Wednesday, March 28, 2012

Biomimicry for Green Roofs, Catching Dew with Plants

Nature offers the finest examples for us to look to when resolving green roof, or any other for that matter, issues.

Yucca, with hairs across the surface edges of her leaves is a highly efficient fog and dew catching plant.

Likewise, with waxy, tough leaves and CAM (Crassulacean Acid Mechanism) photosynthesis, she can serve as the perfect green roof edge wind-break perimeter plant.

Hairs on Yucca's leaf edges allow for fog and dew collection on the Green Roof

Especially relevant during periods of extended drought, dew and fog can add much needed moisture to a green roof.  Because yucca is effectively adapted to low annual rainfall areas we can learn much from studying her botanical physiology. 

Yucca biomimicry tells us high dew catcher surface area to air mass contact is most efficient for air water vapor to occur.

Many yuccas and agaves thrive in hot, dry, windy areas and make excellent choices for green roof plants.

Yucca filamentosa, Adam's needle is a favorite green roof plant of mine, reliably hardy in the freezing cold temperatures, evergreen, very drought tolerant, a dew catcher and the perfect CAM perimeter plant.

Florida's native yucca, Yucca filamentosa ready for Green Roof install

Planted in mass, Yucca filamentosa acts as a green roof parapet, allowing interior plants a more welcoming ecosystem for growing.

Yucca filamentosa also has long hairs growing from the leaf edges, allowing for water vaopr in the air to collect as the humid breezes flow across the plant.

Turbulence is another factor necessary to help drop the condensed air water vapor from the catcher to the green roof soil below.

Success of a nature irrigated green roof depends heavily on sourcing a steady supply of water through rainfall, fog, dew and even frost. Understanding biomimicry based green roof planting layout allows for important air water vapor collection.

Additionally, understanding the principles behind Agave's and Yucca's' water capture successes lie also in an understanding of air humidity. Humidity is the amount of water vapor in the air.

Humidity is an important source of irrigation for nature irrigated green roofs and is often present when rain is lacking. Humidity is often described in terms of ‘relative humidity’ and ‘dew point’.

Relative humidity is the phrase commonly used by weather reporters to communicate the percentage as the amount of actual water vapor in the air divided by the amount of water vapor the air could hold.

A relative humidity of 75% means air contains 75% of the amount of water vapor possibly held.

Dew point refers to lowest air temperature where water vapor remains in vapor form. Once the ambient air temperature reaches the dew point temperature the water vapor condenses into dew or liquid.

Dew and fog reference and collection resources available on the web include; is a great informational resource on capturing dew and fog
A Great & Fascinating design paper about dew catchers
Youtube video on dew catcher constructionAir humidity can be a significant component in the irrigation of any green roof system. Consider those months with lower than average precipitation and check to see if dew occurs frequently. Validate the average relatively humidity percentages.

Think of the times you have walked across a lawn in the morning to find your shoes soaking wet.

Research dew and fog collection websites. Look to the green roof plants you work with to see what species appear to accumulate dew.

Mimic nature. Mimic the Yuccas and Agavaceae.

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