22 Rows of street trees along the curb define a street, especially if they are similar In 1901, Charles Mulford Robinson wrote: “Increased stateliness, impressiveness, and charm are usually secured if the permanent, and therefore larger, trees on any given thoroughfare be of one variety”43 Today arborists recommend against planting the same species for miles, a lesson learned from the Dutch Elm Disease disaster For a few blocks one tree type may be planted, or a mix of types with similar profiles On Mass Ave today there are silver lindens, willow oaks and some London plane trees, as well as some young, disease-resistant American elms Growing these trees to maturity is important to public health Bare pavement intensifies ground-level heat The hot air and sunlight act on vehicle emissions to make ground-level ozone (smog) Federal and city policy has long aimed at lowering smog (see page 20) So rows of shade trees do real work, besides adding beauty Washington’s 144,000 street trees are under the purview of the Urban Forestry Administration (UFA) in the Department of Transportation (DDOT) Though private parties cannot prune or cut down street trees, the UFA welcomes TLC of them by neighbors Such care is needed!44 Fig. 35 How trees help: Rainfall. One-third is absorbed by leaf crown and one-third by trunk and roots, so just one-third flows to pavement and drain. Global warming. Leaves absorb carbon dioxide. Air quality. Tree leaves emit oxygen that freshens ground-level air. Health. Cooler air near trees lowers ground-level ozone (smog). Step 1: Full row of healthy street trees • Watch that the city and its contractors plant trees that are healthy and planted well. If you suspect a problem, report it to the Mayor’s service line (phone 311 or http://311) giving the nearest address. • Water all sidewalk trees 25 gallons per week from May to September, except after heavy rains. • Remove weeds and grass from around the tree. Do not plant shrubs that rob nutrients from the tree’s roots, such as nandina or lariope. • No heavy loads, trash or bikes should be set next to trees. Protect them with low fences that follow city public space regulations. Fig. 36 After weeding the box around a city tree, we spread a thin layer of mulch.
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