When siting curbside planters, maintain pedestrian access and capacity. Bioretention cells must not encroach upon clear walking paths, and cannot impede designated accessible parking spaces or loading zones.
Bioretention planters must be sized to handle the runoff load of the design tributary area (see Bioretention Cell Sizing). Multiple planters may be linked or sequenced to prevent overloading on a single planter.
Bioretention planters must drain within 24–72 hours (depending upon frequency of rainfall) after a storm event to prevent insect breeding and bacteria or algae formation (see Drawdown Time for additional discussion).
The planter bottom should typically be at least 4 feet wide to promote vegetation health. Narrower cells may be implemented in special contexts, such as protected bikeway buffer zones or constrained sidewalks, but must be designed with consideration for plant health, bioretention performance, and implementation cost.
The maximum ponding depth is typically not more than 6–9 inches, with 12 inches from sidewalk to soil in areas with high pedestrian activity to mitigate fall risks. Deeper bioretention cells require more robust barriers, such as fencing or railings.
Where pedestrian activity is moderate or high, at least 8–12 feet of clear width should be dedicated to pedestrian movement between the edge of the planter and the building or property line. Where pedestrian activity is low, a 5- to 6-foot preferred minimum allows two people to pass comfortably.
Provide an edge that can be detected using a cane or other mobility device to prevent incursions, such as a 4-inch curb or low fencing (less than 24 inches tall).
Plants that grow to at least the height of the wall (preferably with some variable height) act as a visual barrier and discourage incursions.
Planters configured adjacent to on-street parking should provide a level step-out zone along the curb to accommodate vehicle entry and exit, and mitigate soil compaction and trampling, typically 36 inches wide from the curb face. Provide regular access paths between the curb and pedestrian through-zone, at least 5 feet wide.
Shorter planters (20 feet or less in length) with regular access paths between the curb and sidewalk can enable narrower step-out width on constrained sidewalks; 12–18 inches of firm, compacted surface behind the curb face can accommodate vehicle entry and access. Provide designated parking and loading spaces with accessible paths and clearances for people using mobility devices on every block and at major destinations.
To provide shade canopy and improve walking comfort, street trees can be integrated into walled stormwater planters, increasing transpiration and water volume managed. Ensure trees have adequate root space for health, and select appropriate species for local climate and cell ponding characteristics. Medium and large trees should be sited outside planters unless adequate clearance from walls and soil volume can be provided to allow for adequate root space (see page 84).
Seating, informational signs, and other urban design features may be utilized as barriers around deeper planters to improve pedestrian safety while also beautifying the streetscape.