Acacia dealbata

Acacia dealbata

Acacia dealbata seems to thrive at Rosemoor, as do many southern hemisphere plants, and there are a number of specimens to be seen in the garden. A particularly fine, well-grown tree grows against a south-west facing wall in Lady Anne’s Old Kitchen Garden with bright , sunshine-yellow flowers and fine, silvery-green foliage. However, by far the most distinctive and admired specimen grows in the Foliage Garden. This is a very unusual form of Acacia dealbata var. subalpina with exceptionally fine, silver-grey, delicate foliage and masses of pale lemon, sweetly scented flowers.

Vital statistics

Common name

Mimosa, silver wattle

Family

Mimosaceae

Height & spread

Can reach 30m (100ft) high and 10m (33ft) wide

Form

Evergreen tree

Soil

Fertile, neutral to acid soil

Aspect

Sheltered position in full sun

Hardiness

Succeeds outside in the south and west of Britain, otherwise requires unheated glass

Acacia

Acacia is a very large genus of mainly trees and shrubs found in many tropical and warm temperate regions, but especially Australia and Africa. Acacias occur naturally in an extraordinary range of habitats from sea level on coastal plains to high sub-alpine zones and from arid climates to high rainfall regions at the rainforest margin. The apparent leaves of many acacias are not actually the true leaves. In many species the normally doubly pinnate leaf becomes much reduced and 'disappears' after the seedling stage to be replaced in form and function by phyllodes - flattened stems which appear like and serve the same function as leaves.

The many yellow stamens are the most conspicuous feature of the flowers, varying in shade from bright to very pale. The flowers are borne in clusters or in cylindrical spikes, which look like miniature bottle brushes. Many species are highly armoured with sharp thorns to protect the foliage from browsing animals. In Africa, the acacias are commonly referred to as thorn trees. The Latin name Acacia also refers to the thorns being derived from Greek word akis, meaning a sharp point. The common name wattle came from the early Australian settlers who would build houses using the 'wattle and daub' method. They would use the abundant, pliable Acacia branches to weave the wall, which was then covered with mud.

Acacia dealbata

The silver wattle is an evergreen tree native to New South Wales, Victoria and Tasmania and was introduced from Tasmania in 1820.

The shoots are noticeably angular in section and bear true, bipinnate, blue green leaves. Harvested as a commercial crop the foliage is often included in cut flower arrangements as material is available all year round and it looks attractively fern-like. The highly fragrant flowers, arranged in large terminal racemes, are borne in spring. They are typical of the genus, being spherical clusters of conspicuous yellow stamens approximately 6mm (0.25in) across.

Acacia dealbata is tolerant of several degrees of frost and it has been known to survive at -10ºC for short periods. It can be successfully cultivated outside in favoured regions of the UK given a suitably sheltered position. In cooler climates Acacia dealbata makes a wonderful specimen in a cool glasshouse or conservatory. Although a tree under natural conditions it is amenable to pot cultivation under glass.

Cultivation

Acacias are basically tough plants but their susceptibility to frost means they can only be grown in areas with relatively mild winters. Frost hardiness seems to increase greatly with age, with young plants much more sensitive to cold than established ones which have attained tree size. Selected clones have shown resistance to short periods of frost at temperatures of -19ºC, though temperatures much below -10ºC will kill most specimens very quickly.

Grow outside in a fertile, neutral to acid soil in a sheltered position in full sun. A. dealbata is well adapted to summer drought but should be watered thoroughly until established. In areas at the limits of hardiness, plant near the base of a sheltered south facing wall

Under glass grow in direct sun in free-draining compost. Water moderately when in growth and sparingly in winter, ensuring that a liquid feed is applied at fortnightly intervals. It has been noted that specimens flower more freely if plunged outside in summer after the danger of frost has passed.

Regular pruning is not necessary except for pot-grown specimens and should be kept to a minimum on established plants. Established plants resent hard pruning and cannot be renovated successfully. Grown under glass, acacias can become infested with a wide range of glasshouse pests including tortrix moth, scale, red spider mites and mealybugs which should be treated accordingly.

If grown outside they are relatively pest free, though any damage caused by frosts should be pruned out to stop any infections.

Propagation

Acacias are easily raised from seed but selected forms must be increased by semi-ripe cuttings. Seeds should be sown in the spring at 20-25ºC (68-77ºF) and take between one and three weeks to germinate if the dormancy mechanism has been broken.

To break the dormancy mechanism, naturally triggered by fire, soak the seeds with scalding water and allow them to soak in the cooled liquid for up to 48 hours before sowing the viable swollen seeds.

Semi-ripe heeled cuttings should be taken in summer. Use a sandy propagation mix and provide a gentle bottom heat at 16-18ºC (60-65ºF) for the best results.

AGM

The RHS Woody Plant Committee awarded Acacia dealbata an Award of Garden Merit and described it as:

'Evergreen shrub to 10m or more, with finely divided grey-green leaves and fragrant yellow flower heads, borne in terminal clusters.'