Open space includes the portion of a lot that is not covered with structures, access roads or driveways, sidewalks, patios, off-street parking or any other material placed on or above the earth which substantially reduces or prevents the natural percolation of water. The open space shall be suitably landscaped with noninvasive, drought-resistant plantings, which may include trees, flowers, shrubs, succulents or ornamental or other grasses, except that where the open space includes wetlands as defined in MGL c. 131, § 40, the requirements of the Salisbury Conservation Commission shall supersede these landscaping requirements.