Rain Gardens: The Ins and Outs

Niakwa Trail Rain Garden along Fermor Avenue. Photo by Mark Bauche

by Mark Bauche, Master Gardener, MALA, SALA, CSLA

Mark Bauche is a Landscape Architect and Master Gardener. Through his work at HTFC Planning & Design in Winnipeg, he incorporates climate adaptation into his designs and seeks out new tools to help landscape architects design more resilient landscapes.

Climate change projections foresee increased amounts of precipitation, especially in the shoulder seasons, and from extreme summer storm events. With all of this additional stormwater, it becomes increasingly important to manage run-off on our properties rather than sending it to the drain, where it can inundate our waterways. One of the most accessible, and indeed aesthetic ways to mitigate this problem is by installing a rain garden.

Rain gardens, simply put, are stormwater catches that hold water for short periods, allowing it to infiltrate the soil. Much of it is then taken up by the roots of water-loving plants, rather than going into the gutters. The plants in a rain garden also act as filters, removing and converting pollutants that might have otherwise ended up in local waterbodies.

But rain gardens are not just functional. They can also be a beautiful addition to your outdoor space. Rain gardens can take many shapes, sizes and styles. In a wet year, they can become very lush quite quickly. They also do not generally require a lot of maintenance. Within a few years of growth, due to the density and spread of the plants, weeding becomes less necessary.

There are a few types of rain garden, differentiated by the level and type of infiltration. Some use pipes and drains to deal with overflows. For the residential applications described here though, we will be looking at full infiltration rain gardens, which allow water to soak into the ground without conveying it elsewhere if it becomes oversaturated.

There are a few things to consider when designing and installing a rain garden. First, how scientific do you want to be about it? If you are looking to catch as much runoff as possible, a bit of math will help you determine the ideal size for your garden. This is done by finding the total area of the hard surfaces that will be shedding water into the garden (roofs and paved areas) and then sizing the garden based on a percentage of that area. In sandy soils this percentage is lower, as infiltration will happen more quickly. Rain gardens in clayey soils need to be much larger and shallower to achieve the same result. There are plenty of websites with formulas and calculators to help you right-size your rain garden if you decide to take this route.

No matter the size though, a rain garden will contribute to catching run-off. Unless you have specific targets for stormwater run-off reduction, you can make the garden any size you like based on your time, labour, and budget limitations. Ideally you want it to be somewhere between 100 and 300 square feet.

Take the time to analyse your site before starting the garden. Look at where water is flowing and standing after rain events. Examine where the sun and shade are. You’ll want the rain garden to be in a fairly sunny place. This will help with evaporation while the water is infiltrating, as well as provide ideal conditions for your plants to grow.

Find a good spot, ideally an existing low area where water is already moving to, but not a spot that tends to pond, as this is an indication that infiltration is not happening there. If there are areas that are ponding, these should be corrected and re-graded toward the rain garden. Also, make sure the rain garden is located at least 10 feet from the house/building foundations, 35 feet from any septic fields and 50 feet from a drinking water well.

Think about what you want the garden to look like once it is mature. Plan out the shape and form before you put a shovel in the ground. Decide on the look and style you are going for. Do you want lots of flowering plants, colour, height, etc. Do want it to look wild or more formal? Determine the species and spacing of the plants you want in the garden. Remember to size and space according to the plants’ size at maturity. You will also want to assess whether you need to amend your soil for better infiltration.

The rain garden should take the form of a saucer, between 3 and 12 inches deep, with gently sloping or terraced sides and a flat bottom. This is key for the rain garden’s proper function. It’s also important to avoid compacting the soil, especially in the basin. That soil needs to stay as permeable as possible.

When it comes to plant selection and placement, it is important to put the right plant in the right place. There are three zones in a rain garden. These go by different names depending on what guide you are using, but generally they are: upland, slope and basin.

The upland zone is the flat area at the top of the saucer. This is where you can plant trees, shrubs or perennials that prefer dry to moist soils. Good species to use here include dogwoods, hawthorns, cranberry, giant hyssop, little bluestem, sedum, and switchgrass, among others. These plants will be occasionally wet for shorter periods but will quickly shed water down into the saucer.

The slope or terrace zone forms the gently sloped edges of the saucer. Plants here will have water trickling past them on its way into the basin, so they need to like moist to wet conditions, and should be able to withstand standing water some of the time. Species to include here are coneflower, swamp milkweed, Lyon’s turtlehead, wild iris, blue fox willows, Joe Pye weed, marsh reed grass, and daylilies, among others.

Finally, there is the basin. This is the wettest part of the rain garden, and depending on your soil conditions, could have standing water for long periods. Plants here need to be able to withstand and indeed thrive in this condition. Species like marsh marigold, sedges, blue ice bog rosemary, and sweet flag will do well in this zone, along with many of the ones listed above for the slope zone.

Don’t be afraid to add some other features like rocks in and around the rain garden. Just remember not to place them in such a way as to impede the flow of water into the basin. Larger rocks should be kept along the top of the garden while a bed of smaller rocks in the basin might help to create some visual interest while still allowing water to percolate through. A bed of rocks leading from downspouts to the rain garden can also create a beautiful and functional feature.

A couple of other things to remember when starting your project: always dial or click before you dig to avoid any unwanted encounters with underground utilities, and be sure not to create any slopes that will shed water onto adjacent properties, to avoid any unwanted encounters with neighbours.

Winter is a great time to get started with planning your rain garden project. Think about where it might go, what form it will take, which plants you want to include. The internet is teeming with great photos to inspire you. In the springtime, when the water comes in, be prepared to send it down into the ground to serve the plants, rather than out to the drain.

Plants for the upland zone:

Dogwood – Cornus spp. and cvs.
Highbush Cranberry – Viburnum opulus var. americanum (Viburnum trilobum)
Giant hyssop – Agastache foeniculum
Hawthorn – Crataegus spp. and cvs.
Little bluestem – Schizachyrium scoparium
Sedum – Sedum spp. and cvs.
Switchgrass – Panicum virgatum and cvs.

Plants for the slope and terrace zone:

Blue fox willow – Salix brachycarpa ‘Blue Fox’
Cone flower – Echinacea spp. and cvs.
Daylilies –Hemerocallis
Joe Pye weed – Eutrochium maculatum
Turtlehead – Chelone spp. and cvs.
Marsh reed grass – Calamagrostis canadensis (spreads by rhizomes)
Swamp milkweed – Asclepias incarnata
Wild iris – Iris versicolor