I. Landscape Guidance

Native plant species are recommended over exotic foreign species because they are well adapted to local climate conditions.  This will result in less replacement and maintenance, while supporting the local ecology.

A list of herbaceous trees, shrubs, and plants native to Philadelphia and Pennsylvania and suitable for planting in stormwater management facilities are included in Table I-1. The list is intended as a guide for general planting purposes and planning considerations. Knowledgeable landscape designers and nursery suppliers may provide additional information for considering specific conditions for successful plant establishment and accounting for the variable nature of stormwater hydrology. Because individual plants often have unique growing requirements difficult to convey in a general listing, it will be necessary to perform additional research to obtain specific information on the plant species proposed in order to ensure successful plant establishment.

Table I-1 lists native and recommended plants, trees, shrubs, and grasses and is organized by Type and  Latin name. Additional information given for each species includes: Common name, National Wetland Indicator Status, hydrologic zone, inundation tolerance, drought tolerance, salt tolerance, mature canopy spread, mature height, light requirements, nativity, commercial availability, and notes to provide guidance for application and selection. For example, some trees are well-suited to landscaped areas that will receive stormwater runoff, while others may not tolerate the additional moisture.

National Wetland Indicator Status

The National Wetland Indicator Status (from Region 1, Reed, 1988) has been included to show “the estimated probability of a species occurring in wetlands versus non-wetlands” (Reed, 1988). Reed defines the indicator categories as follows:

• Obligate wetland (OBL): Plants which nearly always (more than 99% of the time) occur in wetlands under natural conditions.
• Facultative Wetland (FACW): Plants which usually occur in wetlands (from 67 to 99% of the time), but occasionally are found in non-wetlands.
• Facultative (FAC): Plants which are equally likely to occur in wetlands and non-wetlands, and are found in wetlands from 34 to 66% of the time.
• Facultative Upland (FACU): Plants which usually occur in non-wetlands (from 67 to 99% of the time), but occasionally are found in wetlands.
• Upland (UPL): Plants which almost always (more than 99% of the time) occur in non-wetlands under natural conditions.
• A given indicator status shown with a “+” or a “-” means that the species is more (+) or less (-) often found in wetlands than other plants with the same indicator status without the “+” or “-” designation.

Hydrologic Zones

For planting within a stormwater management practice (SMP), it is necessary to determine what hydrologic zones will be created within the SMP. Hydrologic zones describe the degree to which an area is inundated by water (see Figure 4.1-3 for an example of hydrologic zones in a bioinfiltration/bioretention basin).  Plants have differing tolerances to inundation, and, as an aid to landscape designers, these tolerance levels have been divided into six zones and corresponding appropriate plant species have been identified.  In Table I-1, each plant species has a corresponding hydrologic zone provided to indicate the most suitable planting location for successful establishment. While the most common zones for planting are listed in parentheses, the listing of additional zones indicates that a plant may survive over a broad range of hydrologic conditions. Just as plants may, on occasion, be found outside of their hardiness zone, they may also be found outside of their hydrologic zone. Additionally, hydrologic conditions in an SMP may fluctuate in unpredictable ways; thus the use of plants capable of tolerating wide varieties of hydrologic conditions greatly increases a successful planting. Conversely, plants suited for specific hydrologic conditions may perish when hydrologic conditions fluctuate, thus exposing the soil and increasing the chance for erosion.

Inundation Tolerance

Since the Wetland Indicator Status alone does not provide an indication of the depth or duration of flooding that a plant will tolerate, the “Inundation Tolerance” column is designed to provide further guidance. If a plant is capable of withstanding permanent saturation, the depth of this saturation is listed (for example, “saturated” indicates the soil can be moist at all times, “sat, 0-6”“ indicates that the species can survive in constantly moist soil conditions with up to six inches of standing water). Conversely, a plant may only tolerate seasonal inundation – such as after a storm event – or may not tolerate inundation at all. This type of plant would be well-suited for an SMP that is expected to drain quickly or in the drier zones of the SMP. 

Drought Tolerance (N=none; L=low; M=medium; H=high)

The drought tolerance column is meant to provide a way for SMP designers to select appropriate native plants that can survive in hot summer conditions, with a minimum of irrigation. Drought tolerance is defined as the relative tolerance of the plant to drought conditions compared to other plants in the same region (USDA, 2005).

Salt Tolerance (N=none; L=low; M=medium; H=high; U=unknown)

This column ranks the relative tolerance of a species to salt content in the soil. If U (unknown) is displayed, no research was found for that particular species.

Mature Canopy Spread

This column gives the SMP designer a rough estimate of the diameter (or spread) of a tree species’ branching when it has matured. This information indicates what the light conditions will be like beneath the tree for understory plantings; how much space should be left open between the tree planting pit and any vertical structures, such as buildings; how far apart the trees should be planted; and it gives an idea, along with the mature height of the species, of the tree’s growth habit. The mature canopy spread also provides a rough idea for how much leaf surface area will be available to intercept stormwater before it reaches the ground.

Mature Height

This column provides the approximate mature height of plant species in optimal growing conditions.  This height may be reduced dramatically in the urban environment where light, space, and other factors may not be as readily available as in a forest or field setting. However, by providing as much space as possible for a plant to grow and by choosing appropriate species for a planting area, improved – if not optimal – growing conditions can be achieved. For example, a tree planted in a sidewalk pit measuring four feet by four feet may only reach half its mature height, while a tree planted in a four-foot-wide “trough” style planting bed will grow taller and live longer, because it will have greater access to air and water.

Light Requirement

The light requirements for each species are listed as ranges between full shade and full sun. At the bottom of the range – full shade – plants thrive in conditions where they receive filtered, or dappled, light for the entire day (such as under an oak tree). In the middle of the range are plants that grow best in part shade, where they are in full shade for two to three hours during midday.  Plants that require full sun should be sited so that they receive five or more hours of direct sun during the growing season.  Some plants requiring full sun may still do well in a part shade environment, depending on the quality and duration of the light the plants receive when they are not in the shade.

Nativity

A native plant is an indigenous species that occurred in the region prior to settlement by the Europeans.  In this column, each species is located within a range of nativity to Philadelphia. Plants known to have existed in Philadelphia County are native to Philadelphia, while a wider geographic range lists plants native to the state, but not necessarily to the county. The widest geographic range lists a few species native to the United States, but not necessarily to Pennsylvania. The plants listed that are not specifically native to Philadelphia are included because of their demonstrated success within SMPs.

Commercial Availability (C=container; P=plug; S=seed)

Wildflower and grass species often come in a form known as a plug. These are often grown and sold in trays of 50 of the same species. They are essentially very small container plants, with a root/soil mass about an inch wide and two to four inches long. Most species available in plug form are also sold as seed. Often, a combination of plugs and seed will be used to establish a SMP quickly and provide immediate visual interest and stabilization.

Container-grown plants include trees, shrubs, wildflowers, ferns, grasses, and sedges. This is an excellent alternative to the far more expensive balled-and–burlapped (B&B) form of trees and shrubs, although the size of the tree is almost always smaller. Nurseries often provide a few container sizes for each species. 

Notes

PWD has included recommendations for street trees in the notes section of the native plants list and the recommended non-invasive plants, trees, shrubs, and grasses list to assist designers in selection of vegetation most appropriate for the harsh conditions which are often associated in close proximity to streets. It is likely that most of these areas will be hot in summer months until the trees become established.

Table I-1: Native and Recommended Non-invasive Plants

Table I-1 is too large to display in the browser. Download a PDF copy.

Prohibited Non-native and Invasive Plants

Invasive non-native plants reproduce rapidly, degrade, and take over natural ecosystems, and have few, if any, natural controls to keep them in check. Brought in to new areas by people for a specific purpose or by accident, these species have characteristics that allow them to grow out of control and usually favor disturbed sites like areas of new construction. Under no circumstance should they be planted in a SMP.  Because of appealing characteristics, some of these plants are available for sale, and care should be taken not to purchase them. Additionally, the ability to identify and remove them before they can establish themselves is important, as they almost always invade due to their gregarious reproductive strategies. They can be especially hard to get rid of once they take hold. Table I-2 lists common invaders for the Mid-Atlantic region.