Acacia baileyana (Bailey's acacia) This Australian native is the harbinger of spring in San Francisco. It is the earliest tree to flower, putting out brilliant yellow blossoms in January. (As a native of northern New York, I still find it jarring to associate January with spring.) The tree is popular for its feathery, blue-gray foliage, although the "Purpurea" variety has lavender new growth. Bailey's acacia is one of the fastest-growing San Francisco street trees, quickly reaching 20-30 feet in both height and width. Like most fast-growing trees, however, it is short-lived, rarely surviving longer than 25 years. Known as Cootamundra wattle in Australia, this plant is native to a small area near the town of Cootamundra in New South Wales. It is a woody shrub in the wild, but it can be trained to grow as a tree.

Location: 1201 Shrader St./Grattan St. in Cole Valley; also at 236 Ashbury St./Fell St. near the Golden Gate Park panhandle Acacia melanoxylon (blackwood acacia) One of the largest of San Francisco's street trees (to 40 feet in height, much higher under ideal conditions), blackwood acacia is also one of the most common--it was planted heavily throughout the city during the 1960s and 1970s, so many large, mature specimens are now found citywide. The tree is evergreen, with dark-brown bark and dense gray-green "leaves" 3-5 inches long that actually are not leaves but enlarged leaf stalks called phyllodes. (Botanists believe that phyllodes are a moisture-preserving adaptation to a dry climate.) In February and March, the tree produces an abundance of globular, pale-yellow flowers that put out a great deal of pollen.

Blackwood acacia is well adapted to San Francisco's coastal climate and will grow (rapidly) almost anywhere on city streets, as it is not afraid of sidewalks. In fact, just the opposite--this tree's aggressive roots will crack and lift sidewalks, which helps explain why in recent years it has been planted less frequently. Nevertheless, this is a good choice where a large, fast-growing tree is desired. Blackwood acacia is native to the forests of Tasmania and southeastern Australia. It has always been an important timber tree in its native zone--the tree's hard wood made strong boomerangs, clubs, and shields for Australia's aboriginal people. Location: 1 Northwood/Montecito in Westwood Park; also at 740 Masonic Ave./Hayes St.