Can You Have a Rain Garden Without Rain?
The answer is, yes. What is a ‘Rain Garden’, anyway? Rain gardens are planted, shallow depressions or basins that will eliminate run-off from stormwater systems by collecting, promoting infiltration and allowing pollutants to settle and filter as the water drains through the roots of the plants into the soil.
Many of you are probably familiar with the ‘Bioretention Pond’ concept, where stormwater is collected in a larger collection pond planted with native vegetation to help filter the water from surrounding developed areas. Rain gardens may be used earlier on in the stormwater system than the ending bioretention pond point and can keep the water and pollutants filtered closer to the source. Rain gardens can be applied in many different situations and can take any many different forms. Some will simply be used to collect excessive water discharge from the down-spouting of nearby home roofs or paved surfaces, while others will be used in much large systems such as stormwater systems along roadways. When applied to a stormwater system, the form of rain garden typically applied to the site is more of a Bioswale that continues from early on in the swale or drainage system through different cells further down the swales. Overall, allowing a much greater level of infiltration, erosion reduction and pollutant filtration than the typical mown lawn grasses commonly present in stormwater swales along roadsides. Typically, rain garden areas will infiltrate 30% more water than a conventional lawn, or turfgrass stand!
A rain garden installed in the city of Port Townsend.
Another view of the same rain garden. This area is mostly collecting surface water from the surrounding sidewalks.
Another, small rain garden which has an inlet to collect running water from the street. The honeycomb drain is connected with the underground stormwater systems. Note the honeycomb drain for future references…
So, now we know a little bit about Rain Gardens and Bioswales… What are the specific ingredients? Plants can be many types of trees, shrubs, herbaceous perennials, bulbs or grasses. The site should be expected to get large amounts of water at a fast rate, and the plants should be adapted to these conditions and able to take well to the saturated soils for an extended period of time, depending on the drainage. The most effective plants are those that are adaptable to large amounts of water, or periods of dryness in a drought season. This is why I stated that you don’t need rain to have a rain garden, because sometimes the area won’t be very saturated at all, but this depends on the site, function and soil properties/drainage. Also, these gardens can simply be used to collect water from irrigation or other water uses around homes and buildings. In some cases, the water may stay in the area for days and in others, less than a day. Ideally, you want the water to drain and filter as quickly as possible. Just so you know, mosquitoes take four days to complete their life cycle, from egg to mature adult that is able to lay eggs itself. So, if you’re garden is draining the water before four days of still water, then you should be one happy gardener! As said before, plants of all types may be used. Native plants are very great to use in these areas and will certainly increase biodiversity and healthy habitat for many beneficial insects and wildlife! However, in this situation, plant selection should not be limited to natives. The function of the space is most important and as long as the plants are supporting the desired function, it is okay to use ‘non-native’ plants. Quickly spreading, U.S. Native plants are preferred in most cases. The reason we prefer native plants is because there are many non-native, ornamental plants sold today that actually have potential to be invasive to natural habitats, competing with desired native plants!
A bioswale site which was planted about two years ago, maintained by Matthew Berberich Professional Gardening. You can see the larger, clumping grasses, which are Dechampsia caespitosa, tufted hairgrass. The smaller, finer, spreading grass in between those brown clumps is actually the same plant, which self-sowed into the area, filling up the space! This is the goal, to have a dense, thick vegetative cover that is able to slow down surface water flow, and to have a denser rooting area to absorb more water. If you notice, there is an even finer grass closer to the viewer. This is a self-sowing grass, Festuca idahoensis (Idaho fescue).
The same rain garden/bioswale area, with the Washington Conservation Core Training Students who joined us for the day to learn more about installing and maintaining rain gardens! Here they are pulling invasive weeds such as Cardamine hirsuta (shotweed, or hairy bittercress), Taraxacum officinale (Dandelion), Senecio vulgaris (Common groundsel).
A close view of the previously mentioned Dechampsia sp. Other garden plants in these pictures incdlude: Holodiscus discolor (Oceanspray), Aster subspicatus (Douglas aster), Sambucus sp. (Elderberry), Betula sp. (Birch) and Mahonia aquifolium (Oregeon grapeholly).
A closer view of Mahonia aquifolium
After the WCC crew swept through, pulling weeds, cutting back taller grasses and composting. Typically, you do not want to be adding nutrients to the area, as one of the functions is to filter out pollutants and excessive nutrients. But, in the establishment phase of the garden, some compost (a natural form of plant nutrients) should be added to reduce weed competition and aid in plant growth and establishment. For some, this is an acquired natural look, especially this close to the side walks! But, if properly installer and maintained, this is a beautiful area! Remember, this is only the beginning of April, things will really start growing and flowering more soon.
The installation process for the rain garden/bio swale will vary in each site, but there are a few basic guidelines that are followed when installing such systems. Sometimes, the site may already be graded and a swale area may be present. In others, you may have a flat, backyard space that has poor drainage and excessive water buildup from paved surfaces, down-spouting, or both. With the swale, it is important to provide a well-drained soil profile to about 3’ deep. This allows much more drainage and pore space in the soil than typical compacted soil in swale or lawn areas, which doesn’t have good drainage. There are many plants that can have roots down to at least that deep into the soil, especially those adapted to seasonal drought periods! If the soil has poor drainage, it should be excavated down to about three feet below the surface and filled back in with at least 50/50 compost and the native soil. In the Pacific Northwest, it is recommended to add 40/60 compost/sand. On the East coast, this is not recommended, because when you mix clay soils (typical of Southeast Pennsylvania) with sand, it tends to make a concrete-like structure… Bad for plants, bad for drainage! So, I like to use half and half compost/native soil in the area. Actually, I like my coffee black. J When the initial excavation is performed, it is important to make the bottom of the rain garden or bioswale area level for each area. This avoids proper drainage and percolation into the compost/soil mixture. In a bioswale situation, multiple cells, or sections may be used to slow down, contain and infiltrate excess water. Below is a basic design for a bioswale project that I am planning to install this coming fall, 2011. The bioswale contains three cells, and at the end of each cell is an overflow area for the water to flow into the next cell after the previous one has been filled. There is a large swale circling a majority of the house which diverts water from three of the down-spouts off of the main roof. So, during a Pennsylvania rain storm, there can be a load of water flowing through there! I apologize for the poor picture quality. I am not a great photographer, but my wonderful girlfriend, Jessica, and my awesome sister, Gabrielle are really great at it! If you are looking for a professional photographer, go to them!
The WCC crew again. This time, we installed a small entry-bioswale at the beginning of a swale system. The water comes from the above paved surfaces and enters this small space, first. When we got here, there was no trench for the water to flow into, off of the roadway. We excavated the soil to the proper grade and divided a few perennials and grasses from the other rain garden sites!
The finished installation. 3/31/2011
A few rocks and boulders were scavenged from nearby locations to reduce larger debris from entering the piping system, which connect this garden to the next swale area.
This is the outlet from the garden we installed. From this point on, the swales are simply planted with turfgrass species, which we have said before, aren’t as functional as the native perennials, grasses and shrubs.
A picture of a bioswale at Matt’s house, which has been in place for almost two years! The plants in this garden are mostly U.S. Natives, and in the past five weeks I have been here, I have not seen the same water contained in the rain garden for more than a few days. This system is connected to a piping system from a few of the neighbor’s swales. So, when the neighbor is watering his lawn, Matt gets the excess water in his rain garden! Not to worry, though. Any synthetic fertilizers that the neighbor applies to his lawn should be absorbed and filtered into the rain garden area before it reaches the end! This is great because it will reduce the chance of those nutrients or pollutants from entering nearby bodies of water, especially streams and rivers. Very important!
Uh oh.. The next day was supposed to be sunny. But, we didn’t bring our rain coats, so it rained all day! This wasn’t very good because the plants didn’t have a chance to establish, obviously. It wasn’t too bad because the water flowed through the center of the garden and didn’t move any of the plants.
The other side of the driveway, where the water continued flowing down the swale, eventually into the stormwater system and out into the Straits…What we’re trying to reduce with better infiltration and absorption.
There is a build up of silt at the beginning of the garden, which is good. Keeping the silt contained rather than flowing into the pipes will prevent clogging. Soil is made up of a mixture of sand, silt and clay. Sand particles are the largest, then silt, and clay particles are the smallest. Silt has the lowest bulk density of the particles and does not aggregate together like clay does. So, silt is usually the dominant particle the gets washed with rapid water flow.
A different rain garden site which the WCC crew cleaned up the previous day. The rain gardens are working, but the swale area wasn’t engineered properly before we got there to improve the site.
This is the other side of the driveway from the last two pictures. The problem with this site is that the surface area that the water is coming from is too large for these spaces and the swale system does not continue after these ones, and the two sections planted are not connected underneath the driveway. So, the water diverts unto the road and back into the second one. Not bad, at least it’s going back into the appropriate space..
A desired, native shrub in these rain gardens that is similar to one mentioned previously, Mahonia nervosa.
Below are some examples of potential rain garden sites, or problems commonly associated with these sites, before and after installation. Look at the oil on the surface of the water…
The area which would be great to excavate and turn into an attractive rain garden! We were going to work with the Conservation Core to install this, but it was not in the budget and time limited with the crew. This is located at the Fort Warden beach area and would be an excellent spot to educate the how’s and why’s to these gardens and what we’re trying to do!
Remember the honeycomb drains I mentioned before? Well, here they are used again. This is in the city of PT, again. The problem with this ‘rain garden’ is that the drain is positioned too low for enough water to collect in the site. So, minimal water is absorbed into the soil or plants, somewhat defeating the purpose of have plants in the space. If the drain was raised at least 8″ higher, the garden would infiltrate much more water, rather than have it flow into the stormwater drains.
A client that we do monthly maintenance for. This inlet takes water from the parking lot surfaces, down into the rain garden to be filtered! Down spouts also feed into this system.
An appropriately placed drain, higher than the flood level, but below other paved surfaces which collects and filters as much water as possible during rainy periods!
One more poorly engineered rain garden space. This rain garden collects surface water from a parking area that is at least 2,500 square feet of surface area! That is too big for this small garden. Also, this garden is not connected with any drainage systems or overflows. So, the results is major flooding of the entry point and several parking spaces.
Look at that oil! It’s sad that this could have been collected and filtered int he rain garden if the site was engineered correctly.
Well, that is my post on rain gardens and bioswales. I hope it was informative! There is a lot to talk about with this subject and I am excited to continue collecting data from previous installations and installing these systems more, myself! I thought I would post a picture of the Olympic Mountain Range that I took the other morning for you to enjoy! I know I said before that I was going to post about North Olympic Salmon and their habitat/restoration of the habitats. But, I believe I will post that one next! Feel free to comment, and definitely ask questions! Thanks!