for fertilisers marked as suitable for acid-loving plants or for hydrangeas specifically. Avoid high-phosphorus multipurpose feeds.
Reapply mulch annually (pine bark is perfect)
Avoid planting hydrangeas too close to concrete (lime concentration)
Retest soil if flowers begin to turn purple or pink
When selecting your hydrangeas, purchasing from a reputable supplier makes all the difference. One Click Plants offers a range of blue and blue-ready hydrangeas, along with detailed advice to help you grow.
Growing blue hydrangeas is a rewarding blend of gardening know-how and natural chemistry. With the right species, careful soil management and consistent aftercare, you can enjoy abundant blue blooms year after year. Whether you’re creating a cool-toned border or adding a statement shrub to a patio, blue hydrangeas offer timeless beauty and elegance.
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Early autumn or spring is ideal in the UK. The soil is warm enough to encourage root establishment but cool enough to avoid heat stress.
Hydrangeas prefer consistently moist soil. Water 2–3 times per week in dry periods, using rainwater where possible.
A pH of 5.0–5.5 is optimal. Below 5.0 will deepen the blue, while anything above 5.5 may result in purple tones.
Take softwood cuttings in late spring or hardwood cuttings in winter. Root them in a mix of ericaceous compost and perlite for the best results.
Most blue hydrangeas bloom from June to September in the UK, with some varieties producing flowers into early autumn depending on weather conditions.
