Native Plant Selection Modeling, Designing Coastal Green Roofs Part One

Native plants and wildflowers can contribute to the design of a permaculture garden, green roof or container garden on an apartment balcony.  This video shares many of my design hacks and tips for plant selection.  Though the video was made to be an aid to green roof designers, the fundamental design principles are important for any garden or landscape design.  Hope you enjoy and learn from the video.

Native Plant Patterns and Historical Rainfall Trends, Predictors of Green Roof Plant Success

Climate and weather patterns are the most significant determining factors of what plants will go on your green roof project and so, rather than turn on the TV I like to look at historical weather maps.

National Weather Service's Historical Rainfall Maps

Green roof plant modeling process considers light and wind to be the two primary design variables for factors affecting green roof plants.  Sunlight relevance to green roof plants and for that matter even ground level landscape designs is usually referred to in terms of 'Photosynthetically Active (and Reactive) Radiation, or PAR.  Too much PAR and the plants can burn, desiccate and wither.  Too little PAR and the plants fail to grow.

Along with PAR is the photosynthetic pathway of the green roof plant and a host of other survival mechanisms such as photoperiodism, phototaxicity and phototropism.

Wind impact too is a design variable that must be accounted for during green roof plant layout.  Strong desiccating winds can harm green roof plants with as much severity as PAR overload.  Wind can pull so much water out of a leaf that the plant's vascular system will be overwhelmed and interestingly, no matter how much green roof irrigation is added to the planting bed, the plants still die.

Micro-irrigation usually alleviates the stresses of long droughts and so on those green roofs, available rainfall impacts may not be as much a controlling design variable (though still extremely important) as wind and light.

Still I find it very interesting to study rainfall patterns across the U.S. and across the world.  Nature has laid out and sorted the different types of vegetation across our continent in a manner relating to wind light and also according to rainfall amounts.

Yes, it is a simple and very broad generalization to say that following Mother Nature's lead supports project landscape or green roof plant potential success.  

When I look at the above map depicting historical rainfall amounts published by the National Weather Service, I see three main, broad patterns.  The Northwest and the East (red and orange areas) receive most of the rainfall across the U.S.  Broad leaf dicots and C3 monocots fill these regions.  Florida and the Central Plains (green areas) receive less than average precipitation and are vegetated with great stretches of grasslands.  Here in Florida the pine flat woods which make up much of the state are filled with C4 ground cover grasses such as the Andropogons and Sporobolis species (if you live in Pensacola though you may want to choose wetland plants for your green roof due to all the rain they have been receiving lately).  Finally, the areas depicted by the least rainfall amounts (less than 20 inches per year - light blue geographic regions) are inhabited by cacti and other succulents.

So if I were designing a green roof for an area outside of Florida I'd think of this map first.

I may or may not end up following Mother Nature's lead after examine a complicated host of other factors, including client intent and if I choose not to follow then my green roof planting design better be spot on in producing the ecology my selected green roof plants will require.

The roof is a seriously harsh place to grow plants.  Use of Mother Nature's millennia of trial and error as guidelines for selecting green roof plants is smart green roofing.  It is hard to beat local native plants on the roof or across the ground.

Green Roof Plant Design - Understand Heliotropism and Paraheliotropism

Understanding photosynthesis processes in green roof plants is crucial to the green roof designer and we have discussed the difference between C3, C4 and CAM photosynthesis processes in prior notes, including;

• CAM Plants for Green Roofs, Effects of Heat and of Humidity
• Adaptation and Green Roof Plants

• Green Roof Vegetables - A List of Recommended C4 Plants

   

   

   

Under Intense Sunlight Leaves Fold to Prevent Desiccation

Yet nature is complicated and does not limit herself in resisting environmental stressor conditions to just the above three types of photosynthesis processes.  In fact, some plants have evolved other survival mechanisms to help prevent dessication and to conserve water, especially in arid and hot climates.

Some plants, and many of these are excellent species to use on Green Roofs, can move, open or close their leaves to prevent dessication.  This is commonly referred to heliotropism and paraheliotropism.

Cowpeas and other plants for example possess the ability to adjust their leaves position to either increase or decrease the amount of leaf surface area receiving direct sunlight.

Under optimal Sunlight C3 Leaves Open Widely

Under Optimal Sunlight Conditions C3 Leaves Open Widely

Heliotropism is the term used to describe a plant's actions when it orients leaves to receive more sunlight and thereby increasing photosynthesis capabilities, i.e. - more CO2 fixation.

Diaheliotropism describes a plant whose leaves actually track the sun'd path, maximizing solar contact.

Paraheliotropsim denotes a plants actions similar to the above photographs.  During paraheliopropsim, plants may fold or move their leaves to either;

• Minimize solar contact, or
• Minimize total leaf surface area,

either way reducing water loss and preventing desiccation.

Though C3 plants such as the beans - and most other of the world's food plants except the grasses, maize, and sorghum - do not possess as complicated a multi-cellular Calvin Cycle as C4 plants and are more susceptible to drought, they - through Heliotropism biomechanisms they do possess their own unique desiccation prevention mechanisms.

Green Roof designers take note!

Green Roofs and Living Walls are Really Just an Adaption of Raised Bed Planting Theory

If you understand raised bed planting theory you know most of what living roofs and walls are about.  Many times people think there are secrets to green roof and living wall design.

In fact, the corporate world has tried to patent many systems that are merely raised beds.

So expand your design considerations today by thinking of green roofs and living walls, not as a mysterious, high-tech approach to growing plants.  Think of your next design project where you need plants (be they flowers, food, natives or landscape specimens) as a raised bed.

Green Roof projects are like Raised Beds!

Questions to ask yourself include;

• What will your soil media be comprised of?
• Should be lightweight, well-drained not made from exotic synthetics heated in ovens to thousands of degrees for expansion or molding.  I like simple things, like sharp sand.
• What are your raised bed (or living roof/wall) side walls made of?  
• Stay away from materials that are flammable, UV degradable or contain toxins.
• What plants do you want to use?
• Check out the many discussions on C3, C4 and CAM photosynthesis included here on this blog.
• What is the root architecture of your chosen plants similar to?
• Do you have deep roots?  Shallow root requirement?
• Root architecture will direct your soil media depth design.
• Try and stick with plants that provide food or are pollinator attractors.
• I like native wildflowers but many designers stick with the tried and true succulents or sedum.
• Food plants are also wonderful for rooftops and walls if protected from winds.
• Where will the irrigation water come from?
• I hate adding water to a roof.  They all eventually leak....but if water is already there, like AC condensate - use it!
• Views
• Plantings are mostly always pretty - make sure they can be seen.
• Access
• Can you get to the raised bed/rooftop or wall garden?
• Dont put a raised bed on top of a septic tank and neither install a rooftop garden on a weak rafter system.
• Common sense
• and more...

The point here is - think of a rooftop design as a great raised bed in the sky.  Don't wait for the right 'system' to come along.  Go for it, design and install!

Green Roofs & Cyclones - A Continuous Vegetated Assemble Design for Hurricane Impacted Areas

The 2012 Hurricane season will be upon us in a couple months, if not sooner.  Cyclones, hurricanes and tornadoes are all important factors to consider when designing a green roof.

Of course no amount of design and construction can preclude extremely high wind damage as we have recently witnessed across the midwestern United States.

Continuous Vegetated Assembly Drawing for Green Roofs in High Wind Velocity Areas

However, common sense tells us that any object on a roof should be permanently fastened in some manner if the building is subject to high velocity winds.  Even then, these extra-velocity winds can rip up heavy items and toss them to the roadway below.

However, though it may be impossible to prevent all wind damage, use of three dimensional geogrid and tri-dimensional non-woven and woven fabrics can provide a platform for green roof plants to anchor themselves in a permanent fashion to the roof.

Choosing the correct bi or tri-axial geogrid for your green roof design can be confusing.  Attaching the geogrid to your roofing system can be even more perplexing.

After years of field trials with a number of popular fabrics, I have seen some work well while others disintegrated under the stress of sunlight or wind or both.

Most are a PPE or other petro-synthetic compound.  Some, more sustainable than the plastics, such as jute and hemp work well under certain conditions yet fail quickly under other environmental factors.

Importantly though, a properly chosen geogrid can not only form the basis for a cyclone resilient roof, but can allow the green roof designer to build continuous vegetated assemblies on slopes.  In fact, I have seen some innovative living walls based on these geogrid fabrics.

The sketch provided here is a depiction of a flat roof continuous vegetated assembly system for hurricane influenced geographic areas.  The three dimensional geogrid can be permanently attached to the roof through a variety of either mechanical of chemical bonding mechanisms.

Keeping plants on a roof is important.  Parapets help.  Good prudence also calls for permanent fastening systems too.

Florida Green Roofs and Wind Impacts on Green Roof Plants

Wind is an oft-neglected but so important design parameter of green roof layout.

Dry, desiccating winds can damage or kill green roof plants faster then drought or a host of other environmental factors.

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.

MetroVerde  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.

Green Roof Design - Biaxial Geogrids, Wind and Water Erosion

Here in hurricane or cyclone alley we have always recommend use of a restraining weave embedded in green roof soil media.

I like the thick three dimensional weaves of non-woven geogrids.  Those offering multi-axial reinforcement are even better.  However sustainability probably lies in coir or hemp and not in the nylons and polypropylenes.

Regardless, three dimensional weaves have a huge impact on erosion control for green roofs!  Check out this video!

Flat, Extensive Green Roof Typical Cross-Section for the Tropics and Hurricane influenced Areas

I receive daily requests for typical cross-sections so I thought I start posting some of the more popular.

The above system is our system we've tested at the University of Florida's Civil Engineering Hurricane Simulator.

If you'd like a pdf version, email me here - kevin@metroverde.com and I'll be glad to send you one.

Brown Green Roofs, Green Roof Forensics

Any green roof designer has had plants die from time to time.  Roofs are one of the most harshest places to grow across and good green roof design involves much more than basic landscaping.

Here in the tropics an understanding of environmental factors is crucial to ensuring green roof survival, beginning with a solid knowledge of plant selection. Importantly, long-tern survival depends not only on appropriate underlying green roof support systems, but soil media and 'Right Plant Right Place' issues also.

The tropics, for instance possess finicky climates with wild temperature swings, horrendous humidity, extreme temperatures, desiccating winds, salt spray, stealth-like killer fungi, hurricanes and more.

This doesn't mean green roofs do not work in the tropics, they do!

Unfortunately, the attached photos depict what can go wrong with a green roof here.

Struggling Green Roof

Sedum is not generally considered a plant suitable for the tropics.  Note I say generally as I am sure there may be exceptions.  Southern Blight fungus and hot, sticky humidity overwhelm sedum during the hot summer months.  The same applies to many Delosperma also.  

Rainfall quantity distribution here is unusual.  In many places receiving 150 cm rainfall each year the majority of that amount may come in only a series of heavy downpours.  Green roof system drainage must be designed to handle extreme water flows, otherwise failure may occur.  Apparently some of the soil media here has ended up in the swimming pool below.

The example shown here did not possess additional irrigation.  Instead as the plants withered, a rotary sprinkler was pulled up to the roof in a futile attempt to save the plants.  Im not sure how long the sprinkler ran, but it did run long enough to erode grooves in the soil media.

Irrigation erosion from sprinkler

A green roof system without irrigation is a environmentally friendly approach, and can work.   However other factors, including appropriate plant selection must be considered.

Soil media must reflect site specific drainage requirements.  Soil media high in fines may blind geosynthetic fabrics which comprise portions of the green roof drainage system, restricting vertical permeability and causing soil media to wash off the roof.

In this case it appears a lack of vertical drainage has caused rainfall to rush across the sloped roof soil media surface, scouring the grit and producing serious erosion.  It is entirely possible that during a tropical storm, such as TS Fay in 2008 where 600mm+ rainfall dropped in some places, the soil media here would end up in the gutters or on the ground unless appropriately stabilized.  Moreover, damp water retention pads and even the smallest of pooled water depressions can lead to hoards of pesky mosquitos.

Green Roof design involves much more than planting inexpensive fertilizer pumped up plants on a roof platform.

Appropriate green roof design takes into account wind, light levels, available daylight intensities, temperatures, invasive aspects, native plant considerations, nutrients, air quality including NOx levels, pests, adjacent potential allelopathic influences, air water vapor and much more.

Investing in a Green Roof Professional's time up front will save significant costs in the long-term.

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

Green Roof Design Guidance Summary

Good brief design guide for UK Green Roofs - click here.

Green Roof Mats Florida - Choosing Your Green Roof System For Florida

There are many good green roof systems available on the market today.  Both tray systems and mat systems are widely popular.  Pre-filled soil bags for the roof and other green roof designs are also available.

With so many choices, how do you know what system will be best for your green roof project?

I am going to offer a few suggestions here.

Importantly, you should first talk with licensed knowledgeable professionals in your area.  Make sure you talk to roofers that have been doing business in your area and have a solid reputation.

Just as important, make sure you involve a local landscape architect for plant selection.

Ultimately your installation should be completed by those who have experience with green roofs.  This will involve a company or team that is knowledgable about both roofing systems and plants.

A roofer that doesn't know plants may put on a good roof system but your plants may die soon thereafter if he or she doesn't understand how to choose the appropriate plants.

A landscaper may have a solid understanding of what plants are more likely to survive the harsh roof conditions in your area, but may not be licensed to install those plants on a roof, or may damage the integrity of your roof during inappropriate installation.

A successful green roof requires a team of professionals.  A good green roofing team can help you select the type green roof system best suited for your geographic area, your cost and your project requirements.

The above photo, for instance shows a green roof mat being installed over white roofing TPO (thermoplastic olefin).  We will discuss TPO as a green roofing component in another blog entry.  Due to TPO's stability (TPO contains a significant amount of stable polypropylene - ppe) the material is used frequently in roofing (and may make for a good root barrier in some applications).

Mat-based green roof systems have different preferred applications than tray systems.  Both have their place in Green Roofing.

Here in Florida, plants tend to grow quickly due to the extended growing season (some year-round) and mild climate.  Mats tend to allow plant root systems to grow radially all they want to.  Mats also allow the plants to 'move' across the roof over time until they find just the right spot for the species' requirements.

Another advantage of the mat system is it's light weight profile and interlocking ability to allow roots to grow through the mat weave - holding plants in place during storms or on slopes.

Again, your green roof project should involve a number of team professionals - those who understand plants and those who understand roofing.

As always, email me with your green-roofing questions and I'll use your questions here on the blog to help share green roofing data and information!

Happy Green Roofing!

Kevin