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;

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 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.

Green Roof Plants, CAM Photosynthesis Minimizes Drought Effects

In prior columns we discussed how some cold tender CAM plants, such as many of the succulents, can be damaged by freezing temperatures.  CAM plants are called CAM plants because they possess a specific form of metabolism called Crassulacean acid metabolism.


I love to use the CAM plants on green roofs because they can survive very long periods of tropical and sub-tropical drought without much in the way of rainfall.
The CAM metabolic process helps succulents and other CAM plants survive in dry, arid regions by working to keep stomata closed during the day when high temperatures and hot drying winds.  If temperatures are high, the sun is bright and stomata are open the plants can rapidly dehydrate.  

CAM plants open stomata at night when temperatures are cooler and solar radiation minimal.  When stomata are open CAM plants take in carbon dioxide (CO2) and store the CO2 in their leaf cells.  Once the sun rises and temperatures increase, CAM plants close their stomata and take the CO2 absorbed from the evening air and begin photosynthesis, producing the substances the plant requires and also oxygen, O2.

An analogy I like to use in comparing CAM plants to C3 plants is thinking about the difference between a gas guzzling 1970's sedan as compared to a new hybrid-type car with a highly efficient engine.  The C3 plants' metabolism is like the 1970's V8 gas guzzler - they both take in lots of fuel, fire up quickly, get to where they are going/growing quickly but are inefficient with respect to fuel utilization.  In the 1970's V8 wasted, unburned fuel leaves the engine as exhaust.  In the C3 plants we see volatilization and evaporation out of the leaf of photosynthesis substances through numerous opened stomata.  Yet both get where they are going/growing - quickly!

CAM plants though are like the highly efficient hybrid electric/petroleum engine.  In addition to conserving CO2 and preventing desiccation by keeping their stomata closed during the day, they are also very efficient at uptaking and using nutrients like nitrogen.  Because a CAM plant's CO2 is limited, the plants have developed mechanisms to become ultra-efficient at nutrient utilization.  Little is wasted.

Because of CAM plants adaptations to hot, arid, dry and drought-like conditions, they make great green roof plants and have been used historically across Europe as such.

Interestingly, some plants like the sedums can switch back and forth between the C3 and CAM metabolic processes depending upon the amount of water and nutrients available in the environment.  This process is called acclimation and is very similar to what we may imagine a grizzly bear's hibernation may be like.

Sedums are considerably more cold hardier than many of the other succulents that are members of the Crassulacaea family and so are popular in colder climates as green roof plants.  Some sedums are so popular for use on green roofs that they have become pest plants, exotic invasive species displacing some types of native vegetation.

Several CAM plants (Agave) surrounded by C3 plants

However, though CAM plants are excellent at surviving heat, aridity, and bright solar radiation and frost if protected, another climatic condition often is a limiting factor.  Though CAM plants, if protected can survive freezing temperatures and certainly can survive long periods of drought, many are susceptible to humidity-heat combination related issues.

Sedums and other succulents are highly prone to fungal attacks during the summer months when the temperatures range between 90 F and 100 F (35 - 37C).

The Southern Blight fungus, Sclerotium rolfsii, also known by the common names 'crown rot' and 'white mold' can decimate a well established roof of succulents or sedums.  Fortunately, southern blight does not seem to bother the succulents during cooler or dryer months.  However the everyday rainfall and resulting high air water vapor combined with high temperatures found during the summer months produce an environment just right for Sclerotium rolfsii, to proliferate.

CAM plants (Graptopetalum) with C3 and C4 Green Roof Plants

But what does all this technical information about photosynthesis and botany have to do with green roofs?

For the nature irrigated green roof, an understanding of C3, C4 and CAM plants and their advantages and disadvantages, their benefits and their limitations, is critical for a successful design.

Green Roof Succulent Injured by Southern Blight

As we continue our discussion of C3, C4 and CAM plants over the next several days we will begin to clearly see how 'Right Plant, Right Place' is important even on green roofs.

Water supplies are limited across the world.  We cannot continue to rely on irrigation use of potable water on landscapes or green roofs.

Designing a nature irrigated green roof will require utilization of a combination of C3, C4 and CAM plants, planted on the roof according to a number of biophysical variables we will discuss.

And so, to date we now understand that CAM plants are ideal for arid, hot areas and can survive drought.  We also know CAM plants may be susceptible to frost or also to fungal attacks encouraged by hot, humid weather.  We know C3 plants grow quickly.

Finally, as we discuss other important factors about plants suitable for a nature irrigated green roof we will begin to develop a sense of understanding as to the type of plant that will work long term on the green roof if planted in associations with other plants and in the right roof location.

Green Roof Plants and Thermogenesis - Strange World of Plants

All plants possess biological systems that directly impact our ecology and the immediate environment surrounding our day to day activities.


After purchasing a truck load of plants on Saturday we unloaded most, however forgot to remove all of the plants from the cab.


Though the night air was cold (6C) when I opened the truck door and climbed in to drive to the market yesterday evening, after dark, I was enveloped with warm, moist air and confused as to why - with the cold dry air outside - the truck windows were fogged over with moisture.  Then I realized the plants were still in the truck, taking in CO2 and pumping moist O2 back into the air.


After spending much of Saturday evening outside taking temperatures with the ExTech IR thermometer, the oxygen and moisture filled truck cab emphasized what I already knew - plant's biological process are complex and have definite effects on their surroundings.


Sometimes we forget just how much plants impact our environment.


However in addition to the wonderful visual greenery (again we sometimes take for granted), plants sequester CO2, produce O2, provide habitat for wildlife in the Urban Core, provide food, fiber and medicine, clean stormwater and provide a myriad of other functions.


All of these factors and processes impact green roofs.  Understanding how these factors interact with the building is important.


This weekend I wanted to gather additional data on heat and green roofs.  My questions were many and included;


* Do green roofs really act as insulation?
* Do green roofs act as a heat sink - storing heat - instead of being an insulator?
* Does green roof plant selection impact the energy efficiency of green roofs?
* Does green roof soil composition impact energy efficiencies of green roofs - and if so, how?
* and a host of other questions.


After spending several hours with the IR, examining plants and green roof systems after dark - and in 6C ambient air, I can say much data needs to be collected, many studies completed and analysis done before we really understand the dynamics of green roofs.


Just as with the fertilizer and irrigation issues (I am always amazed at how some promote green roofs as ecologically friendly and important yet insist for the inclusion of potable water irrigation systems and fertilizer applications), the insulation or heat sink issues just don't seem to be adequately answered.


After collecting temperature data from under green roofs we see a green roof behavioral trend pointing to a heat sink rather than an insulator type system.  In other words, green roofs may tend to absorb heat during the day and then slowly release it back into the atmosphere and building during cooler evening hours.


Yet the complexities of plant species, plant growth characteristics, root systems, stomata to leaf surface area ratios, soil media specific heat qualities and other issues all contribute towards a complex model.


Getting back to the IR thermometer field  foray, some of the more interesting observations we noted were;


* Night time green roof plant leaf temperatures were approximately the same as ambient air temperatures,
* There were variable levels of warmer temperature readings found in the air space under the green roof plant leaves and above the green roof soil media, depending on the time of night and wind exposure - suggesting a level of insulation occurring as a result of leave structure
* The underside of an extensive green roof (3" soil media) stayed 10F warmer than a similar roof with no green roof system - and stayed warmer all night -- up until 5am the next morning,
* Banana plants stayed considerably warmer than ambient air for up to three hours after dark - unlike other plants,
* and other observations.


The banana plant elevated temperatures pointed us in the direction of thermogenesis in plants.  Thermogenic plants are those plants that can generate heat as a result of biological processes. The voodoo lily, Sauromatum guttatum, can generate temperatures of up to 110F - 32C!


There is a great video on thermogenic plants here.


However, the banana plant is not a thermogenic plant and the reason the banana plant stayed warmer than ambient air for several hours after sunset was the plant's high water content.  Water has one of the highest specific heat values of any compound or substance - four times than of limestone for instance.  Because the banana tree was full of water, the solar heat gain experienced during the day only slowly dissipated after nightfall.  Banana trees stayed warmer than most plants after dark because of the heat stored in the large volume if interstitial water within the plant.


It is possible the succulent filled extensive green roofs we are studying that emanate heat throughout the night are behaving like the banana plants.  The combination of green roof soil media and the water therein is absorbing heat during the day - maybe quite a bit of heat - then slowly releasing the heat at night.


The factors involved in modeling this complex heating and cooling dynamic are many and not well documented today.


We think the heating behavior of the extensive green roof is due to water in the extensive green roof plant root systems.  Because the system studied was non-irrigated (nature only irrigation), the soil media was rather dry.  However for heat to continue to be released for long hours, the heat source probably was water - and probably water stored in the underground parts of plants.


We ask ourselves many questions - if water is a significant heat sink and heat source, then do green roofs really act as insulating systems?


If green roofs are heat sinks then how much heat do they dissipate back into a building at night?


Are irrigated green roof systems actually hotter than non-irrigated vegetated roofs or reflective white roofs?  If so by how much?  How much cooling does plant transpiration and evapo-transpiration on irrigated green roofs?


There are many questions to be answered.


As an industry we need to sponsor and encourage more study of green roof thermodynamics.


I'm sure that over time a design model will be developed and accepted by the industry.  in the meantime - data sharing is crucial and important.

Nature's Living Wall Plants

Every season brings a new face one of nature's finest display of living wall plants growing on the stone walls of Castillo De San Marco, St. Augustine, Florida.  St. Augustine is the oldest colonized city in the U.S. and plants have been growing on the hard shell facade of buildings here for centuries.  The fort is an educational paradise for any green roof or living wall designer.

Irrigated only with dew, fog and rain, the wall plants grow out of sheer vertical rock with little or no soil.

Wildlife are drawn to the diverse range of native plants in droves.

Native plants play an important role in supporting worldwide biodiversity heritage.

For a great blog post on just how important native plants are for supporting biodiversity, read the post here at Wildlife Garden.

One of the many ways to learn about native plants in your local downtown area is to 'look up'.  This week I again, spent several hours walking in the moat of the old Spanish Fort in St. Augustine.  In my opinion the National Park Service has it backwards - they charge for going inside the fort but allow you to walk for free in the moat and around the grounds.  The moat is where you can see many, many native and other plants species growing in the coquina walls!

Castillo de San Marco, St. Augustine

As you can see in the above photo, most plants grow underneath the downspouts on the walls.  Though the downspouts provide water primarily when it rains, they also collect dew and fog from surrounding areas and funnel the water to the plants.

Interestingly, learning about green roof and living walls plants from the fort offers insight into those plants that not only do well under the hot Florida sun and with no additional irrigation, but also the plants shed light on soil media composition.

Coquina shell and the limestome mortar have a quite high pH level.  High pH is usually one of the toughest issues to work with on green roofs and living walls.

Samolus valerandi

Florida Green Roofs: Pteris vittata & 2 Cuban anoles

Wildlife seeks out plants, especially those providing resource benefit such as food or nectar or shelter.  Native plants are best suited at providing the most optimum level of ecological benefit to those wildlife endemic to an area.

In otherwords, planting native plants on green roofs and living walls encourages and supports native populations of insects, birds, reptiles, amphibians and other wildlife.

Many popular landscape plants used on green roofs may not offer the same level of resource benefit.

Understanding natural systems through frequent field observations of your local native plants broadens design capabilities for both green roofs and living walls.

Floria Green Roofs: Limestone & Coquina Walls are Harsh Ecosystems

Green Roof Plants, C4 Species Subject of Increased BioEngineering Interest.

Often times daily inquiries arrive as to what type of plant should be installed on a particular green roof in the Tropics.  Knowing plant photosynthesis is a foundational prerequisite to basic green roof design.


One does not merely 'put' landscape plants on a roof unless those plants have been modeled to show potential survival success.


We've discussed C3, C4 and CAM principles in previous articles.  I now see a major trend towards the development and use of C4 plants in world agriculture.


Plants with C4 photosynthesis processes are now the focus of bioengineering efforts of the worlds foremost plant industry experts.

The C4 Plant Biology Symposium held in Shanghai examined the opportunities of genetically and biologically engineering C3 plants to possess some of the C4 qualities of drought and heat resilience.

The Symposium abstract pdf offers significant insight into the interest of C4 plants' capabilities to survive where C3 plants may not.  Of course, most of the interest now lies in areas of food production (think rooftop permaculture), however horticultural paths, including green roof plant development will soon become a part of the C4 genetic engineering efforts.

As a biodiversity purist I emphatically prefer and recommend use of open pollinated native plant species on green roofs and open pollinated seeds for rooftop permaculture.  But we must take note that the industrial agriculture world understands the benefits a C4 plant offers and we need watch advances in genetic engineering, tracking potential impacts to native plant species and ultimately biodiversity issues.  Keeping abreast of industry trends is important to ensure continuation of biodiversity practices too.

One very clear fact is this:  C4 plants are much more hardier than C3 plants against wind desiccation and dehydration.

Brown, dead Poeaceae C3 species and green, alive Alliaceae C4 species subjected to 48 hr. 1.75 M/S winds

The photo here is of a trial green roof panel, removed from the roof after being subjected to 48 hours of low humidity (approximately 40% relative humidity water vapor) 1.75 meter per second winds, and approximately 12 degrees C average temperature.

• Absorbing CO2 into the mesophyll - 

• Once the CO2 is in the mesophyll it combines with a 3-carbon compound called PEP, phosphoenolpyruvic acid to form a 4-carbon substance called oxaloacetic acid - hence the C4

• The C4 is moved into the deep bundle sheath layer of cells away from the leaf surface

• Inside the bundle sheath the C4 breaks down into the C3 base and eventually PEP for the Calvin Cycle food production

The panel is planted primarily with a cool season annual Poaceae C3 species.  There are also several Alliaceae family C4 photosynthesis plugs int he panel.

Remember, the C4 plants, because of their internal structure, loose on average only 1/3 the amount of water during the Calvin Cycle and photosynthesis as C3 plants do.  C4 plants are much more resistant to dessiccation from strong winds.  This is because C4 plants generally conduct photosynthesis by:

Because the oxaloacetic acid (C4) is moved into the deep bundle of sheath layer cells away from the leaf surface, the C4 plants loose less to transpiration activities from wind and sun action.

Yet there are many additional considerations to integration of C4 and C3 plants on a green roof, including growth rates.  Because C3 plants conduct photosynthesis much more rapidly (remember the analogy between the C3 and C4 plants as a comparison of a large V8 engine vs. a smaller hybrid engine), C3 plants will provide green roof coverage quicker.

Understanding how to integrate C3 and C4 and then CAM species is key to a successful green roof design.  Knowing how green roof plants utilize photosynthesis is key to a successful green roof.


Too many times we in the green roof industry have focused on stone and soil media, plastics and drainage, pumped up additional irrigation and added more and more fertilizers when all we really needed to do is put the right plant in the right place.


Green Roofs should really be about the plants and biodiversity.

Green Roof Gardening and Small Scale Rooftop Permaculture

Reprinted from Florida Permaculture Blog:


Rooftop permaculture may be the long term sustainable trend for green roofs.  Where as green roofs may have many benefits, including insulation, mitigation of urban heat island effect, cleansing of stormwater, support for biodiversity, educational opportunities and more, growing food within the city can save food fuel and transportation costs, create permaculture benefits, cool structures, save money and provide much needed sustenance within inner cities.

Florida Permaculture, Rooftop Gardens - Mustards & Garlic Chives

With advances in green roof and permaculture technology, cost effective and lightweight growing systems can be created and installed for food production across balconies, patios, rooftops and windowsills.

Self watering and systems employing fog nets, dew catchers and condensate reuse will take the place of non-sustainable potable irrigation water.

Florida Permaculture, Greens and Garlic Chives on the Rooftop

Additionally, there is so much permaculture information available on the great world wide web the need for fertilizers can be easily replaced with proper and informed design principles utilizing nitrogen fixing plant species.  Our rooftop tomatoes growing alongside legumes were so much larger than those in our ground level gardens.

Importantly here in Florida (and other places), rooftop gardening eliminates many of the soil borne plant root pests such as nematodes.  Nematodes can devastate garden vegetables, stunting their growth by as much as a severe drought would.  Nematodes generally cannot survive in the hot temperatures typical of green roof soil media.

Florida Permaculture, Clover feeds the Greens on the Roof

Food plants such as those shown above, will have the advantage of first view by pollinators.

Your roof will become alive with butterflies, dragonflies, moths, birds, bees and more.

Tree frogs and anoles will soon take up residence, creating a wonderful integrated pest management systems as they eat volumes of the common household fly, mosquitoes, roaches and other pests.

Consider planting veggies on your roof as the next permaculture project.

Florida Rooftop Permaculture, No Room for a Garden? No Problem

Rooftop permaculture may be the long term sustainable trend for green roofs.  Where as green roofs may have many benefits, including insulation, mitigation of urban heat island effect, cleansing of stormwater, support for biodiversity, educational opportunities and more, growing food within the city can save food fuel and transportation costs, create permaculture benefits, cool structures, save money and provide much needed sustenance within inner cities.

Florida Permaculture, Rooftop Gardens - Mustards & Garlic Chives

With advances in green roof and permaculture technology, cost effective and lightweight growing systems can be created and installed for food production across balconies, patios, rooftops and windowsills.

Self watering and systems employing fog nets, dew catchers and condensate reuse will take the place of non-sustainable potable irrigation water.

Florida Permaculture, Greens and Garlic Chives on the Rooftop

Additionally, there is so much permaculture information available on the great world wide web the need for fertilizers can be easily replaced with proper and informed design principles utilizing nitrogen fixing plant species.  Our rooftop tomatoes growing alongside legumes were so much larger than those in our ground level gardens.

Importantly here in Florida (and other places), rooftop gardening eliminates many of the soil borne plant root pests such as nematodes.  Nematodes can devastate garden vegetables, stunting their growth by as much as a severe drought would.  Nematodes generally cannot survive in the hot temperatures typical of green roof soil media.

Florida Permaculture, Clover feeds the Greens on the Roof

Food plants such as those shown above, will have the advantage of first view by pollinators.

Your roof will become alive with butterflies, dragonflies, moths, birds, bees and more.

Tree frogs and anoles will soon take up residence, creating a wonderful integrated pest management systems as they eat volumes of the common household fly, mosquitoes, roaches and other pests.

Consider planting veggies on your roof as the next permaculture project.

Florida Green Roof Irrigation Systems - Underground Cisterns and Rainwater

EcoRain Tank Modular Rainwater Harvesting for Green Roofs

We've talked about water crises and irrigation of green roofs many times before.  Designing a green roof irrigation system to run off of rainwater instead of potable water is easy.

One of the biggest disadvantages of large rainwater tanks before has been the freight expense of bringing a large tank from the manufacturer to the site.

Many times the cost of freight exceeded the cost of the tank - and there are not many tank manufacturers so the tanks may have had to been hauled across the country.  Long freight hauls create large carbon footprints.

However, several new structural Rainwater Harvesting Systems are now available on the market that are collapsable and can easily be assembled on-site.  For a video of a very large underground rainwater collection system in Dallas, Texas click here (opens in a new window).

Florida BMPs: Rain Tanks Rainwater Harvesting System

One of the more green subdivisions in Gainesville, Florida, the Madera Subdivision, utilizes underground collection and storage of stormwater in several of their homes.

Collection of rainwater and subsequent irrigation of green roofs can be a valid method for allowing larger buildings on Urban Core lots previously designed with much smaller impervious allotment.  This applies also to historic residential areas where renovations are occurring. 

Importantly, the Green Roof design needs to anticipate periods of natural drought where stored rainwater may be depleted.

The use of potable water should be only used in certain instances, and the green roof plant selection should reflect species that can utilize the stored rainwater, however are also drought tolerant.

Florida BMPs: Rain Tanks Rainwater Harvesting System

Typical Underground Rainwater Storage Systems for Green Roof irrigation are designed and installed with little effort.

First the seasonal high groundwater level should be determined.  This can be accomplished with the civil engineer or geotechnical expert.  For the homeowner, simply take a shovel, I prefer a set of post hole-diggers, and dig a small hole down until wet soil or water appears.

The Green Roof Underground Stormwater Harvesting System should be primarily installed above the Seasonal High Ground Water Level.

Size your storage system to hold a 24-hour storm event quantity for the surface area of the roof you are collecting from.  ARCSA has a great website about sizing and designing systems - click here - opens in a new window.

Modular Rain Tank Unit

You can use solar pumps to pump the water back up from the rainwater harvesting cistern to the green roof. 

Unpackage your modular volume systems and assemble on site, placing them inside the liner within the excavated cistern area (refer to the video above for illustrations).



Remember, we a growing population on this globe and conservation of all water resources is important.  Ensure your green roof plant designer is using species that are friendly to conservation of water!

As always, feel free to contact us with your questions!