Rain Gardens: Experiments with Planter Boxes

In Halifax’s North End, a grassy patch is a rare thing, at least along the street. And anyone in the area could tell you how much murky rainwater pools everywhere during a hard rain. A low-impact development enthusiast might say “Sounds like we need more rain gardens!” But the problem here is that rain gardens cannot be placed near building foundations because of the way they pull water down into the ground. This might lead to more basement flooding. So where, in this dense urban landscape of paved surfaces, can we put the water?

A typical North End garden and downspout.

One intriguing option is the stormwater planter box. It’s essentially a rain barrel filled with gravel, sandy soil and plants, which collects and holds water, filters it, and then slowly releases it at a different time than the rest of the rain. Cities all over the world are using them in dense urban areas as a low-impact development tool. These cities include: Melbourne, Australia; Portland, Oregon; Lancaster, Pennsylvania; Philadelphia, Pennsylvania; and Los Angeles, California. Why not here in Halifax?

 

A stormwater planter box in Philadelphia.

One thing you’ll notice is that nearly all the cities mentioned get minimal snow and ice (with the exception of Pennsylvania). So a key question for these stormwater planter boxes is how they perform in and recover from the winter. If they emerge from winter still functioning, then we have found a valuable answer to this tricky question of where to put rain water in a dense urban environment.

The Ecology Action Centre Coastal and Water Team has had an opportunity to build one test stormwater planter box in the Bloomfield area of the North End with generous funds from our local RBC branch on Almon St. If you want to replicate the experiment in your own space, here are the steps to take.

Step one: Choose an appropriate location. Select an area where you can easily disconnect a downspout and re-direct it into the stormwater planter box. This will look like a relatively flat empty space at most 2 meters square right next to your house. The two photos above are good examples of excellent spaces to use.

Step two: Find or build an appropriate container. You will need a container that can be water-proofed, and can have drainage holes cut into it, or can be fitted with a drain pipe. Ideal and easiest is an old water tub like the one below, because it is already assembled and water-proof. A polyurethane wheelbarrow would also work great, if you don’t mind the aesthetic of it.

If you like a wooden look, you can try to find one already assembled, like this one. You will need to water-proof it (read below for your options on how to do that) and cut a hole for the drain pipe.

If you cannot find an appropriate container, then you can build your own.

When selecting a container, think about the volume of water it will receive. You should calculate the drainage area from the part of the roof that drains into the downspout you will disconnect and send into the planter box.

This table of sizing guidelines is from Melbourne Water.

Drainage Area m2	Planter Box Area m2
50	1
100	2
150	3
200	4

Most North End homes will be in the 50 m² or less category, which means that most containers you will find will be an appropriate size.

We did not find an appropriate box to use, so we built our own. We chose cedar wood because it is naturally weather resistant. Another good choice is hemlock. You can use pressure treated wood, as it is much less expensive but consider that it is toxic. We could only find 2” x 6” lumber, which in retrospect is a little too sturdy for this purpose, and resulted in a very heavy box. 1” x 6” lumber would be perfect. You can make your own construction plans, or use basic carpentry books that feature outdoor/garden projects to find a plan.

Step three: Obtain the materials for making the box drain properly. If you’re using a wheel barrow, all you need is a drill to cut holes at the lowest point in the slope. Otherwise, you’ll need two drainage pipes: one for the bottom of the box, and one for overflow. We used a 3” ABS and a 1.5” ABS pipe for the bottom and overflow respectively. You can find these in the plumbing section of home and construction stores.

To connect them, we used an ABS Tee-Wye like this one. We capped the end of the 3” pipe inside the box so that it would not clog with gravel and soil. For the ends that are outside the box (the top of the overflow pipe and the bottom drain pipe), we used window screen held on with a hose clamp like this one. This ensures small animals cannot get inside the pipe. For the bottom drain pipe, it is easiest if you can find one with pre-drilled holes. This is usually made of PVC. We couldn’t use this because the store didn’t have the other pieces that would make it all fit together. Because of this, we had to drill our own holes. If you drill your own holes, make sure they are at least a centimeter in diameter. We made several rows of holes about 2 cm apart, staggered, on the top half of the pipe.

Step four: Cut a hole in the box for the pipe.

Use the end of the pipe to trace the hole. We used a jig saw to cut out the hole. You can borrow tools like this from the newly opened Halifax Tool Library!

Using a jig saw to cut a hole for the drain pipe.

Checking that the drain pipe fits through the hole.

Step five: Cut the drain pipes to the proper length. The bottom drain pipe should be along the full length of the bottom, and be able to fit into the Tee-Wye piece, which will have another short piece (about 7”) of 3” pipe stick out of the hole in the box. Mark the length, and cut with a small saw like a hack saw, or hand saw. It’s a good idea to cut into a container so you can easily clean up the shavings. We just happened to have a kids’ wheelbarrow on hand for this purpose!

Cutting the drain pipe to the correct length.

Step six: Waterproof your box. We were lucky to find a durable plastic liner without any holes in the hardware store’s waste pile, so we decided to use that. You can also use sheets of PVC liner taped together, or you can also use a PVC pond liner. We placed it in the box, and then cut a small X next to the hole for the small drain pipe. To seal the liner to the drain pipe, we shoved the small piece of the drain pipe into the Tee Wye connector in a way that put the liner flaps from the X cut inside the Tee Wye connecter at the edge of the box next to the hole.

Placing the waterproof liner in the box.

Step seven: Move the box to its final location. It should be at least 30 cm away from the house foundation, and in a location where you can connect a downspout to the box easily.Make sure it is level and stable. We needed to do a bit of leveling of the resting area. To do this, we used a shovel, bricks, and a level to get the box set, and once it was, it would not budge.

Step eight: Prepare for proper drainage. Without proper drainage, the water will remain in the box and stagnate, which can kill the roots of the plants. To help angle the drain pipe, we placed a small brick at the up-slope end to ensure that the pipe was angled down slightly. To avoid the pipe filling with the gravel which goes at the base of the box, we stapled the rest of the window screen material around the drain pipe. Then we filled the first 15 cm of the box with washed gravel in such a way that supported the slight angle of the drainpipe. We shaped the gravel to slope down toward the pipe in the middle, and then placed a piece of landscape fabric over the layer of gravel and pipe. The landscape fabric serves several functions. It helps keep sand and soil from the top layers from clogging the gravel and drain pipe, and because it is semi permeable, it helps to direct water flow into the pipe. We cut small holes in the fabric over the holes in the drain pipe to help water drain into the pipe. Note that all of these elements are parts of the experiment. If you decide to take on building a stormwater planter box of your own, you can decide what elements make sense to you, observe how they work, and let us know what you find!

Drain pipe ready for draining with brick for the angle, and window screen to keep the gravel out of the drain holes.

Gravel fill around the drain pipe, and the overflow pipe with its window screen cap.

Step nine: Fill the box.

Once the basis for good drainage has been set up, it’s time to continue filling the box. With the advice of several avid gardeners, we chose to mix a coarse sand and commerical potting mix together. There was a higher sand ratio at the bottom of the box and lower at the top where the plants will sit. If you want to do tests of drainage for different ratios, you can take a couple of unneeded buckets, cut holes in the bottom, and fill them with different mixes. Then put an equal amount of water, but enough to saturate them, into each bucket and observe how quickly they each drain. Soils in a container are too fine and dense to provide adequate drainage, so potting mix is recommended. To provide some nutrients and improved absorption, we added a layer of natural bark mulch on top.

Step ten: Planting!

This too will be an experiment. We chose to use the very hardy and durable Sedum spectible, which are very popular around Halifax. We felt this was a good plant to use as a test run to observe and learn more about the planting conditions in the box through the seasons. If they perform well, then we will try other plants as well. The plants in the container need to be cold tolerant to last through the winter in a box that’s less insulated than the ground, and to tolerate a variable amount of water. You do not need very many plants to fill a small planter box- maybe 5-6 plants.

Step eleven: Disconnect the downspout and redirect it into the planter box.

You can use a hacksaw to cut the downspout. To make the downspout reach the box, you can use an accordion downspout extension tube like this one.

The finished box!