… signs, placed by the Boston Parks and Recreation department, that read in part: “Help me grow strong. Newly planted trees require 15-20 gallons of water once a week … a small thing, but having healthy trees in the city are important because they help combat urban heat islands and air pollution,” she said. Nicholas reached out … but when there’s a particularly hot summer, it’s great to have this additional help,” said Greg Mosman, city arborist, superintendent of tree maintenance, and tree …