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In its traditional sense, Acacia is a huge genus of over 1500 species, found throughout the warmer parts of the world. The greatest diversity is in Australia which has over 950 species, making it the largest genus of vascular plants in that country. However, Acacia is also abundant in Africa (> 130 spp.), Madagascar (c.100 spp.), the Americas (c.270 spp.) and the Asia–Pacific region (> 55 spp.). Acacia species are primarily trees and shrubs, but also, rarely, lianas or herbs. The branches are slender or thick and contorted, rarely winged, armed or unarmed; Acacia spines may be prickles (curved and distributed along the stems) or modified stipules (usually straight and restricted to the nodes). The leaves are alternate, bipinnate or reduced to phyllodes; rarely scale-like or completely absent. Phyllodes have the appearance of simple entire leaves, but are actually derived from the petiole; they may be foliose or reduced to spines. Foliar glands are present on the petiole (and sometimes on the rachis also) in most species. The inflorescences are axillary, simple or racemose, and rarely paniculate. The flowers are arranged into globular heads or cylindrical spikes and are typically yellow, cream or white, rarely red; they may be unisexual or herm aphrodite, and 4- or 5-merous. The petals are greatly reduced or absent and have been largely replaced in the role of attractant by the numerous stamens. The fruit is a legume (pod) and these are extremely variable across the genus; they may be dehiscent or indehiscent (Chapman et al. 2001a, 2001b, Lewis et al. 2005). Many Acacia species have symbiotic relationships with ants, which protect the plants from herbivory. The ants are rewarded with the provision of food (for example, extrafloral nectaries, protein packages on leaflet tips) and shelter (swollen thorns in which the ants can nest).
It is generally accepted that Acacia in its current format is an assemblage of disparate groups that should be split into several segregate genera. However, the various solutions suggested for its dismemberment have caused major disagreements. One proposal, submitted to the International Botanical Congress in Vienna (2005), is that the name Acacia be conserved for all species in subgenus Phyllodineae, which would include all but nine of the Australian species (Orchard & Maslin 2003) – but not the type species of the genus, namely the African A. nilotica (L.) Delile. This proposal greatly reduces the number of name changes in pros pect (as most species are Australian), but would mean that all African Acacia species would fall into either Vachellia (about half the species) or Senegalia – a move not popular with African botanists, who consider this to be an unwarranted reversal of the normal rules of botany wherein the type species has precedence (Moll 2005). American species would fall into Vachellia, Senegalia, Acaciella and one other as yet unnamed genus. The proposal was accepted by the Congress, but it remains to be seen whether the new names enter into common usage.
Acacias are iconic trees for many parts of the world, hence the passions aroused by the Australian name grab. The Australian national flower is the Golden Wattle Acacia pycnantha, while in Africa thorn trees epitomise the bush: brutal, but beautiful. Sadly the African species are mostly too tender for outdoor cultivation in our area, although they flourish along the Mediterranean coast and in southern California.
For horticultural purposes the Australian wattles fall into two groups: those with leaf-like phyllodes, of which A. melanoxylon R. Br. is probably the best known, and those with pinnate leaves, exemplified by A. baileyana F. v. Muell. and A. dealbata Link. The generality with which A. baileyana is now grown in England (with an Award of Garden Merit from the Royal Horticultural Society) is in extraordinary contrast with the notes in the last edition of Bean (1976a), where a specimen that ‘once grew’ in Cornwall is the only one cited outside Ireland. Rising temperatures and generally milder winters have undoubtedly played a part in this, but perhaps some thought should also be given to the possibility that seed is now imported from colder locations in Australia, and that home-grown seed from surviving plants favours the hardier individuals. One of the most beautiful small trees that has become freely available recently is A. baileyana ‘Purpurea’, a stunner in which deep-purple pigmentation combines with the bluish colour of the leaves to great effect. It fully deserves its Award of Garden Merit. It is probable that other Acacia species will become available from time to time as seed is imported and crops are raised by nurseries. There are also a few shrubby species, already in cultivation, that are not covered here. For such plants the Mimosaceae volume of Flora of Australia (Chapman et al. 2001a, 2001b) should be consulted, and there is extensive coverage in the New RHS Dictionary of Gardening (Huxley et al. 1992). The website of the WorldWideWattle project (www.worldwidewattle.com) is also very comprehensive and well illustrated. Several wattle species are weedy and invasive – causing massive problems in South Africa, for example – and this should be borne in mind, even if this scenario is improbable in most of our area.
In general Acacia prefers a lime-free soil, and good drainage and full sun are essential. Seed provides the best method of propagation, although semi-ripe cuttings can be taken and rooted with bottom heat. Seeds should be scarified or soaked before sowing in warm conditions. Transplant with care and plant in their final positions when quite young; pot-bound plants should be avoided.
A very large genus of trees and shrubs found in many tropical and warm temperate regions, but more especially in Australia and Africa. In Australia the acacias and eucalyptuses are the dominant genera of woody plants, and of the former there are now about 400 species recognised. The best known of them in this country is A. dealbata, the 'silver wattle', sprays of which are imported as 'mimosa' in winter from the south of France in great quantities to the London flower market.
The leaves of acacias are normally doubly pinnate, but in a large number of species this type of leaf disappears after the seedling stage and becomes reduced to a mere development of the leaf-stalk – what is known as a 'phyllode'. These phyllodes are of one piece, flat and leaf-like, often of considerable size, and perform the same functions as ordinary leaves.
The flowers of all cultivated acacias are of some shade of yellow, varying from bright to very pale, and they are borne either in ball-like clusters or in cylindrical ones like tiny bottle-brushes. The stamens constitute the most conspicuous feature of the blossom, which is very different in structure from the typical pea-like flower of the family.
The species selected for the descriptive notes that follow are the most important of those that are, or have been grown in the milder parts of the British Isles. They include the majority of those most valued by Australian gardeners and are a good representation of the genus as a whole. None is so hardy as to be quite exempt from the risk of severe damage or death in hard winters, but A. dealbata, the best-known and most widely grown of the acacias, comes through most winters in the south of England with wall protection and is common as an open-ground plant in S. Cornwall; many others might be tried on a sunny, sheltered wall, where, thanks to their rapid growth, they may give many seasons of flower before the next killing winter supervenes. It should be added that many of the species have a wide natural range and might yet yield hardier forms than those now in cultivation.
The acacias are easily raised from seed but selected forms must be increased by cuttings of half-ripened wood and struck with gentle bottom heat. They will grow in any good garden soil provided it is not excessively limy; A. longifolia and A. rhetinodes are known to be exceptionally lime-tolerant.