Yet what is the second most important design factor in creating and ensuring survival in green roofs? For extensive roofs designed to be employed in dry and arid climates using primarily C3 and C4 plants, the secondary most important green roof design factor is Wind Exposure.
The green roof system is an extensive system consisting of a Colbond mat, a root and water barrier layer, a high organic content (compost based) yet well drained soil and the plants.
The system is partially surrounded (enclosed) on the primary windward side by a fog and dew catcher. The dew catcher is a woven fabric and allows approximately 80% light penetration and reduces wind flow by approximately 75%.
As you can see, the plants protected from primary winds by the fog catcher out-preform those subject to direct wind exposure. In fact those protected plants have shown an approximately 300% increase in biomass production.
The plants are a mixture of those possessing C3 and C4 photosynthesis capabilities. In this trial there was no significant difference in biomass production for the wind sheltered plants based on photosynthesis metabolic types.
Placing an anemometer around green roof system revealed an almost continuous daylight hour wind source from the east and northeast with average speeds running 3.5 Miles Per hour or 1.5 Meters Per Second . A good source for average wind speeds in U.S. cities is available through NOAA.
Understanding of wind exposure is best gained by standing on a roof. Rooftop ecosystems are manifestly exposed to significant and continuous winds, much more unprotected than ground level gardens. If you have never stood on a roof you cannot imagine how much more wind flow exists five to ten meters above the ground.
Wind stresses the C4 and C3 plants through desiccating actions, stealing vital fluids from the mesophyll and interstitial layers just beneath the leaf's surface, slowing photosynthesis. Stomata begin to remain closed, throttling the intake of CO2 and bringing the Calvin Cycle to a halt. With the inhibition of photosynthesis the plant biomass production also decreases.
C3 plants suffer the most. Plants with C4 metabolisms however show somewhat more growth potential under wind-stressed conditions albeit not much.
So what does this mean?
It means afford wind protection to your green roof. Understand that if your green roof has a wind exposed area then the green roof plants will suffer desiccation.
For flat roofs, parapets, research has shown, serve as excellent wind breaks and can effectively reduce wind exposure by as much as 90%.
On exposed sloped roofs CAM plants can serve as wind breaks - remember CAM plants generally keep their stomata closed during the day anyway.
Adding more fertilizer or more water wont solve the problem. Look to the wind. Understand your sites wind directions and intensities.
Your green roof will love you for wind design consideration.
Finally, on a personal note, at first I would have thought water was the second most important design factor after light levels. But we have found wind to have more significant influence on plants than hydrology. However rooftop gardens and green roofs are ecosystems and it is not easy to fully separate the forms and interactions comprising the complex web of life forming a growing greenroof.
Wind issues are critical because they limit water.
Happy Green Roofing! And as always, email your questions or comments.