The Silver Wattle tree (species name: Acacia dealbata) is native to the southeastern Australian states of New South Wales, Victoria and Tasmania. In its native range it is reported to grow to an impressive 30 meters (98 feet) tall. In southern Europe however where the images in this post were taken the "Mimosa" as it is called in Spanish usually only reaches a height of about 20-30 feet.
Probably the most striking feature of the Silver Wattle is its bright yellow flowers that are blooming right at the moment (early March) in Madrid, Spain. The flowers are clustered in racemes and the puffy little flower heads are globose in shape and similar to the Golden Wreath Wattle that is also common in Spain.
The leaves of the Acacia dealbata are bipinnate (they are like a scaled down version of the Jacaranda leaf). Another tree that has very similar leaves and is sometimes confused with this tree is the Persian Silk tree. The White Popinac also has bipinnate leaves and globose flowers. The image above was taken in the "Polvoranca" park that is located between the Madrid suburbs of Leganes, Alcorcon and Fuenlabrada. The picture illustrates the typical rounded crown of this tree species.
The bark (at least on young trees) is greyish-brown and smooth (see image below).
Other Australian trees that can be found in Southern Spain are...