A well designed tree guard keeps people, dogs, and objects out of the tree pit but leaves plenty of room on the street side to allow cars to open their doors and passengers to step out. It allows rainwater to flow into the pit to feed the tree.  The pits can either mulched or planted with small bulbs or shallow rooted annuals and small herbaceous perennials. Here is the plant list from NYC's Parks Dept.  Click here

 
 
 
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http://www.christinasgardens.com/
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