There are many reasons to build a rain garden. Rain gardens help filter out pollutants like bird guano from stormwater and turn them into nutrients for your garden. They help reduce the draw on local aquifers to irrigate our gardens and allow those aquifers to be replenished by the natural water cycle. Because of this, rain gardens are essential in the fight to reduce stormwater pollution’s impacts on river systems, which in turn, end up in our beloved ocean. Stormwater is one of the largest sources of environmental pollutants entering the world’s oceans today. Rain gardens can even help reduce the populations of mosquitoes and other biting insects who rely on stagnant water to breed. As climate change brings diseases like malaria further north, managing mosquito populations becomes less a luxury and more a public health necessity.
Apart from purely utilitarian reasons to build rain gardens, the hobby gardeners and organic farmers of the world will find that rain gardens also provide an opportunity to sculpt a beautiful new aesthetic which conventional gardens simply cannot match. To get started, all you need is a ruler, a level, and a calculator.
Doing The Math
Rain gardens capture the rainfall from impermeable surfaces that flows across your property. Then, using nothing more than the natural slope of the land it collects and disperses that water into the garden and out into the local watershed beyond. If done properly, this water should collect and drain away within 24 hours of any given rainfall. Of course, to capture all the water falling on your roof, driveway, patio, and other impermeable surfaces you’ll need to know roughly how much water to expect.
Start by figuring out how large the total area of impermeable surfaces you are working with. Measure all the roofs on your home, garden sheds, barns, and other structures. Add that to any paved or concrete surfaces, and you will have the total area squared where rainwater isn’t being absorbed directly into the ground. Next, find out the average annual precipitation for your region. Historical data in the time of climate change is somewhat unreliable, so using the average over the last five years will give you the best picture of what to expect. Once you know these figures, you can easily calculate the average rainfall per square meter you can expect to collect. If you know that amount, you can figure out how large your garden needs to be, as well as what plant species you will be able to support in the garden.
Remember, not all surfaces will drain to the same garden space, and so it may be useful to figure out which spaces flow onto which parts of your property, especially in “flat” areas where the land will actually slope off in different directions. You will want to mark off the areas where you plan to put your garden and figure out the best path to guide water to those spots. It’s at this step where you need to also decide what shape you will use in your garden design. The most common are circular or kidney-shaped, but the lay of the land will play a big role in determining what is actually possible for you.
Engineering the Space
Water flows down the hill. This is the most important consideration for your rain garden because you will need to work with your space’s natural slopes to collect the water. Depending on what you are trying to grow, and how much water you expect to fall, you will want to dig a series of holes in the garden which flow from highest point to lowest point to make sure that your garden gets maximum irrigation. When planning this out, remember that the lower end of each pool needs to be geo-engineered into a berm to hold the water from flowing any which way, so you will likely be using the earth you are digging for this purpose.
These berms provide an excellent growing environment for any water-loving plants and will add that multilayer aesthetic mentioned earlier to any garden space. While the walls of your reservoirs should allow water to pool, you also want to make sure that there is a place for that water to flow out. The best way to achieve this is to dig an eighth of a meter deeper than the reservoir pool and fill in that extra space with a permeable aggregate like crushed gravel or crusher dust. This will allow water to flow through the absorbent topsoil and manure layers, bringing nutrients down to the roots of your plants before flowing out and into the local aquifer. Once the porous aggregate has been put in place, remember to test that your drainage layer works properly. You will want to repeat the test after each layer of earth is added to the reservoir in order to ensure there is adequate drainage, or else you will end up with a swamp instead of a garden.
It is advisable to put in rain barrels to store extra water for drier weather. Rain barrels are best positioned higher up in your garden to allow them to collect the maximum amount of water, as well as to simplify the process of using that water when you need it. Most designs feature a spigot release which allows the reservoir in the barrel to flow down into the topsoil directly. This allows osmosis and gravity to move the exact amount of water the garden will need, conserving your rainwater supply and eliminating the need for any sort of pump or pressure system. To turn off the water flow, simply close the spigot on the barrel and let the water levels build in the barrels again. Remember to install enough barrels to store a third of the rainfall on your property from any given rainfall. Using the annual precipitation on the impermeable services you calculated earlier, you can figure out how many barrels you will need for this purpose.
Plant and Grow
Once you’ve hooked up the rain barrels, filled in the reservoir with earth, and made sure the drainage works properly, it’s time for the part that every gardener loves- sewing the garden. Remember that taller plants need more water, but can block their smaller cousins from getting enough sunlight if they are positioned wrong in the garden. For this reason, it’s important to know which plants will have the best time to grow in your garden. It’s usually best to have a large variety of indigenous plants in your garden to make sure that your garden isn’t introducing invasive species into the local ecosystem. Whether it’s an edible garden, floral, or a botanist’s mad science; your garden will always be more resilient if you plant indigenous species.