From day to day I doubt many of us stop to think about soil.
What an amazing thing, this soil, which lies bustling with life just under our feet.
Ubiquitous and silent, soil has been here for thousands, sometimes millions of years, and will be here long after we are gone.
But for the most part we don’t even notice this non-renewable resource, unfortunately at our own peril.
So what exactly is soil, and what does any of this have to do with rain gardens?
Soil has a structure comprised mostly of sand, silt, clay, and organic matter, in which exists arguably the most complex ecosystem on the planet.
Soils contain our finite supply of small weathered particles derived from the large parent material, or original rocks, which support our rooted plants and by extension life.
Within a soil’s structure you will find bacteria, fungi, algae, protozoa, earthworms, insects, small vertebrates, and roots all living together.
These organisms come together to form symbiotic relationships of creation and decay known as the soil food web as they move through the soils, creating vast networks of channels and pathways.
So now what does this have to do with rain gardens?
The current prevailing thoughts on how to design a rain garden, especially in medium to fine soils, center on the practice of deep excavation of the native soils followed by replacement of these soils with any number of different mixes of sand, topsoil, organic matter, and/or gravel.
These coarse engineered soils are recommended in a majority of the rain garden diagrams you will find on a Google search, but they are all slightly different in what they specifically call for in their engineered soil mix.
It is difficult to find where this soil replacement practice originated, or exactly who is creating these soil mixes, but it is here and has, unfortunately, been accepted as the standard.
A rain garden with coarse engineered soils over fine subsoils:
– does not increase the infiltration rate of the subsoils.
– creates a textural interface resulting in a perched water table.
– discourages deep root growth through this textural interface.
– has a smaller water holding capacity than a rain garden with medium to fine soils.
– destroys the soil structure that may have taken thousands of years to develop.
– kills living soils and replaces it with “dead soils.”
– creates large volumes of soil waste.
– consumes large amounts of energy to construct.
– is not cost competitive.
– raises the level of expertise needed to install.
– should only be considered if an underdrain is needed to move the filtered water onto another location, in which case the installation may be more accurately described as bioretention.
A rain garden instead should:
– be designed where water sources, location conditions, basin dimensions, and plant requirements correlate with each other.
– employ “Right Plant, Right Place.”
– have minimally amended soils solely for healthy plant establishment.
– use the plants to increase the infiltration rate of the soils with their roots.
– utilize the overflow elevation to increase ponding depths as the root systems develop, if needed.
– take a holistic sustainable approach to development and work with nature to heal our soils rather than improve our stormwater issues only to add to a different development issue.
I’ll be the first to admit that these engineered soils diagrams look cool and seem like they would be a good idea, but the science isn’t there.
They do not ultimately increase infiltration.
They do not increase water storage.
They do not encourage deep root growth.
So then why are we doing this?
The reason why this is such a big problem is that these specifications, or “specs”, are being Googled and adopted by municipalities.
That is a problem because in order to install a rain garden you may need a permit, and to get that permit you have to install your rain garden to these specs, no matter the site conditions.
And these specs call for a scope of work that may not at all be necessary, will not be cost competitive with traditional drainage, and will be beyond the skill level of the average homeowner.
Therefor a rain garden gets overlooked and a traditional drainage solution is implemented.
Instead of an “engineered soils standard,” a rain garden should consist of a design process that evaluates the site conditions and then uses basin dimensions and plants selections to match these conditions.
This may include minimally amending the soils with local compost solely for healthy plant establishment, which allows Mother Nature to do the heavy lifting for us.
When you use tough plants that can thrive in your soils and climate, preferably natives, the root systems will drill down through the soil, constantly casting off dead cells that add organic matter, create air spaces and water channels, and breathe life into even the poorest of post-construction soils.
In cases of very poor or damaged soils, the elevation of the overflow should start low to allow plants to establish deeper root systems in the basin, and then the outlet should be raised over time to increase the ponding depth as the maturing plants can handle the larger amounts of water.
My next issue with “engineered” rain gardens has to do with their holistic sustainability.
While these “engineered” rain gardens are being installed to address stormwater issues created be our development processes, they are contributing to other detrimental aspects of development at the same time.
Indiscriminately killing and removing soils regardless of its composition is not a sustainable practice and creates large volumes of soil waste.
Furthermore, by using the soils on-site you don’t need to bring in any materials, most notably top soil.
A large portion of top soil comes from farm land scraped by developers before building their structures.
They do this because they can get money for the top soil.
Stop buying top soil and they will stop scraping top soil.*
* I will talk about this practice of land scraping more in depth in a future post, but for now, the developers are not the bad guys here; it is a matter of us wanting the lowest cost homes possible and not valuing the soils our homes are built on until we hire a landscaper who then goes and buys our posthumous soils back for three times the price.
Lastly on the sustainable front is the energy consumed and emissions created from the construction of these engineered rain gardens.
If you combine the excavation and hauling away of the native soils, hauling and installation of the new material, and the embodied energy of the new materials’ production and transportation, the “engineered” rain garden will most likely leave a much larger carbon footprint that will ever be sequestered by the biomass in the actual rain garden.
A potential net loss for the environment.
In closing, our continued adherence to an “engineered soils standard” is the single greatest hurdle for rain gardens to overcome before they can see mainstream adoption, and before they can begin to add to the measurable strides being made by other ecological practices that help mitigate our disruptions to the hydrological cycle caused by our development.
Until this standard is abandoned, rain gardens will be relegated to novelty status and only be used when it is directly mandated by the municipality.
The only time engineered soils should be considered is when a perforated drain tile is being used underneath the rain garden.
The coarse engineered soils would work well to slow, filter and cool the water as it travels down into the drain tile quickly, moving rain water through the rain garden faster and allowing more water to be handled by the rain garden.
This option might be utilized when there is limited space and a high design volume, or where the underlying soils have no infiltration; both of which you are more likely to find in large scale commercial projects.
It could be said that this use of engineered soils and a drain tile would be more accurately labeled as bioretention rather than a rain garden.
Engineered soils attempt to offer a way to bypass the knowledge needed to evaluate and understand a site, and then design to it.
Rather than trying to change the Earth into our own personal container planter, we should use the knowledge available to us to design our rain gardens with a holistic sustainable approach that diminishes our hydrological disturbance while also limiting the need for other detrimental aspects of development during the rain garden’s construction.
To do this, we design our rain gardens in a way that balances our water sources, location conditions, basin dimensions, and plant requirements; using soil amendments as normal horticultural practices call for in a given situation solely for healthy plant establishment.
Easier said than done right?