Mulga is a landscape shrub or small tree originating from outback Australia and is well suited to dry climates. It long introduced into Kenya, South Africa and the south-western United States.
A highly variable species, it has 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 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 a few as a result of poor fruit-set.
Mulga is a commonly cultivated small tree in dry-area residential and urban landscapes, such as poolsides areas, traffic islands and various 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 hard and heavy, averaging out at around 1000 kgs per cubic meter and is closely grained, with attractively contrasting shades of brown.
The roundwood lengths, though small in diameter, have traditional use in aboriginal woodcraft and is sought after for decorative woodturning and for making tool handles. The larger diameter stems are cut for poles and posts, particularly fence-posts, firewood and for making charcoal.
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 of 11% to 16% of their dry weight but are deficient in sulphur and phosphorous, and are only moderately digestible, requiring supplementing with other feeds. It is reasonably 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 9 to 19 °C, annual highs of 22 to 35 °C, annual rainfall of 300 to 600 mm and a dry season of 10 months or less, extending to 12 months with irrigation or groundwater.
New plants are usually started from seed, which to remain viable for up to ten years if they are dried and stored under cool, low humidity conditions.
Performs best on free-draining clay-loam, loam, sandy-loam loamy-sand, sand and limestone or gravelly soils of 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. As a nitrogen-fixing plant, it thrives on nutrient-poor soils.
It is listed as a weed in at least one reference publication but there does not appear to be any reports of it as a serious weed anywhere in 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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