Planting a Successful Rain Garden - Part 2: Garden Size and Water Flow
Natalie Britton
As we explained in part 1 (link) of this series, rain gardens help capture stormwater on your property to reduce pooling water and address stormwater pollution. You can read that first blog to learn how to determine the right location for your rain garden. In part 2, we’ll help you determine the correct size for your rain garden and manage water flow.
- Natalie and Liliana, CRWC Green Stormwater Infrastructure (GSI) Technicians
Liliana Holyfield
Unlike standard gardens, rain gardens must be a precise size so that they can handle the amount of rain and runoff your yard receives. Soil type also plays a big part in determining the size of your garden.
Soil Type
The goal of a soil infiltration test is to ensure that your garden is the correct size for capturing and draining water within 24 hours. Your garden also should not be so big that you have to overwater during drought seasons. Each soil type drains at different rates. You can use the following formula to calculate the size of your garden.
We then multiply that number with the square footage of the roof area on the connecting downspout to get your garden size.
Soil type (see below) x sq. ft. of roof area connected to downspout = garden size
Soil type = sand (.19), loam (.34), and clay (.43)
We’ve had a couple of experiences where a soil infiltration test completely shifted a rain garden design. In one instance, we conducted a site assessment during the rainy season in early May. To quickly estimate the soil type, we started with a simple touch test. Sandy soils will easily glide off the palm, while more clay-dominant soils can ribbon and stick together. Due to the sample’s ability to make a short ribbon, we assumed it was loam. About a month later, we came back to the site to complete a soil infiltration test. We began by calling Miss Dig before creating a 12-inch hole near the garden location. We then filled the hole with water and let it completely drain.
This step saturates the soil before the test. We filled the hole again with water and waited for about 15 minutes until it completely drained a second time. If it takes less than 6 hours for the hole to drain, it’s likely that the soil is sandy. Thanks to this test, we knew that the soil was in fact sand and not loam. As a result, we had to reduce the homeowner’s original 67-square-foot rain garden to 37 square feet. If we hadn’t completed the soil infiltration test, the garden would’ve been too big to sustain itself without the constant need for water.
Water Flow
Water flow is important to your garden because water willfunnel into it directly from one or more downspouts. As roof runoff flows into your garden during rain events, your deep-rooted native plants will help filter and reduce pooling within 24 hours. There are multiple ways to connect your downspout to your garden including a gutter or a downward-sloped rock swale. You can find guidance on this topic through Master Rain Gardeners' free webinar.
Visit our website to learn more about native plants and join us for our native plant sale on TBD. And don’t forget to check out our rain barrel sale now through Sept. 6.
Conclusion
It’s important to determine the correct size for your rain garden and to manage water flow. Be sure to perform a soil infiltration test to help you calculate the correct area for your garden and to funnel water directly into your garden.
FAQ
1. How large should my rain garden be?
Use this formula to determine the size of your garden.
Soil type (see below) x sq. ft. of roof area connected to downspout = garden size
Soil type = sand (.19), loam (.34), and clay (.43)
2. How do I determine my soil type?
Began by calling Miss Dig before creating a 12-inch hole near the garden location. Then fill the hole with water and let it completely drain. This saturates the soil before the test. Next, fill the hole again with water and wait until it completely drains a second time. If it takes less than 6 hours for the hole to drain, it’s likely that the soil is sandy. Heavy clay soils may take up to 24 hours to drain and loam should take 6 - 12 hours.
3. How do I direct storm water into my rain garden?
There are multiple ways to connect your downspout to your garden including a gutter or a downward-sloped rock swale.
4. How do I learn more about native plants?
Visit our website to learn more about native plants and join us for our native plant sale on TBD.