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.
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 aluminium sulfate (sold as Hydrangea Blueing) to the soil or potting mix to lower pH and increase the amount of available aluminium. Flowers will be pinker where dolomite lime has been added (sold as Hydrangea Pinking). White hydrangeas don’t change colour in relation to soil pH or aluminium levels.
You can test your soil with a pH kit available in-store, or simply wait to see the flowers that your plants produce. If the colour is not what you hoped for, increase the pH for pink flowers by adding lime, or decrease for blue with Hydrangea Blue (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).
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. Alternatively plant them in an area that’s shaded by trees for most of the day (but don’t plant them 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. 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 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. Plants wilt when the weather is very hot or if the soil or potting mix has dried out.
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 below image 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.
Don’t be afraid to pick the 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.
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!