Hydrangeas

Hydrangeas were my grandmother’s favourite plant and the stars of her summer garden. I vividly remember as a child picking the big blue flower heads to use to decorate the house for Christmas and learning about plunging the cut stems into a bucket of water to keep them from collapsing. Before they went in the vase we bashed the base of the stems to break them to let them take up water so the flowers wouldn’t wilt in the vase.

As my grandmother knew, these easy-going, exuberant plants provide weeks of colour in summer gardens and are long-lasting for use indoors too.

Hydrangeas have had a reputation for being old fashioned, but their popularity is experiencing a resurgence as a new generation of gardeners fall in love with these romantic blooms.

Hydrangeas are true chameleons, with many changing their colours based on the soil's acid content.

Flower colour and shape

The main hydrangeas in Sydney gardens are forms of Hydrangea macrophylla. They usually have blue to purple-blue flowers as these colours predominate when hydrangeas grow in acidic soils (soil pH of 5-5.5). In areas with more alkaline soils (soil pH of 6 or above), hydrangeas produce pink flowers.

This colour variability is due the plant’s reaction to concentrations of aluminium ions in the soil. Hydrangeas can be made bluer by adding an aluminium sulfate, such as Yates Hydrangea Blueing Liquid Aluminium Sulfate, to the soil or potting mix to lower pH and increase the amount of available aluminium. Flowers will be pinker where a dolomite lime has been added, such as Yates Hydrangea Pinking Liquid Lime & Dolomite. White hydrangeas don’t change colour in relation to soil pH or aluminium levels.

A red lacecap hydrangea.

You can test your soil with a pH testing kit available in-store, or simply wait to see the flowers that your plants produce. If the colour is not what you had hoped for, increase the pH for pink flowers by adding lime, or decrease for blue with aluminium sulphate. Just remember that changes to the pH need to be made slowly, and should begin before July for your December blooms.

There are two main flower types: ‘mop tops’, which have flower heads that are a mass of small petals (technically sterile bracts); and ‘lace caps’, which have a centre of sterile flowers surrounded by ‘petals’ (again sterile bracts).

Hydrangea care

Hydrangeas are hardy, easy-to-grow shrubs that happily tolerate shade and moist conditions. In fact, they tend to suffer sunburn in full sun and dry out in the wind. The extreme ultra-violet levels that cause us to have high rates of skin cancers are damaging to hydrangeas, too. UV exposure can burn flowers and leaves. To provide protection, give them an easterly to southerly aspect with shade for most of the day - morning sun is fine, but they need shelter from harsh afternoon rays. Alternatively, plant them in an area that’s shaded by trees for most of the day (but don’t plant them directly under trees where they’ll be dry due to root competition).

Hydrangeas can be grown in the ground or in large containers with good quality potting mix - we recommend Supersoil Professional. Make sure the soil has good aeration and is free draining with high organic content. Where soils are hard to wet, apply a wetting agent to improve water penetration into the root zone.

These plants have low fertiliser needs but will always benefit from an application of slow-release fertiliser with micronutrients in spring. Use a low-phosphorus fertiliser for blue hydrangeas.

When in full bloom give hydrangeas deep, regular watering - they love to be kept nice and moist. Plants wilt when the weather is very hot or if the soil or potting mix has dried out.

Powdery mildew on hydrangea leaves.

Hydrangeas and powdery mildew

Hydrangeas are prone to powdery mildew, which is a fungal disease.

Powdery mildew starts off as silver-coloured spots on the top of the leaf before developing into black spots, with leaves eventually turning yellow.

The image to the left shows the different stages of powdery mildew on hydrangea leaves.

To stop the disease from spreading, pull off and throw away affected leaves, then apply Eco-fungicide - an organic fungicide for the control of powdery mildew - together with Eco-oil.

Pruning

Don’t be afraid to pick your hydrangea flowers to display indoors - they look spectacular and the plants love trimming. Hydrangeas also grow best with the correct pruning regime. Hydrangea macrophylla has flowers on old wood in early summer. To keep plants compact and neat prune them in stages. Stems that haven’t flowered and those holding burnt blooms can be pruned in mid summer. Flowered stems are deadheaded in autumn. The hard prune is done in late winter by cutting flowered stems back to the first pair of live buds. Old stems can also be removed at this time, cutting them back to ground level. These older stems are five to six years old and may have peeling bark. Removing them helps to rejuvenate the plant. Leave stems that haven’t flowered unpruned at this stage.

Some new varieties of hydrangeas are repeat flowering - for example, Endless Summer forms. They bloom from summer to autumn with flushes of flowers and may need deadheading after each flower flush.

If you're cutting hydrangea flowers to display inside, there's a trick you can use to keep them looking fresher for longer. Once you've trimmed the stems up, dip the very ends into boiling water for no longer than 30 seconds, before putting them straight into cold water in a vase. The hot water helps to clear away sap from the end of the stem, which can stop water reaching the blooms and cause faster wilting. The cold water stops the process preventing heat damage further up the stem.

When to buy

Hydrangeas are sold in flower in spring and summer. A pot of hydrangeas in flower is an ideal Christmas or hostess gift! Click here to view Flower Power's range of hydrangeas.