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Friday, June 28, 2013

Water Wise?

We have spent a lot of time boasting about the native plants on our green roof, and how they're naturally drought-, wind-, and sun-tolerant, so are ideal for this rooftop setting.  I've also mentioned a few times that Laramie is quite arid – we typically receive around 11 inches of precipitation (including water weight in snow!) per year – only 1 inch more than what classifies a place as a desert. 

So what's with the sprinkler?

As much as we'd like to think that we're perfectly mimicking a natural prairie on our green roof, it's obvious we can't do it in every way.  We can't let pronghorn loose on the roof (though that would be sweet!) and we can't set fire to it (oh goodness, can you imagine the ruckus?).  

And we can't recreate all of the benefits that groundwater, soil depth and topography bring to plants in a natural setting. 

Native v. Natural

We need to be careful when we describe the green roof as mimicking a local prairie.  To clarify:  We use native plants.  We don't pretend this is a natural environment.

In a natural prairie, plants have incredible root systems that, depending on the plant, can grown many, many inches, even feet, into the soil to access groundwater and moisture that our 8-12 inches of green roof soil medium can't provide. 

Topography can also play an important role in a short-grass prairie, providing nooks and crannies for plants to grow in, protected from constant wind and sun and benefited by piled snow and rain run-off. 

The green roof is also surrounded by concrete and brick – increased evaporation due to heat island effect is likely a player as well.  And while we're on the topic of evaporation, look at the color of the soil on the roof compared to the native prairies.  We suspect the dark color absorbs more heat from the sun and evaporates moisture more quickly than in prairie soils.

Natural prairie soil

Green roof "soil"

Furthermore, this isn't just a green roof, it's also a showcase garden.  As such, the aesthetics do matter to some extent.  In order for the Berry Prairie to look as beautiful and healthy as possible, we want to give it an extra boost with supplemented water every so often. 

We also have an drip irrigation system set up, which allows for us to soak the roots when the soil dries out.  We can verify whether the soil is saturated, moist or dry through manually trenching or coring the soil, or through the soil probes we have set up throughout the roof.  The probes are set 4 inches below the surface and measure soil temperature and moisture every hour of every day.

Water Usage

With all this said, we try to be really careful to water only what we need to in order to ensure the health of the plants.  We have watered 4 times so far this summer using a sprinkler and the drip system.  It's a manual process, not automated, so if it rains or if it's cool, we don't need to turn on the sprinklers.

What do you think?  Is it right to water our green roof?  How much, how little, how often, and at what cost?

Written by Brenna Marsicek, Biodiversity Institute

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