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DESIGN PALETTE Generally, if utilities and future street construction measures are considered, subsequent pavement and utility construction can take place with minimal damage to the trees. (Examples of this are Broadway, Second Avenue, and University Way in Seattle.) Funding for comprehensive tree plantings could be shared by City and property owners through some form of agreement or Local Improvement District(LID) mechanism. Landscape Establishment and Management While street trees can provide substantial benefits to a downtown, they require favorable horticultural conditions and maintenance in order to thrive. Therefore, this plan recommends that any street tree planting program be accompanied by adequate resources to ensure that the planting is done properly and the trees are maintained. Described below are some of the considerations that must be addressed and estimated maintenance costs are in the appendices. • Planting Beds: Plants in the urban context are most benefited by providing a suitable, well draining rooting medium generous enough in size to supply sufficient water when the plants need it. Soil blends should consist of granular sandy soils, organic matter and nutrients. Trees require a planting-bed dimensions of at least five to ten feet wide by three to eight foot deep, depending on the needs of the individual tree species. Where trees are chosen to fit existing built conditions, planting-bed width is a primary criterion in their selection. Additionally, the City Departments of Public Works and Parks have found that non-woven plastic pit liners reduce the likelihood of root damage and are, therefore, recommended. Larger widths will further enhance tree health and longevity. • Irrigation and Water Needs: All plants require irrigation at least for a plant-establishment period of two to three summers. This is also the case for plants chosen for their tolerance to drought. Deep periodic waterings will generally help plants develop the deep healthy root systems that make them survive dry periods. Irrigation can be provided by automatic low- volume drip systems or manually filled manifolds at individual trees, plants beds, or planters. It is recommended that new street tree plantings, including those required of new development, include an automatic irrigation system. • Tree Protection: Tree grates and permeable-paving blocks can help create a smooth and safe sidewalk surface, allow water to reach the soil, and protect trees from soil compaction caused by pedestrian foot traffic. This compaction can restrict the soil's ability to hold water and oxygen where roots can reach it. Choose grates or block patterns with inner diameters that will accommodate the maturing trunk size, that knock out to allow for future growth, and that can if be removed without damage to the valuable mature tree. -- \v*, Choose tree-grate design in conjunction with other site pf �� A « furnishings; they can add an important aesthetic element ><-< cog ) ,- to the streetscape and a reinforce neighborhood character and identity. • Tree Spacing: Space trees by at least the width of their •< mature canopy (in coordination with street-lamp " - -' placement). Trees in planting beds rely more heavily on extending their roots deeply into surrounding well drained . gravel and crushed rock base layers, ideally growing underneath the sidewalk and parallel to the curb for the - Figure 28. Tree grate. 24