A common characteristic of bioretention cells is that they have shallow earthen slopes, less than 2H:1V. The design of a bioretention cell allows for several different planting zones. Bioretention cells are suited for institutional, commercial, industrial, and residential multi-unit/multi-story land-uses. They are can be sited in large landscaped areas, parks, parking lot islands, or any areas where there is space for shallow earthen slopes and the multi-zone planting aesthetic is appropriate.
There are two basic categories:
- Exposure to roadway or parking lot runoff. Runoff is contaminated with deicers and vehicle pollutants. These can take on several forms, including parking lot islands, traffic islands, roundabouts, or cul-de-sacs and are often used as snow storage location
- Select salt tolerant grasses, other herbaceous material and shrubs.
- No exposure to roadway or parking lot runoff. These receive runoff from rooftops or areas that use no deicing salt and have low pollutant exposure, such as courtyard bioretention.
- Practices allow for a greater range of species selection.
Other selection factors:
- Most bioretention cells will be situated to receive full sun exposure.
- Facilities with a deeper media bed (greater than 1 m) provide the opportunity for a wider range of plant species (including trees).
- The inclusion of vegetation with a variety of moisture tolerances ensures that the bioretention cell will adapt to a variety of weather conditions.
- Proper spacing must be provided for above-ground and below-ground utilities, and adjacent infrastructure.
Selection of plant species suited to tolerate the varied conditions common to green infrastructure is essential for the success of a planting plan. A plant’s ability to tolerate flood conditions is further correlated to its age, adaptation to the site, and condition. A well-established plant has greater reserves to withstand flood events. While it is recommended to leave the LID practice offline until plants become established (one to several years), in most instances this may not feasible. Measures incorporated into the LID practice such as erosion and sediment controls and pre-treatment cells can moderate flows enhancing survival potential. In all cases, soil surfaces must be stabilized prior to allowing flow to enter the LID facility.
Selecting plant material that will be appropriate for a particular site should take into consideration the local context or setting. Some species, especially rare or potentially aggressive species, are not always conducive to the type of site or objective of the project, especially in areas near or next to natural communities. For example, a green roof that is isolated from all natural areas may be ideal for regionally rare species that are specially adapted to conditions of high exposure and shallow soils. Among the questions that should be asked are:
- Is this LID site located within, adjacent or close to a natural area? Or, is this LID site location isolated (i.e. beyond 120 m of natural feature) from the natural environment?
- What is the composition of the nearby natural area?
Native, introduced and rare plants
The goal of planting design for LID practices is to achieve a sustainable vegetation community that is tailored to the ecological qualities of the site and the aesthetic considerations of the landowner. Plant selection for LID practices is predicated on the principle of ‘right plant for the right place'. Many LID practices are carried out within a highly urbanized context that poses unnatural stresses on plant growth and survival. This guide provides general recommendations to direct species selection. Landscape professionals should use this information to generate specific plant lists that are tailored to the conditions prevalent on site while addressing surrounding urban and natural land uses.
Native and introduced species
Native plants have co-evolved with the local ecosystems and natural processes. They are genetically better adapted to local climate, soils, insects and diseases of the area, and may require less maintenance to ensure health and survival. Working with native plants helps protect local native biodiversity, allows the LID feature to function ecologically while creating a more diverse, naturally-beautiful, landscape. Where conditions for growing native plants are inhospitable, diversifying the planting palate with introduced species may have a more successful result. In addition to native species, many introduced plants are grown in nurseries and garden centers and are readily available to landowners.
Some introduced species escape from gardens and other managed landscapes and begin to reproduce in the wild. Invasive plants are typically non-native plants that out-compete native species. These species lack natural predators, grow aggressively and reproduce rapidly and can be problematic in the natural environment. Invasive species have not been included on the recommended plant list for LID practices and should not be planted in any situation.
- In all applications of LID, designing with native plant species is the preferred and strongly recommended option.
- In areas containing, adjacent to, or within a linkage to an existing natural heritage feature, native plants should be used exclusively. In areas regulated by a conservation authority, native plants will be required for approval.
- In settings that have restricted or harsh conditions for plant growth (e.g. limited root volume, unusual patterns of drought and/or inundation, exposure to salt, sun, wind, shade and pollutants) introduced species can be used to substitute/augment native species selection. Introduced species shall be chosen to be functionally and aesthetically appropriate for the location.
- Introduced invasive species should not be planted as these can compromise the function of an LID practice, the ecology of nearby natural areas and can lead to higher maintenance costs. Non-native invasive species have not been included in the LID plant list.
- Except unless otherwise noted, the use of cultivars of native plants should be avoided.
- When sourcing native plant material, especially from commercial nurseries and garden centres, always refer to the scientific (botanical) name for the desired plants. Confusion over cultivated varieties of plants can be avoided by asking questions of the staff to ensure that only the correct native varieties are sourced. Species that are often mislabelled and other common concerns are noted in the Bioretention: Plant lists.
Some of the native plants in the LID plant list are ‘rare’ and/or ‘uncommon’ within the Province of Ontario . STEP do not support the incorporation of rare native species in planting plans and designs unless locally sourced material can be obtained and there is little to no threat of escape into naturally occurring populations. Genes from plants adapted to other areas can inadvertently introduce harmful traits that weaken the ability of the native population to thrive. Rare species are not always appropriate for LID practices depending on the project objectives or the characteristics of the site, especially in situations where rare species may be planted near natural communities. In these situations, the opportunity for genetic contamination of native populations is higher and the planting of rare species is unadvised. Some native species are rare because they require specific habitat conditions in order to grow which may not exist or be able to be restored easily. The benefits to landscaping with native, common plant material is that plantings may be better adapted to a wider range of conditions, and may tolerate more disturbance. A higher success rate and lower maintenance requirement may be achieved.
The method and collection of native plants is an important consideration when sourcing rare plant material. Ecologically responsible methods of seed collection and propagation ensure that wild populations are not over harvested and that they have the ability to keep adapting over time. Seeds should not be collected or existing plants relocated from the wild for use in LID practices. When sourcing material from growers and nurseries, the plant material origin should be confirmed as well as its method of collection and propagation. Recommendation: In areas containing, adjacent to, or within a linkage to an existing natural heritage feature, common native plants should be used exclusively. In areas regulated by a watershed authority rare species are not suitable for planting unless the local source/provenance is documented and approved. Rare species should only be used sparingly when there are no suitable common species available and reasons for selection should be provided. Where rare species are included in a design, it should be ensured that the plants are procured from locally adapted seed sources. Biologists, botanists and ecologists can help guide the sourcing and selection of plant material. Nurseries that specialize in native plants should be contacted to source material.
A plant community is not a static entity; rather it is always growing and changing in response to natural processes and disturbance events. When utilizing a plant community as an analogue for design, it is essential to understand the role of agents that affect the composition of the plant community over time. Designers and managers may need to mimic or compensate for the natural processes in a management plan. Consideration should be given for the impact of natural processes and disturbances to the long term vision of the LID landscape design. Natural ecosystems develop and are maintained through natural changes such as the onset of shading, and disturbance events such as:
- Seasonal storm events / flooding outside the design norms
- Prolonged drought
- Microclimate change and progression - sun / shade / moisture
- Plant competition, successional growth, invasive behaviour
- Weeds entering from other sources
- Pests and diseases
- Changes in soil chemistry / nutrients
- Development of soil structure
- Maturation and decline of plants