Tuesday, February 22, 2011

Green Roof Dead Zones - Why Wind Breaks are Necessary

Green Roof, Extensive - Nature Irrigation via Dew

Green Roof, Extensive - Wind Damage
 Wind is the silent killer of Green Roofs.  Wind can cause more acute damage to green roof plants than a lack of light.  On a chronic basis, light will eventually contribute to stress or death - light is required for photosynthesis.  However strong, desiccating winds can cause plant vascular failure and dehydration within a matter of hours.

The above photos show two edges of a small trial extensive green roof.

The top photo portrays the amount of available water vapor in the form of dew.  As is usual for Jacksonville, we experience prolonged droughts.  It has not rained for a week and we are well behind in our 52 week rainfall accumulation.  However water vapor usually offers a steady supply of condensation on a daily basis.  Click on the top photo for an enlarged view of the green roof plants and the dew condensed on the leaf blades.

The bottom photo shows the northeast corner edge of the 50mm thick green roof.  This edge takes the initial wind impact of most daily winds as the other corners are more protected.  Winds at speeds of only 2 meters per second can evaporate plant leaf water stores faster than the plants vascular system can resupply the leaf with moisture.  Without water, CO2 cannot be absorbed and photosynthesis may not occur.

Additionally, desiccating winds can collapse leaf structure, rupturing vacuoles and other cells causing permanent and possibly fatal damage to the plant.

Surprisingly, many designers do not look to wind as the cause of green roof plant problems.  Yet the wind phenomena is not only restricted to green roofs.  I was speaking to a group of professionals last week and a landscape architect from Orlando mentioned how on busy highways where automobile induced wind currents continually hit landscape plants placed in the median areas, those plants are difficult to keep alive.

Parapets are one answer but how do you prevent the wind induced green roof 'dead zones' from occurring on an exposed roof?

The answer lies in proper integration of wind resistant plants, such as the CAM plants we've discussed.  CAM plants can serve as wind break plants for C4 and C3 interior placed green roof plants.  Remember, for best biodiversity results follow the 10/20/30 rule.  Of course one can design a green roof with wind resistant plants from only one Genus, such as only Sedum, but biodiversity opportunities suffer greatly when rooftop monocultures are used.

All wind exposed green roofs will have perimeter dead zones.

Depending on the strength and water vapor content of the wind, this 'dead zone' can be from 50mm to 50cm in width or more.

Understanding how the wind travels across the roof, and the combined interactions with available light are crucial to placement of CAM plants as wind breaks.

Finally, pumping additional water and adding fertilizer will not solve wind issues.  Know your prevailing wind directions across the green roof.  Design properly with wind resistant plants as wind breaks and you can minimize the effects of winds across green roofs.

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