Wind, wind & wind across Green Roofs in Arid Areas

The central states along the Mississippi have been receiving more than their fair share of rain recently as evidenced by the US Army Corps of Engineers blasting of levees to alleviate flooding.

MetroVerde Florida Green Roof, Breaking Ground

Here in Jacksonville we've had little if any rainfall for weeks now.  The saying April showers brings May flowers does not hold true for the geographic climate here.  As weather fronts push down and across the US they loose momentum and dissipate just before reaching Jacksonville.  All the dry weather is rough on green roof plants.

However, being situated on the Atlantic Ocean we are constantly exposed to high winds.  When the weather fronts come through they drop all their moisture west of Jacksonville typically but retain enough wind turbulence to stress unprotected green roof plants.

Up on the Breaking Ground Green Roof for most of yesterday I watched in amazement at the amount of water the dry winds were stealing from the plants as breezes whipped across the flat roof at 4 or 5 meters per second.

Of course the CAM plants were fine and this is why we suggest planting a belt of CAM plants around unprotected green roof perimeters.  CAM plants generally keep their stomata closed during the day.

C4 plants like the Poaceae hold their own, as long as they are established.  The corn, lemon grass and native grasses don't seem to mind the desiccating winds though I am sure after time they too would suffer without some rainfall.  I love the wave action native grasses produce in the wind.

The Asteraceae too do well, again as long as they are established and again, most are considered C4 photosynthesis possessing plants.

But the C3 plants can dry out so quickly.

Because we are doing an install I water the plants to help them settle into their new home.

Yesterday the wind was so drying and strong I could hold the hose to where the stream of water was flowing vertically up and evaporating or being stolen by the wind before the droplets could ever make it back down to the plants.

Watching the C3 plant leaves move in the wind I could see the same occurrence happening, water being quickly vaporized out of the leaves just as the droplets from the hose where also 'gone with the wind'.

Sitting there with the hose I witnessed some of the more tender vegetables quickly wilt even with the hose  water directed at their roots.  The plants vascular system could not keep up with the wind induced evaporation of leaf moisture.

Without water in the leaves plants can not conduct photosynthesis.  Without photosynthesis plants die,

Wind impacts are so significant across green roofs.

In my opinion wind exposure is, with light, availability the most important consideration for green roof design.

We've posted numerous articles here on how to use CAM plants as wind breaks.  Understanding wind impacts on green roofs is critical for long term green roof design success.

Wind and Green Roof Plants

Included here are two photos of the same black-eye pea plants.  One is on a wind protected green roof, the other is on a wind exposed green roof.  Note the wind burn on the wind exposed roof plants.

Both looked the same prior to the 48 hour wind storm just leaving (killed 40 persons in the southeast US during tornados).

We recorded between 10 and 20 Km/Hr winds on a continual basis for the 48 hours with almost non-existent water vapor or air humidity.

Florida Green Roof, Wind Protected Green Roof Plants

Florida Green Roof, Wind Exposed Green Roof Plants

Wind can burn or kill green roof plants quickly, taking an otherwise beautiful planting and turning into a brown mess within a matter of days.  Even with more than adequate irrigation water applied - simply because the vascular system of the plants cannot keep up with the demand for water in the leaves.

CAM plants and those plants with stomata remaining closed or closing under lack of water conditions must be used as perimeter wind break plants on green roofs with out parapets or other wind protection if the green roof is not otherwise sheltered.

Understand the different types of photosynthesis green roof plants have.  For a primer, check out the many articles we have posted before on CAM, C4 and C3 plants.

Other helpful sites are included here;

Great Irish Gardening blog article on wind damage in Ireland to garden plants.

North Carolina State University has another brief yet informative note on wind desiccation of plants.

Very interesting and informative Permaculture Wind Break Site.

Remember, your green roof site may receive plenty or precipitation or irrigation water, but if it is constantly exposed to desiccating winds the plants will experience the effects of wind damage.

Top Two Green Roof Design Issues

 Light availability is the most important green roof design issue.  Without light photosynthesis does not occur.  Without photosynthesis plants do not grow.  The green roof plants must have sufficient light levels for  the Calvin Cycle to occur, turning CO2 into complex carbohydrates for energy and growth.

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.

Rooftop permaculture

The above photo is a rooftop trial bed of winter salad mixes (the plants were started from plugs and are approximately 2 months old).

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.

Kevin