Welcome to Part 2 of the series.
At this point you should have a specific location selected for your rain garden with help from Part 1.
Now we need to evaluate your prospective location’s drainage with a combination of observations and analysis.
Calculating exactly how well a location drains can be a complex and involved process that requires a knowledge of hydrology and soil sciences.
Unless your rain garden requires a very detailed analysis, a few simple tests can give us the information we need to estimate the site’s drainage.
First we will make a few general observations of the surrounding land.
– Are you at the low area for the surrounding hundred feet or so, or maybe a little plateau area on a large hillside?
Water from runoff is probably collecting in this area.
– What kind of turf is growing and how well is it doing?
Bentgrass means a sunny wet location, great rain garden potential.
– Are there ruts from mowers in the turf?
Even if they are currently dry, it is wet there for some period.
– What kind of shrubs and trees are growing?
If there are Willows, Cottonwoods, or suffering evergreens growing there are probably saturated soils. You might not be aware of it, but you probably know what trees grow around water and which grow up on dry hills even if you don’t know their names.
– Are you close to any surface waters like a lake or river, stream, creek?
This surface water can help show the localized water table.
Soils contain the metals Iron and Magnesium.
These two metals are responsible for most of the coloring of your soil.
If the soil is reddish or yellow, the metals are oxidizing in the drier soils.
If the soil is blueish-grey, the metals are not oxidizing because they are saturated for a good portion of the year.
The more blueish-grey spots in the soil, the longer the soils are saturated through out the year.
It can be difficult to decipher soil color in the uppermost layer of soil known as top soil.
This layer can be dark brown or black because of the organic matter deposited over time.
You will want to get below this layer to see a clearer soil color.
This may be anywhere from 3” to 9” down in the Chicagoland area; less for developed lands, more for virgin soils typically.
Soil texture can be very helpful in understanding the drainage conditions of your rain garden.
Soil has three main size categories; Course (Sand), Medium (Silt), Fine (Clay).
Course soils allow the water to move through the soil much quicker than fine soils.
To test your soil texture there are two different methods I would recommend, the “Texture by Feel” flowchart and the “Jar Test.”
If you want a quick on site test I would use the “Texture by Feel” flowchart below.
This is modified from S.J. Thien. 1979. A flow diagram for teaching texture by feel analysis. Journal of Agronomic Education. 8:54-55.
Once you have your soil classified, you can use the Soil Texture Triangle below to decipher your soil composition.
But if you have a little time and want a fun science project, especially if a little one is involved, I would recommend the “Jar Test” test.
For this project you will need:
– One Clear Jar, preferably with a flat bottom and straight sides, which I don’t think exists, but just one with minimal contours would be good.
– Clean Shovel
– Clean Bucket
– Pulverizing materials
– Permanent Marker or tape
Dig a roughly 1 foot by 1 foot hole at least 6 inches down and toss this top soil aside, you can replace it after you are all done.
Then at this 6 inch depth, dig down a little further and remove about a half shovel full of soil into a clean bucket.
Let this soil dry out over night, or maybe even a couple of days, on some newspaper in the garage.
If you want to cook it in the oven to dry it out quickly that works too.
Pulverize the dry soil.
I put my dry soil in a used plastic cool whip container and use the end of a 2″ by 2″ to crush it, but that’s mostly because that is what I found in the garage the first time and it worked.
This pulverizing is very important so that you get the soil as fine as possible so that it settles out properly.
Remove any rocks or pebbles.
Once you have your soil sufficiently pulverizes, fill up your jar about 1/2 full with soil, and then fill it all the way up with water.
I have read to use detergent to help separate the soil particles.
My experience with this was that it expanded the entire soil profile and I ended up with bloated and in accurate soil quantities, I don’t recommend it.
After your jar is completely filled with soil and water, put on the cap and shake it for a good minute vigorously.
Set your jar down on a level surface and set your timer for 1 minute.
As I set it down I like to give it a couple quick light little spins for the first 10 seconds on the table-top without lifting it up to evenly spread out the sand that is quickly settling, this gives a nice clean settling line.
After 1 minute mark the line of settled out soil with either a permanent marker or piece of tape, this is your Sand Layer.
Set a timer for 1 hour, at the end of which you will mark another line which will be your Silt Layer.
After about 6 or more hours you will have a pretty good settling out of the Clay Layer, although it can take days to completely settle out.
Measure each layer and then divide that unit by the entire soil profile and that will give you your soil composition.
In general, the higher your clay content, the slower your rain garden will drain.
This means a wetter rain garden for a longer period.
If your clay content is above 50% I would recommend mixing in some organic matter like compost to help with plant establishment.
It is sometimes recommended to excavate and replace existing clay soils down to a depth of 3 or more feet with engineered soils that drain quicker.
Although these soils themselves drain quickly, the water stops as it hits the textural interface of the existing clay soils below.
I have a strong opinion on these “deep excavation” rain gardens and why they should be the exception instead of the norm, which you can read about here.
So after finishing this post you should have:
– Your water source(s)
– Your water source calculations
– Your location details
With this information we can now begin to Design your rain garden.
I have begun work on the Design Series and will be publishing soon.