It is unlawful to plant in any public parking strip the following trees: Poplar, willow, cottonwood, fruit trees (except ornamental types), nut trees, ailanthus, mountain ash and Oregon or big-leaf maple, or any other tree the roots of which cause injury to the sewers, sidewalks or pavements, or which breed disease dangerous to other trees or to the public health. No tree shall be planted within two feet of any sidewalk or pavement unless the planting is accomplished in accordance with a comprehensive street tree planting program approved by the city for the subject street. Poplar, willow, American elm or cottonwood trees planted anywhere in the city must be placed at least 100 feet away from public sewers. (1957 code § 8.15.050.)