Blue hydrangeas: how to turn hydrangeas from pink to blue

Pink hydrangeas are a common sight, but there is something rather special and rare about blue hydrangeas. Read on to learn how to change a hydrangea’s colour and how to make sure it does not revert back.

The soil’s pH influences not only what plants you can grow, but also the colour of some plants and flowers. For example, the widely grown mophead hydrangea (Hydrangea macrophylla) has the unusual ability to bloom in a different colour depending on what type of soil it is grown in. These mophead flowers, which are usually pink, can actually turn blue if grown in the right soil conditions.

What causes pink hydrangeas to turn blue?

The colour of hydrangea flowers is dependent on how much aluminium is available in the soil for their roots to absorb. To get a pink hydrangea to turn or stay blue, you will need to grow it in an acidic soil with a pH of 4.5 to 5.0, which allows the plant to absorb more aluminium. Garden soils in the UK typically have a pH range of 4.0 to 8.5, with 7.0 considered neutral. Unless your garden soil is naturally acidic, it can be easier to achieve the blue colouring by growing a hydrangea in a container. That way, you can use an acidic soil or easily add a soil acidifier to the container. For growing hydrangeas in pots, we recommend using our Plantura Organic Ericaceous Compost, which has been specifically developed for acid-loving plants and has a low pH of 4.6 to 5.4.

How to maintain a low pH of the soil and keep your hydrangeas blue

When planting blue hydrangeas, it can be very beneficial to amend your garden soil with an ericaceous compost. This compost is often used for other acid-loving plants like azaleas and rhododendrons and typically have a pH of 4.0 to 5.0. Even if you are not fussed about having blue hydrangeas, a slightly acidic or neutral soil is recommended because the shrubs can struggle to absorb iron if the pH is too high and alkaline, resulting in chlorosis of the young leaves.

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For a pink hydrangea to turn or stay blue, the soil must be acidic with a pH of 4.5 to 5.0. Aluminium can be artificially added to the soil as an acidifier to achieve a faster and more intense colour change. This is available to home gardeners as aluminium sulphate or hydrangea colourants. Apply per the manufacturer’s instructions; the aluminium sulphate is usually diluted in water and applied regularly when the shrub comes into bud. The resulting colour change can be intense and rapid. However, if misapplied, the pH can be lowered beyond the desired level and can cause aluminium to build up to almost toxic amounts.

No matter what colour you may want your hydrangea to bloom, hydrangeas benefit from being fed the correct nutrients. A specialised hydrangea fertiliser, such as our Plantura Hydrangea Food is ideal, as it contains nitrogen and potassium to support growth and flower production as well as magnesium and iron to help prevent any foliage yellowing.

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Peat moss has traditionally been used for acidifying soil, including areas where hydrangeas are grown. However, due to environmental concerns, peat composts will soon be removed from the market for home gardeners.

How to check the acidity of the soil

Knowing the pH levels of your soil is the starting point for growing blue hydrangeas and pH testing kits are readily available. To test the pH of your soil:

• Put about 100 g of your soil in a jar.
• Add 100 ml (distilled) water and mix thoroughly.
• After 10 minutes, dip a pH test strip into the soil solution and read off the result from a colour scale.

The results obtained this way are not completely precise, but they will provide a good indication of the soil’s pH. For a more accurate measurement and if testing the soil regularly, a soil pH metering device can be a good investment.

Tip: to avoid altering the soil for other nearby plants, blue hydrangeas can be grown in a large pot or container. Alternatively, it may be easier to only grow plants that prefer ericaceous soil in a bed or border all of their own. Other acid-loving plants that can grow well alongside hydrangeas include azaleas (Rhododendron) and Japanese andromeda (Pieris japonica).

Can you change the colour of white hydrangeas?

The colour of white hydrangeas such as the popular Hydrangea arborescens ‘Annabelle’ or Hydrangea paniculata cultivars cannot be changed. Only the pink or red Hydrangea macrophylla cultivars can turn blue or dark purple respectively.