Mulga is a landscape shrub or small tree originating from outback Australia and is well suited to dry climates. It has been introduced into Kenya, South Africa and the south-western United States.
It is a highly variable species, with up to ten naturally evolved varieties reported in its native range, from multi-trunked shrubs 2 to 3 m ( 7 to 10 ft) tall, to trees up to 10 m ( 30 ft) tall. In cultivation it is more commonly a small tree 3 to 5 m (10 to 16 ft) tall with a round- to wide-spreading crown of ascending branches.
The leaves are lance-shaped and have a blue-green or silver-grey colouration that contrasts strongly with the usual browns and greys of dry landscapes.
The flowers are bright yellow, in dense clusters resembling small bottle-brushes and come into bloom after a shower of rain or intermittently under irrigation, as is common in gardens and landscapes.
The flowers are followed by flat seedpods up to 5 cm (2 in) long that become brown and resinous when mature, although they are sometimes absent or are only few in number as a result of poor fruit-set.
It is a commonly cultivated small tree in residential and urban landscapes in dry areas, including poolsides, traffic islands and other types of urban plantings. It is also sometimes planted to prevent soil erosion, especially on low fertility soils on account of its extensive root system and nitrogen-fixing abilities.
The wood is very hard and heavy, averaging out at around 1000 kgs per cubic meter and is finely grained with attractively contrasting shades of brown.
Though small in diameter, the roundwood has traditional use in aboriginal wood craft and is sought after for decorative wood turning and for making tool handles. The larger diameter stems are cut for poles and posts, particularly fence-posts as well as for firewood and charcoal making.
It is an important bee-forage plant in its native range, visited by honeybees, though not for nectar but as a source of high protein pollen for feeding to their larvae.
In some areas the leaves are a valued livestock fodder and forage during the dry season. They have a crude protein content reported at 11% to 16% of dry matter, but are deficient in sulphur and phosphorous and are only moderately digestible, requiring supplementing with other feeds. It is fairly resistant to grazing by livestock, having the resilience to re-leaf and recover.
Grows naturally in dry subtropical and tropical mid-elevation climates, generally in areas with annual lows of 10 to 19 °C, annual highs of 20 to 36 °C, annual rainfall of 200 to 600 mm and a dry season of 5 to 12 months.
New plants are usually grown from seed which are reported to remain viable for up to ten years, if they are dried and stored under cool, low humidity conditions.
A nitrogen-fixer, it will thrive on nutrient poor soils. Performs best on free-draining clay, loam, sand and limestone or gravel with an acid to alkaline nature, generally with a pH of 5.0 to 8.5. It is very drought tolerant and although intolerant of waterlogged soils withstands seasonal flooding.
It is listed as a weed in at least one reference publication, but there does not appear to be any reports of it being a serious weed anywhere the world. Instead, it is most commonly reported for its re-seeding habit leading to the formation of dense thickets and for its vigorous root growth, which can block sewerage and other underground pipes. In Australia, a minimum planting distance of four meters from any underground pipes is recommended.
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