Saturday, April 2, 2011
Rain Gardens 101: How to Choose a Site
So you like the idea of a rain garden. Capturing runoff, filtering pollutants, growing native plants, supporting biodiversity and recharging groundwater all appeal to you. But where do you put one?
Well, the first thing to do is take a close look at your property. Is there a natural depression or bowl-like part of the garden or yard? Have you noticed water pooling there when it rains? If so that's a prime candidate. And lucky you--less digging!
Even if you don't have a natural swale (i.e., depression or bowl-like area), look for places that become waterlogged in the rain. Maybe there is a spot by a downspout that puddles, even when it's not a torrential downpour. These places are already collecting water so by planting a rain garden you can help along the process of the ground absorbing the water.
Let's say there's no place like this on your property anywhere, and rain seems to drain away from everywhere quickly. Well, then you can choose a site in the vicinity of a downspout and plan to dig the depression yourself (more to come on that).
One thing to note: Your rain garden needs to drain completely in 48 hours or you run the risk of hosting a mosquito breeding ground! So before breaking out the shovel, test your location by digging a small hole 6-8" deep and fill it with water. If it drains in a day or even less, you're good to go. If the water is still there after two days you should look for a site elsewhere.
In my experience, I found places in my garden where extensive puddling was keeping the ground bare, and I planted water-loving yet drought-tolerant plants (more details on those in another post). My rain garden is in a spot where the gutter overflowed and the downspout is nearby, so with even a mild rain this patch of dirt turned into a huge puddle. It seemed to me a natural fit. (See the original site in the photo above.)
Similarly, in my front border there is a spot where another gutter overflows (yes, we do clean our gutters but it's nearly impossible to keep up with all the mature trees around here).
I didn't dig a rain garden per se, in fact this was before I planted my actual rain garden. I just planted things such great blue lobelia (L. siphilitica) and Joe-Pye weed (Eupatorium maculatum) to help the absorption.
The point is to listen to what the land is telling you. If there is a spot where water naturally collects, then it's a good bet that site can support wetland-type plants. But, if you don't have a naturally soggy spot you can still increase rain collection and absorption by creating one. Keeping it relatively close to a downspout will allow you to direct more water into it, thereby increasing its ability to do all the good things rain gardens do.
The Big Caveat
The prevailing wisdom is to site a rain garden 10 feet from your house. I learned this from two different presentations on rain gardens (one by with Wild Ones with a speaker from Prairie Rivers Network and one from the local Wildflower Preservation & Propagation Committee). The sensible thought is to keep water from pooling near your foundation and basement were it could cause a leak. If you've located a spot kind of close to your house where you'd like to plant a rain garden, think long and hard about digging from that spot a few feet out to keep the rain garden away from your house. You can dig the rain garden at 10 feet away, extend your downspout with a plastic extension piece and run it down a path of river rocks (more details on that later too).
Now, in all honesty, my rain garden is not 10 feet from my house. You can see in the pictures that it's right next to my house. Why? Because my house is on a slab (so no basement to flood), and I tested this spot for permeability with the 6-8" test like I mentioned earlier. The water drains here in barely a couple hours, so I was confident that drainage wouldn't be an issue. Plus, I made a relatively tall berm against the house to keep water from saturating the foundation.
So yes, it can be done but I wouldn't recommend this across the board to anyone and everyone interested in a rain garden. You have to evaluate your site based on your conditions and your house!
So what did we learn?
1. Pick a spot that already collects water or a place sort of by a downspout where you can dig a depression.
2. Make sure the water drains from your site in 48 hours or less.
3. Evaluate how close the rain garden will be to your house and keep the 10-ft guidance in mind.
Next up...how to actually dig the rain garden!
FYI: Dr. Stacy James from the Prairie Rivers Network says April and May are the best times to create a rain garden because the soil is soft but the weather is not too hot!