Follow me down the rabbit hole known as sustainability; to the land where questions lead only to more questions.
Fear not—sustainability is not about finding definitive answers—it’s about equipping ourselves with the most current information and tools available to craft the best possible solution with the resources at our disposal.
When it comes to the sustainability of rain gardens, the two most important aspects to evaluate are your plant and soil choices.
In this post I will go through the most common choices for plant and soil selections and how those choices are seen on the spectrum of sustainability.
Plant species that have been present in the region before large-scale human migration. Here in the United States, that is usually defined as prior to European settlement. These deep-rooted plants are preferred because they are adapted to the local climate requiring little to no fertilizer or water once established (if properly sited). These plants have also coexisted with the local wildlife for many millennia and feed many of the beneficial insects that keep our ecosystem functioning.
Native plants that have been selectively bred to have certain “desirable” physical characteristics such as a certain bloom color, foliage color or size. These plants typically retain their ability to thrive in the local climate without additional water or nutrients but it is still being researched whether the change in plant characteristics effects the plant’s wildlife value. It is thought that the change in color and shape of flowers may deem it unrecognizable or inaccessible to the beneficial insects that usually feed on the species.
These are plants that we consume such as vegetables, fruits, or nuts. You may not want to use low growing food when run-off from streets or other highly polluted sources are running into the rain garden. Below are some raspberries that I installed in a roof run-off rain garden a few years ago that are thriving in the conditions present.
Plants that are not native to the region but do not require additional inputs such as watering and fertilizing to survive. These plants are also deemed as non-invasive, meaning they will not escape into our open spaces and take over. Invasives are covered below.
Plants that are not native to the region and require special care to survive our climate with inputs like water and fertilizer.These are not preferred, but if they mean that someone who would not normally install a rain garden will do so, then as long as the inputs are minimal and it is non-invasive, it is still a net-positive in my book.
Annuals only live for one season and require the most inputs from resources like water, fertilizing, and labor. These require replanting every year and are typically not considered for a rain garden.
These plant species, typically exotics, will outcompete native plants and create mono-cultures in our ecosystems. These mono-cultures are typically not consumable by our native wildlife. Now you may be asking, “Who would landscape with these plants?” Well for those who are not familiar with plant species, invasives can quite often be used unintentionally. This is easy to believe because most invasives were brought here because of their beauty in a different region. I saw this use of the beautiful, but highly invasive, species loosestrife on my way home from work one day.
In a Native Soil Rain Garden the goal is to design the plant choices and basin shape to the existing soils and water conditions with only minimal soil amendments solely for healthy plant establishment (if necessary). This concept of extreme low-impact development requires much less labor and materials than deep excavation rain gardens, and creates substantially less soil waste. I go into more detail how to select your plants and basin dimensions in my section on Designing your Rain Garden.
A Deep Excavation Rain Garden is typically employed when a certain volume of water is required to be stored below the surface in a rain garden. These types of gardens have their place; specifically when installed on public lands where liability of standing water may be an issue. With the practice of removing a large volume of the native soil that is then replaced with any combination of sand, stone, compost, and/or gravel; this practice consumes a large amount of resources and produces a large volume of dead soil. For a long time, deep excavation was considered the standard in rain garden design, but the definition of what a rain garden is has been expanded beyond this single specification in recent years. You can go here to find out more details on why deep excavation should not be the standard.
An under-drained rain garden will have a perforated drain tile installed running horizontally about 3-4 feet under the soil that will move the water that has been filtered by the plants and soils onto another location, typically connecting into the existing storm water system. This may be necessary to meet a certain requirement by a municipality and, in my opinion, may be more accurately described as a vertical bioswale than a conventional rain garden. A majority of the water in these under-drained rain gardens is cleaned, cooled, and slowed before moving on to the storm system, with only a small amount staying onsite and slowly percolating into the surrounding soil.
It’s All Good
Often conversations on sustainability quickly get mired in what exactly is the most sustainable material or practice. When factors such as toxicity, embodied energy, cost and life-cycle are included, it is rare that the choice is clear.
Rain gardens are no different;
- Not all rain gardens have to include natives.
- Not all rain gardens should use the native soils.
- There is almost always a place for all the different design considerations.
It is more important that we have the discussions and experiment with the possibilities than it is that we find the exact design that we require to be repeated time and time again.
That’s boring anyway!
Do you have a certain plant or plant combination you like to use in your rain gardens?
Do you have a soil mix you would like to recommend?
Let me know below!
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Assistant Ed.: Stephanie Sefton/Ed: Bryonie Wise