Hydrangea
Family Saxifragaceae
Approximately 23 species, and many named cultivars.
Used as a screen plant, or in garden beds, as a tub plant, or for cut flowers.
They are excellent for south and east facing walls (opposite in the northern hemisphere), and good in acid soils and can tolerate clay soils in partial shaded locations.
Most hydrangeas grow to around 3 metres tall, with large showy flower heads, generally in summer and autumn. Some have attractive autumn foliage. Generally deciduous, though some can retain leaves under mild conditions.
A protected position in warm climates is preferred. Provide protection from hot, dry winds. Hydrangeas grow in most temperate areas and into the sub-tropics. They prefer a position protected from hot sun in warmer climates. They will generally grow well in both the sun, if plenty of moisture is available and there are no hot, dry winds; or in the shade. A fertile, moist, well mulched soil, in partial shade, and protection from drying winds gives best results.
They respond to organic fertilisers or compost applied at the start of the growing season. Flower colour will change according to soil pH (acidity). Blue flowering heads are retained by keeping the soil acidic (eg. mulching heavily or applying regular doses of aluminium sulphate). Red colour is retained by liming the soil, or adding superphosphate.
Regular feeding with a liquid fertiliser tends to encourage much larger flower heads.
Irrigate hydrangeas frequently in dry weather: although they can wilt easily, they will recover if the soil is kept moist.
They should be pruned after flowering, with the branches that have flowered being cut back to 2 or 3 double buds near the base of the shoot. Remove weak shoots at the base or any disease affected parts. Vigorous shoots that haven't flowered may be left intact. In most climates major pruning is in winter and can be severe removing all but a few short, thick stems.
Hydrangeas grow well with most deciduous trees.
Propagate by cuttings at anytime, but generally hardwood cuttings taken in winter.
While a large variety of pests and diseases can infect hydrangeas, they are generally easy to grow and rarely require a serious pest control effort. In warm climates allow space for good ventilation between hydrangeas and other shrubby plants (to minimise mildew). They grows well with azaleas, Gardenia, Iris, Lonicera and plants which like a moist acid soil; and have been observed growing well and pest free interplanted with Artemisias (eg. wormwood), and a ground cover of mint (ie. peppermint and apple mint).
Cultivars
Named cultivars are more commonly grown than species.
H. arborescens grandiflora -to 1m tall, hardy, large white flowers.
H. macrophylla serrata ‘Greyswood’ – generally only grown to 1.5m with stunning flowers frequently tinged in pink or soft blue.
H. paniculata -at 6 to 8m tall is generally too large for small gardens, despite being hardy. It grows well in any open position.
H. paniculata ‘Grandiflora’ – Very large white blooms.
H. petiolaris -is a climbing plant that can grow very large if it has something to climb on; though it’s size is normally controlled by pruning. It can be trained as a standard on a stake.
H. quercifolia (Oak Leaf Hydrangea) –to 2m tall, white flowers, attractive oak shaped leaves can have an colourful autumn foliage.
H. quercifolia ‘Snowflake’ – large white bracts.