Cougar Canyon Elementary School
Rain Garden 2 (2009)
11664 Lyon Road, North Delta

From the rain garden “live & learn” files

Cougar Canyon Elementary School Rain Garden 1 had always functioned so perfectly, that when it came time to create a second one just the other side of the parking lot driveway, we were confident that it would work fine. So confident, in fact, that no overflow drain was provided.

Delta Engineering created their usual (or so we thought) imitation streambed, contoured the banks for planting, and arranged for safety fencing to be installed.

Imitation streambed completed, fence going in. The sedges (Carex 'Ice Dance') are excess ones from Cougar Canyon Elementary School's first rain garden; we stored them here temporarily - some went elsewhere, others reappear in different locations in this garden.

In September 2009, students planted the garden with a generous supply of shrubs and groundcovers, purchased by Delta. Three deciduous trees were slated for later planting, once the weather became cooler and wetter.         

 All appeared to be fine. Until the first heavy rains, that is. 

Oops! What was this lake? Why wasn’t the garden absorbing water quickly, as the first one had?

Was it the absence of trees? (8 evergreens and 3 deciduous in the first garden, none yet in the second one) Did we need more evergreen sedges and shrubs? Was the fire hydrant leaking?

While we pondered the possibilities, we also learned from Delta that the structure of Rain Garden 2 was actually quite different from Rain Garden 1, despite their similar appearance. Rain Garden 2 featured a very large volume of drain rock, wrapped in geotextile (landscape fabric), down the centre of the garden. By contrast, Rain Garden 1 had been entirely filled with highly absorbent compost/soil – with just a skim coat of gravel on top.

But aren’t rocks good for drainage?

The accepted wisdom is that lots of “drain rock” is good for drainage. So it was with great trepidation that streamkeepers and students set about the exhausting task of manually removing the tons (or so it seemed) of drain rock and their geotextile wrapper. Then we backfilled with highly-absorbent landfill compost, topped with just a thin layer of gravel.

To our relief, this fixed the problem! There’s still another 1/3 the length of the garden to be “liberated”, but it is now draining well into the subsoil and groundwater table below.

So perhaps rocks aren’t so good for drainage after all – at least not infiltration drainage. If the water has somewhere to go, then a thick layer of rocks can keep our feet dry while the underlying water flows away. But if we need to absorb water on site, rocks do not appear to be the best answer.

Do crows know best?

Rocks displace water. Every smart crow knows this! Just read about Aesop’s fable “The Crow and the Pitcher” – a story confirmed by experiments with modern-day corvids.

http://en.wikipedia.org/wiki/The_Crow_and_the_Pitcher

In contrast to rocks, soil with lots of organic matter does not displace water, instead it soaks up water like a sponge. The more roots and soil organisms, the more water the soil can soak up, and slowly release down to the subsoil and groundwater.

Now that 2/3 of the geotextile (landscape fabric) and drain rock have been replaced with highly absorbent compost, the garden absorbs rainwater runoff just fine, even though it lacks any overflow drain.

Beware the overuse of rocks in rain gardens!

In addition to displacing water, overuse of rocks may perhaps also inhibit absorption in 2 additional ways:

(1) Their sheer weight may compact the underlying soil so tightly that millions of small porosities are squeezed shut, thus preventing water from trickling down easily.

(2) Rocks (and geotextile) are not nearly as good habitat as soil and compost, for the countless roots, invertebrates and micro-organisms that also contribute to creating porosities into which water can trickle.

A surface cover of gravel and/or boulders is great for aesthetics, and for protection against erosion in places where water may flow vigorously. It’s also good as a maintenance path, from which weeding and trimming can be done.

But underneath that thin layer of rock, soil rich in organic matter makes for the most effective rain gardens.

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