Designing A Tropical Rain Garden & Managing Storm Water Run Off

It started as a drizzle, quickly turned into a downpour, and before we knew it homes were underwater and whole roads were washed away. It’s truly mind boggling how quickly rain can go from a blessing to a curse. Water is an incredibly powerful force, and this past spring’s record breaking rainfall and tragic flooding have driven that point home for all of us here on Kauai.

Now imagine a neighborhood road that winds up a ridge line, Kalihiwai Ridge (where we are located) is the perfect example of this and there are many others all over the island. Along that road are lots of houses, some may be large, expansive homes and others might be small and modest. No matter the size, the average house has a roof, at least a few gutters, and possibly a paved driveway that might be fairly steep in some sections. When it rains all of the water from that roof gets channeled into the gutters and flows down and hopefully away from the house. Water also collects on the paved driveway and ultimately most of the water from both the roof and the driveway gets routed out to the street. Depending on a number of factors (square footage of impermeable surfaces, amount of rainfall, slope, elevation etc.) the water coming off a property and onto the street might be a meager gallon or it might be several bathtubs full.

But now imagine that happening at every house in the neighborhood, water collecting off roofs, flowing over driveways, and converging in the street with the water from every other property. You can see how easily you end up with a small river at the bottom of the hill and a major flood at the bottom of the valley. Now add to that the factors of slope and the speed of the water moving and you’ve got erosion, which causes major problems on both the home scale as well as island wide.

The ways we design, build and develop our homes and landscapes play a major role in the flow and force of water. Roofs, buildings, concrete, pavement and bare soil all contribute massively to runoff and erosion. In most cases, people’s approach to dealing with storm water and runoff is to get it away from buildings and off property as quickly as possible with very little thought given to the effects it will have down stream. Many people also opt to use an emergency water removal service from a local company. Either way, there is no doubt that poor storm water management was a major factor in the terrible flooding experienced by so many.

We can use this:


to prevent this:

Fortunately there are lots of things we can do both individually and collectively to better manage runoff and stormwater, and we can integrate them into our landscapes in a beautiful, productive and regenerative way. Just imagine all that water, instead of running off the surface and into the ocean, carrying our precious topsoil with it, being sunk back into our aquifer, getting drunk up by plants or being caught and stored for future use. In this post we’ll outline some of the key concepts for dealing with surface water and we’ll also give you some great information on how to build a beautiful Rain Garden.


When it comes to surface water on your property (be it from excessive rainfall, existing swales that cross property lines, water coming off a roof or parking area and especially gutter downspouts) the rule is Slow it, Spread it, Sink it.

Firstly, you want to slow the water down. Water that is moving quickly picks up sediment and small rocks and carries them along, the effect of this increases exponentially the farther downstream you get and can end up becoming so powerful that it causes landslides. The more straight and narrow a channel is the faster the water moves through it. To slow water down you want to give it a wide, curving path. Think of a river, rarely do rivers move in straight lines, they flow over the landscape in snaking curves, following the path of least resistance.

Water naturally prefers a slower, winding course.

Secondly, you want to spread the water out over a greater surface area. Water moving in a concentrated area has more speed, force and potential to cause erosion. But water spread out moves slower and has more chances to absorb into the ground. Again a wide, curving path is the best course.

Finally, you want to sink the water back into the ground so that it can be used by plants or filtered back into the groundwater to recharge our aquifers which is where our drinking water comes from! One excellent way to do this is with a Rain Garden!

A Rain garden is beautiful landscape feature that collects excess stormwater and sinks it into the ground while passively watering the surrounding plants. If you have ever walked through a marshy meadow you can imagine the lush beauty of a rain garden, green grass, vibrant plants, birds playing, frogs ribbiting, dragonflies flitting about and the soft sounds of water trickling through.

You can design your own gorgeous rain garden in a just a few simple steps!:

  1. First you need to locate the source of water, be it a gutter downspout, the low spot in the driveway or the spot in your yard where all the water from a slope collects. Then you want to figure out the best adjacent location for the rain garden. This will need to be a spot that absorbs water easily and is close enough to the source of water to make for an easy job. You can pipe water into the rain garden directly or just channel it there using slope. You want to make sure the placement of your rain garden is down hill and at least 10 feet away from any buildings. You also want to be at least 20 feet from any septic systems and make sure there aren’t any utility lines running in the area as well.
  2. Once you’ve located the source of the excess water and figured out the best location for the rain garden you can start to dig! You really want to make sure the spot can absorb water and won’t just turn into a stagnant puddle, so make sure you don’t choose an area that’s already a stagnant puddle! To test for soil percolation, dig a hole 2 feet deep by 1 foot wide and time how long it takes for 8 to 12 inches of water to disappear. For example, if 8 inches drains in 12 hours, the rate is 8 inches divided by 12 hours, or 0.67 inches per hour. A rate higher than 0.5 is great—your rain garden needs to be just 18 inches deep. If the rate is lower than 0.5 you’ll have to dig 30 inches deep. If the percolation rate is less than 0.1, the site isn’t suitable for a rain garden.
  3. Next you’ll need to determine the size and shape of the rain garden. As a general rule in a well draining area, a rain garden that is 10% the square footage of the roof will handle about 99% of the roof water. You might need to do some calculations to determine the average amount of runoff you’re dealing with. Find out what the high ends of your averages are and plan for those. If you’re short on space or puzzled about how to calculate the size, you can always put in a small rain garden and figure that the good it does will at least be better than what’s happening now. If you want an impressive-looking garden, make it at least 150 square feet. Ovals, kidneys, and teardrops often look best, but rain gardens can also be long and skinny. Lay a garden hose on the ground to test possible shapes. Once you settle on a design, decide where the water will flow in and where any overflow will exit. Mark the shape with spray paint. On your lawn, mark 18 inches farther out for sod removal, since grass has a way of creeping into planting beds.
  4. Now you can really get to digging! Cut out the sod or any other existing plants and remove the soil to the appropriate depth for your drainage rate, somewhere between 18 and 30 inches. Depending on the size of the garden you can shovel it yourself or you might want to consider hiring an excavator. Create a flat bottom so that water will percolate down evenly. If the rain garden is on a slope, you can pile some of the excavated soil into a berm on the low side to retain the water. For stability, stomp the berm soil down well and make the base at least 2 feet wide and the top at least 1 foot wide. The peak of the berm should be at least 6 inches higher than the water level when the rain garden is full.
  5. Next, channel the water into the rain garden. Either place the garden adjacent to the flow of excess surface water or dig a trench for a pipe that will carry water from one or more gutter downspouts to the rain garden. (Note: If you can corral helpers, this can be done at the same time you excavate the rain garden.) Install the piping. Rigid piping with smooth walls is the most durable, but corrugated tubing is easier to work with; get the kind without perforations so dirt and plant roots can’t work their way i over time. Extend the piping into the rain garden basin by a foot or so. Line the area underneath with stones to prevent erosion. You can also place stones over and beside the pipe to hide it and to keep corrugated tubing from curling up. When all the piping is in place, fill in the rest of the trench with excavated soil.
  6. Then you want to start layering the soil back into the rain garden. Fill all but the top 6-12 inches with a well draining mixture of equal parts sand, lava cinders, compost and native soil.
  7. Now comes the fun part, planting! You’ll need to choose your plant selection based on three “zones”. Zone 1 is the center of the rain garden, where the water stands the longest. Choose plants for this zone that love water and can handle being submerged or soggy for some time. Zone 2 is the area around the center, these plants will need to be able to handle some standing water but also periods of dryness. Zone three is the edge of the rain garden and should be drought tolerant plants. Here is a handy list of plants for each zone that are suitable to our climate. Note that the best options are often native plants as they are the most well adapted to this variety of conditions:

Plants for zone 1:

  • Kalo and colocasia varieties
  • Pennywort
  • Horsetail
  • Papyrus
  • Heliconia
  • Ginger
  • Bacopa monieri
  • Sealing Wax Palm

Plants for zone 2:

  • Carex grass
  • Ahuawa
  • Alocasia
  • Heliconia
  • Ginger
  • Sealing wax palm
  • Licuala palm
  • Raphis Palm
  • Palapalai fern

Plants for Zone 3

  • Kupukupu fern
  • Ti plants
  • Vetiver
  • Coleus
  • Comfrey
  • Pohinahina
  • Pikake
  • Kou
  • alahe’e
  • Hibiscus

You can choose just one type of plant for each zone or as many as you can fit! Have fun with it, play with different combinations of color and texture and create something you love!

8.  Once you’ve made your plant selections for each zone, get them in the ground! Proper planting means digging a hole that’s just slightly deeper and wider than the pot or root ball of the plant, add a little more compost to the bottom of the hole, pop the plant in, sprinkle some fertilizer around the edge of the root ball and cover the rest up with soil! Then add some mulch around the whole garden area and if you like you can fill any open areas with some decorative river pebble to complete the look. Water your plants in well once you’ve finished and keep an eye on them in case they need additional water. Once the plants are established they shouldn’t need anything but the rain but if you start this project during a dry spell you might need to water every once in a while until things have grown in.

9. Enjoy! Rain gardens are fantastic places to be in nature, observe the wildlife in your yard and feel good about making a positive impact on your environment!

For further research I highly recommend the Hawai’i Residential Rain Garden Manual.

Now you know everything it takes to DIY your own rain garden and help prevent excessive storm water from becoming a catastrophic flood! If you still have questions or you are daunted by the idea of taking on a project like this just give us a call! We can walk you through the process or you can hire our design and install team to create it for you!

Just remember, Slow it, Spread it, Sink it!

This article was written by Zoli, Senior Sales Associate and Landscape Designer at Seascapes Nursery.

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