November 7, 2007

Tree Walk Wednesday: Accoutrements of a new street tree




Only sometimes are the ideas for our posts planned in advance. This week's Tree Walk Wednesday post is a good example. On a recent stroll in Somerville (a city across the Charles from Boston), I walked by several newly planted street trees. The site evoked memories of administering Boston's street tree planting program: reviewing tree requests, selecting trees, watching while the contractors planted the trees, and inspecting the trees several times after they were planted. I was also struck by how different the Somerville trees looked than new street trees I have seen in Berkeley and Oakland. The significant differences are the size (dbh = diameter at breast height) of the tree, the tree gator, and the water & aeration loop.



The trees in Somerville are at least 2" in diameter. Nurseries typically use a caliper measure taken 6" above the trunk flare, but other tree professionals calculate tree size by measuring diameter at breast height at approximately 4.5' above the trunk flare. The new street trees I have seen in Berkeley and Oakland are approximately 1" in diameter.

The water & aeration loop (second image - white tube with black lid) is commonplace in street tree plantings in Boston. I have not noticed this for East Bay tree plantings. The water & aeration loop is installed around the root ball with an extension above the soil level. Ideally, the loop enables air exchange as well as direct watering of the root ball. There are some challenges with this system: if the lid is removed, the loop can become filled with rubbish and debris, thus preventing efficient watering and air exchange. The third major difference between the Somerville and East Bay trees is the tree gator. The tree gator is a watering bag manufactured by a company of the same name. For each inch of diameter, a tree requires approximately 10 gallons of water.

According to treegator.com, a full 20-gallon gator bag not only slowly releases water over a 12 hour period, it also provides sufficient water for a week. Water is necessary for trees to establish, that is, grow new roots beyond the original root ball. East Bay tree planters might forego tree gators because trees planted in the winter receive adequate - and in some years more than adequate - rain water. East Bay cities are not the only cities that do not install gators. The City of Boston - to my knowledge - does not install gators with its new street trees. The California community forestry alliance, California Releaf, recommends treegators for low run-off watering.

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