Gardens

RHS Garden Hyde Hall

Plant of the Month: April

Acacia pravissima @ Hyde Hall

Our Dry Garden has proved very popular over the last few years and illustrates beautifully to our visitors the vast range of inspiring plants you can grow without needing to water them. Being on a sheltered southwest-facing slope we are also able to grow a great selection of plants that can be regarded as not fully hardy. One great example is Acacia pravissima, a fantastic small tree that can be used in a smaller garden as its maximum height is only 3-8m. It also has distinct, small, grey, triangular leaves and during spring it is covered in small spherical yellow flowerheads. These look just as attractive in bud as they do when they are in full flower when they cover the tree with glistening yellow blooms.

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 leaves become much reduced and 'disappear' after the seedling stage to be replaced in form and function by phyllodes - flattened leaf stalks, 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 have 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 pravissima

This half-hardy species is found growing in the hills of south-eastern Australia.

It is a small tree or spreading shrub with pendant branches and small, approximately triangular, grey-green phyllodes, 0.5-2cm (0.25-0.75in) long. The name pravissima means 'very crooked', referring to the phyllodes.

Flowers are produced in spring, in extended racemes. They have profuse, fragrant, bright golden yellow, spherical, fluffy flowerheads, 5mm (0.25in) across.

The prostrate form ‘Golden Carpet’ spreads to 4.5m (15ft).

Acacia pravissima is a good tree for a small garden, if adequate shelter can be provided.

AGM

The RHS Woody Plant Committee (formerly Floral B) awarded Acacia pravissima an Award of Garden Merit on account of its good foliage and flower.

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. 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 placed 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 heel 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.