In the previous post I claimed that planting design for stormwater management applications should involve all of the same considerations as any planting design project. This post will elaborate on that by laying out a basic design framework of goals - context - means, and exemplify that in an imaginary but reality-based urban development project (or maybe it's preparation for my field experiments… who knows).
The multiple benefits of urban vegetation are becoming more widely recognized, as nature-based solutions and biodiversity are increasingly encouraged in policy and implementation. City plans on different levels often promise green streetscapes and vegetated rooftops, and ponds and swales are becoming mainstream in urban drainage schemes. On the other hand, we're not really changing the way our cities are structured, neither are the financial stakeholders budging from their goals for profit marginals. In my personal experience, it often means that the green is there in the plan, until we come to the actual design process where available space for vegetation just keeps diminishing as infrastructure is mapped onto public space. But the clients are insistent: they want nice, multifunctional spaces, within the specified budget and physical site constraints. It then becomes the designers' job to balance out project goals with the contextual restrictions by specifying suitable means.
So let's say we have the following goals for urban green in this imaginary area: Biodiversity support, year-round visual interest and spatial differentiation, participation in stormwater quality and quantity management, ease of maintenance and the experience of a "high quality green urban district", whatever that may mean.
The context is densification of a city-center-adjacent district, where existing industrial buildings are replaced by housing aimed at the urban upper middle class. The construction company as well as the municipality are expecting good profits from the project, hence the branding as "high quality green urban district". Fine. But the emphasis on profit also means that the streets where the vegetation should be placed become narrow and thoroughly shaded as commercially viable building volume is maximized. Some of the streets become wind tunnels, whereas others are expected to become sweltering hot in the summer due to poor ventilation and urban heat island effect. The catchment area for the streets is not especially big, and the annual precipitation is relatively low (let's say 600 mm/ year for good measure), so the need for stormwater retention capacity during most rain events is not especially high. On the other hand, a rain event of a magnitude that is only expected to occur every 10 years will cause the streets to flood to a depth of a few centimeters, unless it's swallowed by both stormwater piping and street planting. This doesn't count as a cloudburst event and it shouldn't damage buildings or impair transport considerably, but it's best if it can be avoided. The soil is thick, hardly permeable clay in some areas, and in other areas it's rough super-permeable filling material. Snowfall is fickle in the winter, so the municipality uses a mixture of ploughing, sand and salt as anti-slip measures. Average winter temperature is -2 °C and average summer temperature is 15 °C. Vegetation period tends to start mid-April and end in mid-October. Spring is drier than the rest of the year, when precipitation is relatively evenly distributed across time.
The means that can be used to fulfill the goals within the site context are dependent on the available resources and any given limitations. Money (both for design, construction and management) and space should be considered as stricter boundaries, whereas there is a lot of leeway with design strategies and inspiration (Of course, it is entirely possible that imaginations are being limited by trying to re-create a particular design in every single project, or that there is a chance to extend site boundaries in order to arrive at a satisfactory design solution. But these are probably not the norm). Somewhere in between is the amount of relevant options for plant selection and construction for the planting beds. So let's start hunting for those.
We begin by prioritizing goals together with the client and other technical consultants (in this example just the stormwater people - we don't want to be here all night. Road planners, electric designers, geotechnical engineers and the rest will get their due in later posts). The client's landscape representative posits that the spatial and aesthetic-experiential values are the most important ones, since a) this area is supposed to become an extension of the city center and b) why make bad public spaces when you can make good ones? The stormwater experts want to prioritize stormwater quantity, because they've modeled different cloudburst scenarios and worry about flooding if a 100-year rain event hits. The areas available for infiltration within the project boundaries are too small to make a real dent on a cloudburst volume, but it's the thought that counts. On the other hand, they're well aware of the fact that because the average precipitation and even 10-year rain events don't really provide that much water, it would be good to work with quality issues as well even though the stormwater in the area shouldn't be all too polluted.
The client and the stormwater engineers agree that it's best to use tree pits and other street plantings to combine the best of both worlds, and so they suggest the city of Stockholm's type solutions with biochar, compost and crushed rock. This way the planting beds become well oxygenated, the biochar holds water and catches chemical impurities, the planting beds can be extended into the roads due to their load-bearing capacity, and in the case of heavy rain events they chug plenty of water. Problem solved! The compost will leach nutrients into the stormwater for a bit, but this short-term decrease in quality could be accepted on the behalf of improved chances for successful tree establishment.
For the tree pits the solution is good - there are a few tree options (Acer campestre, Acer pseudoplatanus, some Tilia species, Carpinus betulus and Quercus robur) that are suitable for the local climate and should be able to tolerate the site conditions while staying healthy and producing acceptable growth. Some of them even have narrow-crowned cultivars for use in especially tight spaces. In time the treetops will hopefully reach some sunlight as well, improving their quality of life and thus performance. Of course, it's not ideal for biodiversity reasons to keep repeating the same trees across new city development areas in similar climatic regions with the exact same tough site conditions, but whatever, that wasn't chosen as a main priority here. The tree pit type solution may need to be modified to account for both non-draining and highly draining subsoil so as to balance water availability across the tree pits, but that shouldn't be much of an issue.
For other types of street plantings for stormwater management… Ehhh. I mean, there are definitely some perennials and some shrubs that do tolerate dry, warm shade. None that thrive in it, but survive, sure. They'll stay smaller, bloom less, and have a patchier coverage especially if they're subjected to disturbance, like trampling or the snow plough destroying woody growth. There's an educated guess making rounds among planting design professionals that most plants can take 24 hours of inundation on occasion, which might allow for a share of the species not to be chosen on the basis of specific flooding tolerance. But there's no telling what will break the proverbial camel's back: maybe accumulating salinity in the soil becomes more difficult to cope with for a plant already weakened by shade and drought, or maybe a prolonged flooding hits harder on a low-growing specimen than one from the same species that has been able to hoist its shoots above the flooding level. In short, when you pile and pile and pile difficult site conditions on top of another, the plants are likely to get weaker, thus decreasing their capacity to deliver ecosystem services and even risking premature death. And a dead plant is certainly of little help in stormwater management!
In this specific project, this is at odds with both of the main project goals for participation in stormwater management and increasing the value of urban space: small, weak plants will not be able to play a large role in either water cycling or water quality management, nor will they provide the "high quality green" or the spatial aspects asked for. The spatiality is, besides horizontal configuration, related to height, volume and time; I mean, large spaces of lawn are green, sure, but is the experience of them as "green" as the same as the experience of a forest of the same size? What does it do with the space and its usage if the hedge that was imagined to mature into a height of 1,5 m (eye height for short person, like myself) within 3 years stays stubbornly at 1,3 m (chest height, measured on the same short adult) even after 5 years? Another feature that might influence spatiality is the density and transparency of a volume. A sparsely branched or gappy hedge provides neither the visual or physical barrier that a more opaque one would (is it obvious here that density has to do with resource availability as well as choice of species, and even management? Good). So we really should be looking to provide the vegetation we plant a good fighting chance to increase the likelihood of successfully creating the planned spaces.
"High quality green" is such a badly defined goal that we should probably get back to the client. They show an inspirational picture of Jaktgatan/ Lövängsgatan in Stockholm as their ideal outcome (because of course they do). The overall feeling of luxuriant, overflowing urban nature is a combination of the sheer size, volume and layeredness of the plantings and the plants in themselves. The finer, species-specific details of the plants are bouncing off of each other in time and space: habit, leaf textures, flower shapes and sizes and timing and abundance, the leaves turning all shades of brown and yellow and red, and even later in the autumn and in the winter in the winter some decorative fruit may persist, and the colors and textures and architecture of woody branches can be appreciated as well. "But please add something wintergreen as well", the client might ask, "the winters feel longer otherwise". The high ground coverage of the planted species is also identified as a success factor, as it deters weeds from invading, thus helping to keep the management costs reasonable. The stormwater engineers chime in as well: they have concluded that the planting spaces in this project cannot even hope to swallow enough of the modeled cloudburst water within the project boundaries to make a difference, and that they'll look for alternative ways to flood-proof the new district. Optimizing them for a 10-year rain event will be enough from their point of view.
And so we finally get back to the drawing board… The goals are clearer now, and some of them are difficult to fulfil or capitalize on with the challenges in the context. So what we have left to do is to work on and with the means that we have to bring the goals to fruition within the reality of this specific project.
We cannot influence the climate at large (at least not with this budget!), and tinkering with the microclimate might not be the number one priority of the property developers. They will probably not be interested in cutting building height to let more light in ("we're just following the zoning plan"), and we cannot reasonably argue for giant outdoor lamps either. So the patterns of temperature, rain and wind we just need to live with. Shade becomes an immutable factor, as well. The decision to use salt as an anti-slip measure could possibly be overturned, or it could be limited in scope; we can keep that in the backburner, but I'm thinking it's probably an unnecessary fight to take. If there's salt, adding lots of nutrients could burn the plants even further, and then there's the question of nutrient leaching into stormwater. So what's left for us is water, which luckily is both something that we can influence in several ways, and which has a decisive effect on plant performance and range of available species to use. I will present three of the possible alternatives, and elaborate on each of them in later posts.
Manipulating the inflow of water. Since watering would increase management costs, it would be better to see if we could rather influence local catchment area sizes through grading. On the other hand, grading might enable us to divert cloudburst from the plantings, thus protecting them from flash erosion and long periods of flooding.
Manipulating water retention within the planting bed. The easiest and most obvious way to do this is through choice of planting substrate; if we don't need to optimize the plantings for storing large quantities of water for short periods of time, we could pick a substrate that stays moist for longer periods of time instead. Another option might be some kinds of cisterns or reservoirs where water is harvested below the main root mass of the plants.
Manipulating the drainage system. The amount, size, type and location of drainage measures will influence flooding depth, flooding length and sensitivity to flooding. In this particular project flooding wasn't a huge issue, but in a different project this might be a decisive part of the puzzle of securing goal fulfillment for the plantings.
Adapting plant selection to site is vital for sustainable green spaces in the long term, but sometimes adapting the site to the plants can be just as crucial for securing diverse, healthy and high-performing vegetation. Increasing water retention in the rain bed will improve the growth and performance of the pre-chosen dry shade toughies, and we'll also be able to include some slightly less drought tolerant species (but definitely no moisture lovers) into the mix to be able to get more aesthetic value and biodiversity support.
In my opinion, this is the point of applying the basic design framework: good results (goal fulfillment) can be achieved by employing suitable available means, while having a good grip on the existing reality on site (context). The framework also keeps reminding us that there are no default settings for a project, just like there are no default plants or default site conditions. It is precisely this specificity that makes planting design such a complex affair, and why typologies like "planting for stormwater management" or "green roof vegetation" often limit our understanding of the actual design assignment at hand. And when we don't understand the assignment, we also become blind to many of the available opportunities for ensuring the performance of vegetation as a component of urban nature-based solutions.