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Acacias comprise a large genus of mostly tropical shrubs and trees of the pea family (Fabaceae). There are a few, however, that range quite high up into the mountains of southeastern Australia, and are probably hardy enough to do well in milder sheltered areas of USDA zone 8 and perhaps 7b. Their key features are fast growth and and a profusion of bright yellow flowers, in winter on most species. Also, many of them have very attractive feathery evergreen foliage that is often used in flower arrangements in Europe. The hardiest acacias can probably tolerate temperatures around 8 - 12 F or so without too much damage. I would recommend trying high-altitude provenances of A. alpina, A. dealbata subalpina, A. melanoxylon, and A. obliquinivera. Good provenances of A. filicifolia, A. frigescens, A. kybeanensis, A. mearnsii, A. pravissima, A. retinodes, A. riceana, A. rubida and many others may also prove fairly hardy.
Banksias are members of the Proteaceae family, related to Embothrium, Grevillea, Hakea, Leucadendron, Protea and Telopea. They produce incredible bright cone-shaped flowerheads of various colors, depending on the species. In addition, many also have attractive shiny leaves. Most are shrubs, but a few species can reach tree size under good conditions. B. integrifolia, B. marginata, B. saxicola and B. serrata are among the hardiest and most vigorous, showing promise for cultivation in cooler climates. Other good ones to try include B. aemula, B. dryandroides, B. ericifolia, B. menziesii, B. oblongifolia, B. paludosa, B. robur and B. spinulosa. It must be admitted that they are a bit of a challenge to grow: most don't like being in pots, and they are extremely sensitive to phosphorous and calcium--don't let them have any besides what is naturally in the soil. Also, though some may eventually prove hardy in zone 8, they are quite marginal and will require shelter until further experimentation has been conducted. For a bit more information about Banksia growing, see this site.
Casuarinas (many have been reclassified as Allocasuarina) have not been grown anywhere in the U. S. outside of California and Arizona to my knowledge. They are among the most primitive of dicotelydonous angiosperm plants. Many grow into trees of considerable size, with wispy, primeval foliage. Some, especially Tasmanian species, may be hardy to 12 F or so. In California, casuarinas are referred to as "She Oaks" and known for their ability to do well in tough situations, such as poor drainage.
Callistemons are known as "Bottlebrushes" for their brushlike clusters of bright flowers that form all around the stems for a few inches along the stem. After flowering, the stem continues to grow more leaves. They are in the Myrtaceae family, along with Eucalyptus, Feijoa, Leptospermum, and Melaleuca. Some are quite hardy and have flowered in zone 8 in Washington and other areas. Among those that have done well are C. salignus, C. subulatus, C. viridiflorus and C. violaceous. C. pallidus, C. pityoides, C. rigidus, C. teretifolius, and perhaps C. viminalis are also good ones to experiment with, as well as cultivars such as 'Woodlanders hardy'. Most are shrubby, but a few can reach small tree proportions if trained. See this site for more information about these plants.
Grevilleas grow throughout Australia and range in size from small shrubs to large trees. They are also in the Proteaceae family, and produce very interesting showy flowers. Foliage varies to attractive and lacy on G. robusta, which can grow into a huge tree in California; to gray and velvety on the hardy but phosphorous sensitive G. victorae, which produces bright red flowers in the winter; to needle-like and prickly on G. 'Canberra', another hardy kind that has done well in zone 8 in the South as well as on the West Coast. G. juniperina sulphurea, a yellow-flowered species, has also proven hardy; as well as G. 'Pallida Constance', and there are certainly others that remain unknown like G. alpina. Most are easily grown from seed or cuttings.
Leptospermums, called "Tea Tree" by some, are also in the Myrtaceae family. In general, they are characterized by small leaves and flowers, and they vary in size from small shrubs to medium-sized trees. Though the flowers are small, they can often be remarkably profuse and showy. Some species grow in areas natively subjected to considerable amounts of snow, and are very likely to prove hardy here. They grow easily from cuttings, and their loose, casual form makes them very attractive in a garden of other exotics. Among the following, I suspect some will prove hardy in zones 7 and 8 and some won't, but we'll just have to wait and find out which ones are which. Try L. attenuatum, L. flavescens, L. grandiflorum, L. juniperinum, L. lanigerum, L. laevigatum, L. myrtifolium, L. nitidum, L. rupestre, L. scoparium var. nanum, L. scoparium var. rotundifolium, and L. squarrosum. Here is some information regarding these Leptospermum species and their cultivation.
Telopeas, known in Australia as "Waratahs," are another small group of Australian Proteaceae with very showy red flowers. They do well in a partially shaded situation and are fairly tolerant of a wide variety of soils. T. speciosissima is probaly the best-known species for its use as a cut flower, but it has a reputation for being difficult to grow. T. truncata from Tasmania is likely to be the hardiest species as it grows high up in the mountains and is covered with snow and battered with cold winds in the wintertime. T. mongaensis, T. oreades, and T. 'Braidwood Brilliant' also exist, as well as rare white-flowered and yellow-flowered forms. See this site for further information.
Tree ferns are ferns with trunks. I have devoted an entire seperate page to them, which includes many species not from Australia. See Cold-Hardy Tree Ferns.
Finally, Xanthorrhoeas are worthy of mention as fascinating plants of the lily family--fascinating because, in maturity, they look like a patch of grass atop a slow-growing trunk! Although these "Grass-trees" come from the deserts of Western Australia, they have proven adaptable to the climate of southern England, and therefore certainly stand a chance in zone 8 and up in the U. S., since with very few exceptions we have warmer summers than they do. With age they send up a magnificent tall flower spike.
This is just of sample of the great number of Australian plants with poteneial for horticultural use. Some other favorites that had to go undescribed (since I know less about their potential for hardiness) are Hakea and Melaleuca(try M. squamea), to name just a couple. Unfortunately, my brief and general information about these genera is representative of the prevailing state of knowledge in the horticultural realm. I hope to change that over the next few years by bringing some of these into cultivation and reporting on their performance.
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