No person, firm or corporation, except with the written permit of the Director of Public Properties, shall place or maintain upon the ground in any public highway within the City, any stone, cement or other material or substance in such manner as may obstruct the free access of air and water to the roots of any tree or ornamental plant in such highway. Unless otherwise provided for in such written permit, there must be maintained about the base of the trunk of each shade tree in the public highways of the City at least six (6) square feet of open ground for a tree three (3) inches in diameter, and for every two (2) inches of increase of such diameter there must be an increase of at least one (1) square foot of open ground.