Posted by Kevin Songer, J.D, February 2, 2011, kevinsonger.blogspot.com
The plant shown above is Spanish Moss, Tillandsia usneoides, a Bromeliad. Spanish Moss hangs ubiquitously from Florida trees, especially the Live Oaks, Cypresses, Longleaf Pines and Red Maples.
For green roof designers, adapting a bit of biomimicry based on Tillandsia usneoides can prove very useful. You see, Spanish Moss feeds only from the air. All the plants’ water needs and nutrient requirements are met from scavenging the air. Spanish moss utilizes host trees for support, hanging from the limbs and branches yet not penetrating the bark of the host tree.
As seen in the above photograph, Tillandsia usneoides is quite efficient at capturing atmospheric moisture. Most mornings Spanish moss is covered in dew. Following the principles of biomimicry, we can look to Spanish moss for dew and fog catcher design fundamentals.
|Dew Catcher with woven fabric similar to Tillandsia usneoides|
|Dew is available as a primary nature-based irrigation component|
Spanish moss biomimicry tells us high dew catcher surface area to air mass contact is most efficient for air water vapor to occur. 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. Using biomimicry based on Nature’s Spanish moss design allows for important air water vapor collection.
Additionally, understanding the principles behind Spanish moss’ 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;
- Fogquest.org is a great informational resource on capturing dew and fog
- A Great & Fascinating design paper about dew catchers
- Youtube video on dew catcher construction
Air 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 Spanish moss.
As always, feel free to email us with your comments and questions and Happy Green Roofing!