Stormwater running off a homeowner’s property can cause a number of problems such as erosion, spread of excess nutrients into waterways, infrastructure damage and wildlife displacement. In some areas, governmental agencies are taking notice of this damage and as a result, new building codes are being enacted that require mitigating actions to minimize such damage.

In 2018, my wife and I began plans for building a home on a five-acre plot in southeastern Pennsylvania that we had purchased about 13 years prior. Previously I had been involved with building other residences, but in the intervening years, much had changed with respect to many of the code requirements. Stormwater mitigation requirements were now a significant issue in that Pennsylvania county.

New codes to consider

To comply with stormwater codes, we contracted with an engineering firm to plan for a rain garden that would mitigate the effects of uncontrolled stormwater runoff and provide other benefits.

The plan took into account all the areas of disturbance including a long asphalt driveway needed because the lot was what is known as a “flag lot,” which typically has a long narrow section for a driveway through adjacent lots (the flagpole) leading to a rectangular plot of land (the flag).

The front of our lot is approximately 300 feet from the road with another 50-foot setback to the house. As a result, the driveway with additional parking space was quite large. Primarily because of the impervious surface of the driveway and roof, a large rain garden was mandated by the county codes.

The garden begins on one side of the house, continues all the way around the back and then extends part of the way up the other side of the house. It includes an overflow pit where water exits into an area of riprap and an emergency overflow low spot in the rear of the berm in case of blockage of the overflow pit. The roof gutter drains and sump pump discharge also feed into the garden so that water will return to the water table.

Functions of a rain garden

Aside from the obvious control of fast-moving water to prevent erosion, the rain garden provides other benefits.

First, it returns as much water as possible to the water table through a construction method that includes amended soil in the bottom. The amended soil used in this project consists of approximately 80 percent sand mixed with 20 percent organic material. The garden enclosure retains the water runoff and the amended soil allows it to percolate into the water table.

The second benefit is delivered when the design features trees, shrubs, grasses and flowers that are attractive to birds and pollinators. Our garden plan included an extensive list of native flora that could thrive in the rain garden conditions and provide food and habitat for wildlife.

The ideal choice of flora for a rain garden is not as straightforward as it may seem. First, the plants should be species that are native to the local area and not invasive plants, like English ivy for instance. Locally native plants host native insects like caterpillars that provide birds with essential food when raising their young. When acquiring seeds or seedlings, be careful that all the species are in fact native plants.

Second, the plant species should be tolerant of or adapted to standing water and occasional submersion at the base while also able to withstand dry periods when the permeable soil does not retain as much moisture as conventional soils.

Third, the plants should be varied enough to provide a food supply, whether pollen or berries, that peaks at different times throughout most of the year.

Construction

Construction on the house started in October 2019, but the rain garden could not be started until April 2020 because the grass surrounding the garden had to be growing first. If the grass were not in a condition that could control runoff of rainwater, sediment would flow into the depression, covering the amended soil and impeding its ability to allow water to seep into the ground below.

In the beginning of the construction, the sides of the garden were configured, and the perforated drainpipe that feeds into the overflow pit was covered to prevent the amended soil from clogging the pipe. The soil on the edges of the depression was a traditional topsoil for preservation of grass during rainfall, while the soil in the bottom was the amended soil that would readily absorb water.

First year

In June of 2020, the garden got its first real test. We had a very heavy rain in a short period—and the garden did its job. All the water that would have washed into local waterways was captured in the rain garden where it remained and slowly percolated into the groundwater.

By August of that first year, the grasses had completely covered the bottom of the garden.

Second year

During the first summer, it became apparent that the original plan had a weakness. At the large end of the garden where most of the water entered, there was a steep slope and that—combined with the volume of water—created a fast-moving stream that was causing erosion. To solve that, a causeway of stone was created to slow the water across the underlying soil and prevent that erosion.

If you look at the entrance to the garden you can also see a solution to another problem that became apparent early that summer. When the now-emerging young tender flowers appeared, the neighboring groundhogs thought they had found a salad bar. Before long they had eaten a large section of the new plants.

To solve the groundhog problem, I constructed a low solar-powered electric fence to ward them off. It was a good solution. One day I happened to be watching the garden and saw a groundhog approach the fence. When it touched the fence, it jumped into the air and scurried back into the woods behind the property to find another meal. The fence solved the problem and is only on through part of the year.

During this summer, a number of the original flowers bloomed. By far, the highest percentage was black-eyed susan (Rudbeckia serotina), but some purple coneflower (Echinacea purpurea) and coreopsis were also present. By this time, a significant number of pollinators were regularly visiting the garden. I was surprised to learn from Penn State that there are over 400 species of bees native to Pennsylvania—mostly solitary bees that don’t pose any danger of stinging.

At the end of the summer, we purchased a seed mix designed for this region of Pennsylvania containing a number of perennials. They were mixed with sand and distributed in various parts of the garden in anticipation of even more flowers the following year.

Third year

The summer of 2023 was the third year for the garden, and we were anticipating a more diverse group of flowers. We were not disappointed. It appears we now have a number of new flowers including bee balm (Monarda didyma), goldenrods (Solidago), swamp milkweed (Asclepias incarnata), dianthus, asters and many other natives including wild bergamot (Monarda fistulosa) and horseweed (Erigeron canadensis).

Many more pollinators were now visiting. In addition, later in the summer and fall as the plants matured, many small birds were flying in to feast on the seed pods. Those pods and the dropped seeds continued to attract birds after the plants themselves had gone dormant for the winter.

As with any garden, maintenance is necessary, especially if you want to give the new seedlings a head start in the spring. Just before the new seedlings appear, we go into the garden and remove the remaining dead material that would otherwise cover many of the seedlings. In the wild, that process does not occur, but doing so yields a higher number of flowers throughout the summer.

A rain garden of this size is no trivial undertaking. The initial cost in land preparation including delivery of many truckloads of amended soil is high. However, so are the benefits. Stormwater runoff on our property is now very well controlled and instead of causing erosion, it feeds precious fresh water back into the water table.

Pollinators benefit greatly from the variety of plants throughout the summer. The seeds and berries in the fall feed a lot of birds. And the beauty of all those flowers and birds in the summer adds to the enjoyment we receive when sitting on our patio. It will be interesting to see how the garden progresses in future years. As Martha Stewart would say, “It’s a good thing.”

Top photo: A stone causeway slows water as it flows into the rain garden. A solar-powered electric fence was needed in year one to prevent groundhogs from eating the emerging wildflowers. Credit: Tom Perazella.