When I first began researching rain gardens I found that many of the designs involved excavating and removing on average about three feet of the existing soils and replacing it with an engineered soil mixture that usually included some specified composition of sand, compost, top soil, and/or gravel.
I would see the same deep excavation designs and specifications on different non-profit or municipal websites over and over again.
I began to wonder; who is actually creating these specs?
Although this concept of deep excavation sounded good, and it sure looked really cool, was there really any justification for it, or was this just being googled, copied, and then pasted.
I started to think about what was happening in these specs.
Were these engineered soils actually improving infiltration?
The water is certainly going to drain faster through these coarse engineered soils, but what about when the water hits the native soils again below the engineered soils.
It isn’t going to move any faster than before.
If anything, the water and roots would travel even slower into the native soils below because of the textural interface created from the two different soil types.
So if this deep excavation doesn’t ultimately help with infiltration, maybe it has some added storage benefits.
No, it doesn’t
Fine soils have more surface area and therefore actually have a larger water holding capacity than coarse soils.
Even so, the void space in soils is pretty minimal and very costly per gallon compared to other storage options like a dry-well.
Well the replaced soil helps the roots dig down deep.
No, it doesn’t.
Do we dig out everywhere three feet deep and replace the soil with sand, compost, and topsoil when we put in plants?
Ironically we are discouraging deep root growth because of the textural interface of the soils which is creating a root bound mass in the engineered soils.
In the end I came to this conclusion.
This process of deep excavation creates large volumes of soil waste, consumes large amounts of energy to achieve, raises the level of difficulty to install, and limits the overall implementation of rain gardens due to the excessive construction costs.
Soil amendments should be used as normal horticultural practices call for in a given situation solely for healthy plant establishment.
Rather than the practice of soil replacement, rain gardens should be designed where water sources, location conditions, basin dimensions, and plant requirements correlate with each other.
The only exception to this would be if a perforated drain tile was used underneath the rain garden with engineered soils.
The engineered soils would work well to filter and cool the water as it travels down into the drain quickly, moving rain water through the rain garden faster and allowing more water to be handled by the rain garden.
This would be a great solution to a location with limited space and a high design volume, or where the underlying soils are garbage with no infiltration, both of which you are more likely to find in large scale commercial projects.
This use of engineered soils and a drain tile may be more accurately labeled as bio-retention rather than a rain garden though.
This is the quick version of a longer post that contains more specifics on this topic that you can find here.