People discuss regularly which plants are good for stormwater management applications. Oftentimes during these discussions people wish they could find somewhere a list of suitable species for raingardens, ponds and bioswales. How do you choose the best palette for your project? To be honest, the answer is simpler than you'd think.
In my 10 years in the green industries, the question of "stormwater management plants" has popped up plenty of times, both in my studies and in client projects. Choosing vegetation for situations where water is the defining element is not the norm for most designers and projects (or at least it hasn't been), and as such creating plantings for ponds, streams, bioswales and rain gardens may seem daunting. Which plants work in shallow water, which can take shorter periods of flooding, and are there plants among these categories that can even tolerate periods of drought?
When people ask for references to use in species selection, usually somebody has a document or two to share on the subject. These documents usually come in list format, which then may later be compiled and synthesized into longer lists that are more adapted to relevant climates or specific projects. I would know, since I've worked on both general and specific lists, with a master list that never quite gets done. But it is above all the project-specific lists that have given me insight on how to make the best of any planting assignment in stormwater management solutions. So here's my (almost) fail-proof strategy for tackling these projects:
Just treat them like you would any other planting design assignment.
…That's it. Easy, right? All you have to do is to follow the handy chart below:
Aw. Dang.
This is not to say that stormwater management solutions do not require special considerations that are not as dominant as when doing decorative plantings in less extreme situations. When I started my studies, the focus for planting stormwater management solutions was finding plants that thrived in wet conditions, so the question was "how to plant ponds/ wetlands/ channels". A few years later the focus had shifted into rain gardens, where rainwater from smaller catchment areas pooled shortly before being infiltrated. It was assumed that these would be created in normal garden soils and have average to moist soils for the most part. Since then, the conversation has pretty much always been about urban stormwater solutions being deathly dry for the most part, and then occasionally becoming severely flooded for indeterminate amounts of time. Additionally, garden soils are increasingly being replaced with soils engineered specifically for urban stormwater applications (more on these will follow in later posts).
I believe that the main current that has influenced this view has been the typical urban climate change threats, where relentless summer heat is paired with rare but violent cloudburst events that clog the pipe system for days. With that in mind, let's take a new look at the question. What most people mean by "Do you have a list of plants that work well in stormwater management projects?" is that they're asking for recommendations for planting rain gardens in demanding urban situations where drought and flooding alternate. So to rephrase the original question: "What do you have to add to our knowledge base of ornamental plants with a wide tolerance amplitude on the water availability continuum, including tolerance for periodical extremes of drought and flooding?" Much better! So let’s break this question down into its components.
“Ornamental plants” in this context doesn’t just mean showy, highly bred exotic species, but rather denotes that the plants are chosen for specific, human-centric reasons: beauty is in the eye of the beholder, after all, and can be closely related to functional or moral values (The common aspen tree Populus tremula, for example, is host to a great number of threatened insect species, participates effectively in water cycling between soil and air, and it’s a native species here in the Nordics, as well. Ain’t it pretty?). As such, “ornamental” in this context covers both native and exotic plants of wild and hybrid origin, as long as their inclusion in the planting is intentional, and that they're not primarily chosen for production purposes.
“Wide tolerance amplitude” is a fancy way of saying “it’ll survive anywhere”, or, more specifically, that the plant will survive in a wide range of conditions on a specific continuum, like water availability (No water – deep water), light availability (full shade-full sun) or salinity (no salt – highly saline). Each plant’s tolerance range is always limited, though, and we rarely know where the limits are. “Drought” and “flooding” are not specific points on the continuum either, but rather ranges within the continuum, defined both by absolute amounts and depths of water at any given moment, but also by the timing and length of the period when a certain condition persists. It’s easy to understand that a week without rain might not kill a plant, but that two months might; Another plant might thrive at a water depth of 5 cm in early spring, but will not tolerate wet soils during wintertime. Additionally, the limits are often interconnected in ways that might not always be obvious. A good example here in the Nordics are many shade-tolerant plants. We can often put Hostas and Astilbes into full sun, and they’ll survive or even thrive, as long as the soil is moist enough. If the soil is dry, though, they’ll give up the ghost much faster in the sun than they would in the shade. That means that at the very least their light- and water tolerance ranges are interconnected with each other, but probably even with other factors such as temperature averages and extremes.
The takeaway is that it is, in fact, entirely possible to collect information on drought-tolerant plants as well as flooding-tolerant plants, and then cross-reference these lists to find plants that tolerate both extremes, at least to an extent. But can the results of these studies be called “lists of stormwater plants?” A simple reference to the chart above reminds us that the water availability continuum of sites and the tolerance amplitude of a specific species are just two pieces of the bigger puzzle of a planting design assignment. So even if we assume that the plant-water-relationships are similar across different sorts of stormwater management applications (hint: we shouldn’t), there are still plenty more variables that influence the choice of plants for any given site. Holistic context, for example.
In 2014 I participated in an open design competition for Stormwater Management solutions in the city of Lahti, Finland, as a part of a student team. The city center has a variable topography, so depending on whether the planting was on a roadside slope, in a park at the base of a hill or part of a market square it would capture and retain variable amounts of the rainfall. These three situations are also different from each other in context: median strips are passed by in haste, market squares are expected to be clean and architectural in quality, and in park environments the plantings may be either scrutinized in peace or read as a part of the park's overall plant mass. These considerations led to us developing a plant list for each these three situations as a part of our competition submission. These lists were then subdivided into zones on a mini-moisture gradient, a thought copied directly from a book on bioswales that I stumbled upon during my exchange studies in Germany. Basically, something like this:
During my time in Germany I had, in fact, stumbled upon “lists of stormwater plants” from LWG-institute in Veitshöchheim (see links in the link box). I did extensive cross-referencing between the lists in these documents with other materials that would tell me more about the attributes of specific plant species, and then relating them to the site. Which ones would be hardy enough in Finland? Which were showy enough to be used in high-profile situations? Where would I apply the “mostly dry”-list, where the “mostly wet”-list? Had this been a more detailed project, I would also have needed to check light conditions and light requirements, possible height limitations from safety standpoints, considered whether or not road salt was a problem, and so on.
What becomes apparent through the examples above is that "stormwater management" or even "nature-based solutions for stormwater management" is not just one thing, but multiple different situations with varying positions on the wet-dry- or urban-rural-continuum, and the main problem to solve could be qualitative (water cleansing, aesthetics, habitat value) or quantitative (retention capacity, flow speed, storage function), just to name a few aspects. It’s been the same process in all of my stormwater management-adjacent projects ever since: Analyze the site, define goals and constraints, and find appropriate plants. Just like with any other planting design assignment. As a result, I am quite confident in saying that there are no specific “stormwater management plants”, and so it follows that any lists that circulate in the profession are, in fact, lists of plants chosen for specific sets of circumstances, regional climate usually underlying all choices on the list.
I love lists as much as the next person (or maybe a bit more), and so I’m definitely not saying that lists of plants with certain attributes or functions don’t have value. But they need to be seen for what they are: compilations of plants made according to a specific set of criteria that, partially or as a whole, may or may not be relevant to your project. They will not free you from site-specific considerations and cross-referencing sources, much less from the unavoidable uncertainties with regards to the unfolding of the site and plant performance within it. (If I’m being honest, objective and generalizable knowledge on plant performance in general is very shaky. But I’ll get back to that in future posts.) You’ll find some of my favorite lists on the subpage "Links and resources"; use them wisely.
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