Florida Green Roofs - Know Thy Plants!

Florida summertime is hard on green roofs.  Not only are roofing membranes subject to intense weather challenges, green roof plants also are subject to intense biological and meteorological assaults.

Summer humidity may quickly kill Florida green roof succulents

We have posted notes about Black Rot fungus before here on this blog.  It never fails though that every summer persons will contact me asking why their succulents are turning brown, black or rotting.

Florida green roof succulents battle the Black Rot fungus every summer

The simple answer is to use Florida native plants (preferably evergreen species but a mix of deciduous plants will work too, depending on the green roof location in the state).  Here in Florida native green roof plants usually far outperform horticultural succulents.

Florida green roof succulents rarely become the dominant green roof plant and usually die out

Florida summers bring maximum humidity and maximum temperatures.  With daily afternoon rain showers most rooftops become pressure cookers, steaming green roof plants like vegetables in a hot wok.  If you do not intimately understand Florida roofs and how plants preform there (and this only comes from hands on learning, failures and successes are the best teachers) then your roof design may quickly end up devoid of plants as you stand there, helpless, watching the succulents literally dissolve in the heat and humidity and fungus attacks.

Tropical green roofs are a challenge.  We have listed some great green roof native plants here.

Know how your plants will perform in a pressure cooker before specifying them on a green roof here in Florida.

Why Florida Green Roofs Are So Environmentally Important

Here in Florida surface water can directly flow into the drinking aquifer below the ground in many places.  Here is a video of storm water flowing down into the ground through a karst connection to underground caverns after a recent rainfall event.

Cleaning rainfall runoff with green roofs and other urban greening projects before the stormwater reaches our drinking water supplies makes good sense.

Avoidance of pesticides, herbicides and lawn fertilizers and chemicals is good not only for our environment but also supports a cleaner and healthier place to live in.

Florida Green Roofs, Six Must Have Living Roof Plants

Enjoy the photos of six of my favorite drought, salt, heat, cold and hurricane tolerant green roof plant species!  See more photos of additional plants on the Green Roof Wildflower website.  Wildflowers on #greenroofs are totally awesome.

Florida Green Roof plant, Coral Bean, Erythrina herbacea
Drought, salt and heat tolerant.  Hummingbird plant.

Florida Green Roof plant, Prickly Pear Cactus, Opuntia humifusa
Very drought tolerant, great coastal green roof plant for habitat and pollinators

Florida Green Roof plant, Fleabane, Erigeron app.
Tolerates wet and dry soils, wind and heat tolerant, excellent pollinator plant

Florida Green Roof plant, Blanketflower, Gaillardia puchella
Amazing hardy green roof plant, tolerating salt, wind, heat and other environmental challenges

Florida Green Roof plant, Purple Coneflower, Echinacea purpurea
Hardy, drought tolerant green roof plant that pollinators LOVE!

Florida Green Roof plant, Black Eye Susan, Rudbeckia hirta
Stunning rooftop bloomer that will flower all summer and call pollinators for miles around

MetroVerde's Biodiverse Green Roof. Extensive, Lightweight, Hurricane Designed Green Roof for Urban Core.

Biodiversity in the Urban Core can be truly supported by Green Roofs.  Here the Breaking Ground Green Roof has over two hundred different species of wildflowers, herbs, vegetables and plants, attracting pollinators, amphibians, reptiles and birds. 

MetroVerde Green Roof at Breaking Ground Contracting #Florida-Green-Roof

MetroVerde Green Roof at Breaking Ground Contracting #Florida-Green-Roof

The BGC Green Roof treats and cleans stormwater, mitigates Urban Heat Island effect, sequesters carbon dioxide, fills the air with fresh oxygen, affords educational opportunities and so much more.

MetroVerde Green Roof at Breaking Ground Contracting #Florida-Green-Roof

The BGC Green Roof is a MetroVerde Green Roof, a very light weight system tested by the University of Florida under tropical storm and hurricane force winds with the large wind turbine testing equipment located within the Civil Engineering Department at UF.

MetroVerde Green Roof at Breaking Ground Contracting #Florida-Green-Roof

 The roof shown here has been in existence for approximately four years and is applied directly over white TPO roofing.

MetroVerde Green Roof at Breaking Ground Contracting #Florida-Green-Roof

 Hopefully we will see more and more Green Roofs within the Urban Core as the worth (economic, ecological and social) of sustainable design becomes more and more apparent.

MetroVerde Green Roof at Breaking Ground Contracting #Florida-Green-Roof

Tropical and Coastal Green Roof Design with Native Plants, MetroVerde Design Video

Green Roofs are complicated enough to design and build.  But those affected by salt spray and tropical storms are even more intricate and possibly problematic. 

Watch Part One of our Design Video for Coastal and Tropical Green Roofs.  Each part is approximately thirty minutes and will focus primarily on selection and layout of native plants for coastal Green Roofs.  Part Two will be available for viewing later this weekend.

HVAC Air Intake Covered in Plants - Green Roofs and Living Walls Filtering Air Flow

Newton, the Ask a Scientist, Scientist - available for consultation from the U.S. Government - click here for Newton's Website... - 

...says 53 Liters is the amount of pure Oxygen the average adult needs to survive every hour.  53 liters is approximately 14 gallons.

You can see we use alot of oxygen on a regular basis.  Now picture your bedroom and night or your office during the day.  For estimation purposes we will use an office space of about 5,000 SF with a 9' ceiling and containing 24 employees.  The Office contains  336,600 gallons of atmosphere.

Green Roof Plants filter toxins and produce oxygen

According to Newton the air we breathe contains about 21% oxygen so the office will contain 70,686 gallons of oxygen.  Is this enough to last the 24 employees for a day?  Let's find out.

Each employee breathes 14 gallons of pure oxygen per hour - more if they are active but most office workers are sedentary - so each employee breathes in120 gallons per shift and the office as a whole breathes in 2,880 gallons of oxygen per shift. 

OK - that's plenty of oxygen to start, but within a month without the doors and windows being opened the employees will rapidly use up all the oxygen.  How old is your office?  How old is your house?  When is the last time you've flushed the air in your office or house?

Just think, without the windows being open, you are breathing stale air - air already breathed in many times over by others in your office or house.  This air contains not only stale exhale of others (and suspended germs) but volatile toxins off-gassing from carpet, furniture, paint and other manufactured goods in the office or house.

And the unfortunate part of the whole equation is - we keep our windows shut most of the time.

However, there is a solution - plants!  Plants produce oxygen as a by-product of photosynthesis.  Your personal oxygen machine is as readily available to you as setting a plant on your desk.

I suspect all employees would be happier if air intakes for HVAC systems (most residential systems are closed loop systems) were covered with vines and pumped full of oxygen.

Moreover, plants are extremely efficient at removing toxins from the off-gassing process.

Green roofs and living walls are a key component to filling our sometimes stale Urban Core with fresh oxygen.  Imagine buildings downtown covered in plants and those plants pumping out oxygen daily.
The roof-based vegetated air intake tunnel keeps a building roof vegetated and provides additional benefits, including;

1.  Cools intake air
2.  Shades the roof
3.  Removes airborne toxins, and more!

MetroVerde Intake Tunnel Alternative to Green Roof

 

Additional benefits allow for reducing heat island effect, providing shade, cleaning stormwater, wildlife habitat and much more.

Restoring volumetric green to the Urban Core.

Green Roofs are the key to healthy cities.

Florida Permaculture Plant for Living Walls, Florida Green Roofs and Backyards, Seminole Pumpkin, Cucurbita Moschata

One of my favorite vines this year is the Seminole Pumpkin, Cucurbita moschata. 

Florida Green Roof and Living Wall plant, Seminole Pumpkin (Permaculture Food)

An adapted garden wonder to Florida, the Caribbean and Latin American, this variety of pumpkin or squash is acclimated to the harsh, humid climate of the region. 

Unripe Seminole Pumpkin, resistant to pests

A fast grower who provides ample shade, Seminole Pumpkin makes a great end of summer living wall and green roof plant.

Florida Living Wall plant, Cucurbita moschata

Thriving on neglect and drought, Cucurbita moschata, is ultra resilient to squash vine borers and other pests.  Here she is used as a cover to our geese pen, providing a wall of privacy, security, shade and food.

Seminole Pumpkin creates a living wall and green roof for the Urban Farm fowl

When thinking of drought tolerant plants for tropical green roofs and living walls, they don't just have to be wildflowers.

Seminole Pumpkin is a heavy food producing plant

 Nature has provided us with some awesome  food plants who will thrive well in the permaculture garden and on the hot roofs and walls.

Urban Rooftop Permaculture, Bringing Organic & Healthy Food to the Urban Core

"Gene manipulated corn fields feed people"

The above comment was a response this morning to one of my suggestions rooftop permaculture can help.  It is really sad because some truly believe this.

Fortunately - people around the world are beginning to grow their own food, on patios, in windowsills and on rooftops in the Urban Core.  I believe rooftop permaculture and vertical permaculture is well on its way to replacing the old and becoming the new frontier in volumetric and rooftop green.

Green Roof Rooftop Permaculture - Brassica

Green Roof Vegetables - Broccoli

The broccoli here were pulled from the roof garden this week.  We had the Brassica growing in 4" of light weight highly organic composted and very well drained soil on a 4/10 slope.  You can see just how the vegetables became with no additional fertilizers and zero pesticides and herbicides.


Understanding permaculture principles and integrating those maxims into rooftop gardens can pay off with substantial results.  Organic greens, which cost four or five dollars at the market can be grown in masse on a small roof plot from an inexpensive packet of seeds.
Unfortunately, large corporations view rooftop permaculture and the citizens taking the task of feeding themselves back into their own hands as a serious threat to profits.

But I reject the above statement that "Gene manipulated corn fields feed people" as the only successful approach.

Many people may want to eat GMO corn products.  Yet I believe a large portion of the population may not want to.

So I see a grassroots movement arising to support rooftop permaculture, one where common people can make nutritional decisions for themselves rather than having the State or a Large Multi-national Corporation do so.

Check out just how big the Brassica grew.  Look at the root architecture.  We planted them close together to brace against wind issues and ate the greens daily. The plants adapted to the 4" soil (100mm) dispelling the myths of need for deep dirt to grow.
ECHO, see http://echonet.org is a great resource for rooftop permaculture practices, tips and design guidelines from a cost-effective approach.

Our $2.00 worth of seeds provided a daily bunch of organic greens and broccoli tops that would have cost US $ 5 in the store for six months.  As I see it the numbers work out to be close to $1000 savings in food costs, not to mention the health benefits.  That is just for organic broccoli alone.  Add the collards, mustards, sugar snap peas, pok choy, turnips, potatoes, tomatoes and the benefit of fresh air and gardening companionship and one can easily see the economic advantage.

No thank you to GMOs and to the outdated, stale industry guru's that are old news.

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.