August 23, 2008

Tree Walk: Evergreen vs. deciduous, implications for stormwater

In the northeast U.S. where I grew up, large broadleaf deciduous trees are important green infrastructure for managing stormwater. These trees are in leaf when it rains during the spring and summer. However, in the San Francisco Bay Area, where I now live, deciduous trees are not in leaf when it rains during the winter months. In this place, (broadleaf) evergreens are more appropriate. This distinction was highlighted in a conversation I had with Jane Martin, founder and director of PlantSF, about landscapes designed to manage stormwater.

Martin recently got the San Francisco Department of Public Works to approve a second species of tree - Strawberry Tree - on her block. The Strawberry Tree (Arbutus unedo)is a broadleaf evergreen with fruits that resemble strawberries. As an evergreen, the tree is in leaf during the winter rains. The tree is small statured so its stormwater capture function is lower than that of a larger stature tree but it also provides forage services. The fruit is eaten by birds and the flowers are pollinated by bees. Human can eat the fruit but the taste might be a deterrent. The tree is featured in the City of Madrid's coat of arms; a bear is attempting to eat the fruit.

Not only is the evergreen nature of the Strawberry Tree well suited to the wet winter months in the Bay Area, it also "performs well" in summers characterized by dry conditions. It is recommended by the City of Antioch as well as the Contra Costa County Water District. Other evergreen trees that might be appropriate for sidewalk planting include the Peppermint Tree (Agonis flexuosa); Carob Tree (Ceratonia siligua); Bronze Loquat (Eriobotrya deflexa); Chinese Flame Tree (Koelreuteria bipinnata); and Small-leaf Tristania (Tristaniopis laurina), all of which are listed as "water conserving" trees in the East Bay Municipal District's 1990 publication, "Water-Conserving Plants & Landscapes for the Bay Area."

Related articles and resources

Street Trees: Let’s Think Outside the Wires

Bringing City Trees to Fruition

Friends of the Urban Forest Gallery of Urban Trees

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