Seattle has laws mandating the planting and/or retention of trees when buildings are built. Its regulations strongly encourage the retention of trees that are very large, or large and unusual in Seattle. (There are exceptions that allow the removal of "hazardous" trees, and let individual homeowners remove a few "non-exceptional" trees per year.) Its regulations also list recommended trees to plant along streets, and list trees that are specifically not recommended for planting along streets. Some of the criteria for inclusion on each list are:
Reasons for recommendation
- Trees that provide useful shade and evapotranspiration.
- Attractive leaves.
- Attractive bark.
- Attractive flowers.
- Disease resistance.
- Fall color.
Reasons for not recommending
- Trees that drop fruit or nuts on streets and sidewalks cause messes and stains.
- Trees that are likely to get aphid infestations severe enough that they drip.
- Trees that release large amounts of pollen that many people are allergic to.
- Trees that require irrigation during month-long dry spells, even after they mature.
- Trees that cannot handle frosts, and temperatures that sometimes go down to 15°F (-9°C).
- Trees that are likely to grow large enough to have branches within ten feet of power lines.
- Trees that are likely to have large shallow roots that cause sidewalks to heave.
- Trees that have aggressive roots that damage water lines or sewer lines.
- Trees that are likely to grow large enough to interfere with building roofs and/or foundations.
- "Invasive" species that quickly spread seedlings.
- Brittle limbs that can fall under ice or snow load.
- Conifers are "encouraged on appropriate private property sites", but are discouraged where their lower limbs "can cause visibility/safety problems" for drivers and pedestrians.
Seattle assumes that trees that are on private property belong to the owners of the private property, who are responsible for maintaining the trees, and are entitled to the crops that the trees produce. Trees that are on city property belong to the city. Many trees are in "planter strips" between the streets and the sidewalks. Unless you look up the property lines for a particular neighborhood, it is not obvious whether the "planter strips" are city property, or belong to the adjacent lots.
In practice, maintenance of these trees is split between the city-owned power company and the adjacent property owners. The adjacent property owners are theoretically responsible for keeping an eight-foot tall clear volume along sidewalks, and a fourteen-foot tall clear volume along streets. (In practice, most property owners make a good faith effort to allow passage along the streets and sidewalks. Others do not. The city does follow up with adjacent property owners when there are specific complaints.) Every few years, the city-owned power company severely prunes street trees to keep their branches at least ten feet from the nearest power lines.
In California, most fruit orchards, nut orchards, and vineyards are torn out and replanted on a 20-25 year cycle. This practice maximizes crop yields, by repeatedly having plants that are 5-15 years old. (It also helps keep crops in sync with long-term demand trends.) This practice is not allowed by Seattle's mandates to retain trees that have six-inch diameter trunks.