Australian Arid Plants Garden
Australia has very few succulent plants in its extensive arid and semiarid regions. The Australian salt bushes (Chenopodiaceae family) are semi-succulent. Succulent plants store water in their tissue and therefore require a regular rainfall, which may be quite low, but must never-the-less be regular and dependable. The rainfall in arid Australia is not only low, but also irregular and unpredictable. Drought of varying lengths, broken by short wet times, is the normal condition, and Australian plants have evolved different strategies to cope with the low water availability.
Some plants, especially the daisies in the Asteraceae family and the pea flowers in the Fabaceae family, grow rapidly from seed, flower and set seed quickly after heavy rain. The seed then remains dormant in the soil, often for years, until the next rains arrive.
Other plants, especially the woody shrubs such as Acacia, Eremophila, Grevillea, Senna, etc. reduce their water use by having densely hairy leaves and stems to insulate them, and the hairs are often silvery to reflect the heat. Alternatively they may have viscid leaves with a varnish-like coating to reduce water loss. The stomata , the pores through which carbon dioxide and water pass in and out of the leaf, are often sunken and covered by hairs or waxy coatings to maintain a high water vapor density at the surface of the leaf, even in windy conditions. In prolonged dry conditions the plants become physiologically inactive, and ultimately the leaves may be shed.A third strategy adopted by some species, such as the Mulla Mullas (Ptilotus sp.) die back to an underground rootstock, and then shoot more or less annually depending upon conditions.
The scattered desert trees usually occur where their deep root systems reach underground water. The leaves of the eucalypts and wattles hang vertically to reduce the amount of heat reaching them, and they often have whitish trunks to reflect the heat or thick bark to insulate the inner live tissue.
A characteristic and ubiquitous plant of arid Australia is the spinifex (species of Triodia and Plectrachne in the grass family Poaceae). The leaf blades of these hummocky grasses are hard and rolled so that the stomata are concealed, and are either hairy or resinous to further reduce water loss. After severe drought or fire they will reshoot from the rootstock.
This Garden contains only a few representatives of these plants as most do not grow very well in the more humid conditions of the east coast.
This is a developing collection of the palm-like cycads. They are not related to the palms (which are angiosperms - the flowering plants) but are an ancient and primative group of plants which flourished during the Jurassic and Cretaceous ages 200 to 100 million years ago, and now relect populations are found in Africa, China, Mexico and Australia. Pollen is produced in male plants in small cone-like structures and is transferred to the female plants either by wind or insects where the naked seeds develop in large cone-like 'fruits'. Some also have 'contractile' roots, which as the plant grows pulls it below the ground surface. All parts of the plants, including the seeds, are poisonous. The cycad Macrozamia communis, the (Burrawang Palm), grows naturally on the Gardens site and has been chosen as the gardens emblem.
Plants of Vine Thickets
Vine thickets are a vegetation type which grow in moderately low rainfall areas and are composed of non-sclerophyllous shrubs and small trees related to the dry rainforest plants. This unusual vegetation type is found in northern NSW, northwards from the Upper Hunter Valley to the Queensland border and extends into the "Bottle Tree scrubs" of Queensland. However, it is not of the same floristic composition or structure as the Bottle Tree scrubs. A particularly outstanding tree associated with these vine thickets is Ooline (Cadellia pentastylis). Other common plants include species of Capparis which are usually armed with thorns, and Canthium species. They are correctly referred to as "semi-evergreen notophyll vine thickets".
Very little of these vine thickets now remain, as most have been cleared.
Mallees are small, multi-stemmed eucalypts which sprout from an enlarged root area called a lignotuber. In this way they can survive fire and drought and are reputed to live for a very long time. The lignotuber can also become very large. This small collection will take many years before the usual mallee character develops and takes on the typical appearance. Some other plants associated with this vegetation type are also growing here.