Salt Tolerant Native Plants, Turkey Tangled Frog Fruit, Phyla nodiflora

 Old buildings teach me so much about the wind, sun and salt tolerances of native plants.

Phyla nodiflora, Turkey Tangled Frog Fruit growing out of the old brick walls of Fort Pickens, Pensacola Beach

Turkey Tangled Frog Fruit (TTFF), aka Lippia nodiflora (Verbenaceae) has long been one of my favorite Florida native plant species.  First of all I'm enchanted by its name.  Rumor has it that once upon a time a flock of turkeys got their long legs and feet all tangled up in the Frog Fruit vine-like flexible stems. Supposedly all the turkeys then fell over, practically squishing all the happy frogs who were eating the plant's fruit-like flower buds.  Of course this ruckus caused quite a stir with all the butterflies feeding on TTFF's nectar and they proceed to flit about telling the mockingbirds what was happening.  Well you know, once a mockingbird hears a good stanza, it'll be repeated everywhere. To this day the low growing groundcover as been known as Turkey Tangled Frog Fruit.

From time to time, I've been asked for a list of native plants that may be salt, wind and blazing sunlight tolerant.

There are many reference books that discuss optimal growing conditions for native species, but I always find that the best way to learn which native plants do best in any location is to walk around and look at what plants are growing on older buildings in the specific location you are interested in.

Here is a photo of a marvelously healthy Turkey Tangled Frog Fruit growing out of the brick walls of Fort Pickens in Pensacola Beach.

The only irrigation this plant receives is dew, air humidity, ocean salt spray and rain.  The Gulf of Mexico is only a few steps to the south and the just as salty Pensacola harbor is a few steps to the north.

There is no tree canopy around for shade, just hot, glaring sunlight.

The four or five meter per second persistent ocean winds are salt laden and dessicating.

The old brick walls of this fort are a hard place for any plant to survive.  Yet this TTFF specimen is thriving.

Spending time in the field is the very best way to learn what, where and how native plants need to grow.

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.

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.

Plant Size Considerations for Green Roofs - Size and Weight Are Important!

Sometimes we may be inclined to add small succulents to a green roof planting scheme because we know they may be tolerant to just about every environmental factor encountered on a roof.

Agave are drought, salt, wind, heat and hurricane tolerant, but they will add weight to a green roof!

Succulents can be deceiving with respect to size and weight, especially to those who have not worked long term with these plants and seen their size at maturity.  What may be a tiny, perfectly sized immature succulent may turn into a very large and heavy specimen with age.

Potential Florida Green Roof plant or potential roof problem?

Root architecture too is another important consideration for the green roof designer to take into account.

Some succulent plants develop swollen root tubers for water and nutrient storage.  Additional water means heavier weight.  Moreover, some of these roots grow in a very aggressive fashion and can damage an underlying single ply membrane or asphalt shingle roof.

Potential green roof plants have also have aggressive root systems that can damage single ply or shingles

While some small succulents, like sedum may never grow six feet tall, others can.  Though the succulents and cacti you plant may do wonderfully for the first several years,  over time they may outgrow the roof's planting bed.

Problems arise when these overgrown plants present weight issues with respect to the loading capabilities of a supporting roof structure.  An extensive green roof with smaller plants may weight twenty to thirty pounds per square foot or 100 - 150 kg/sm.  The same extensive green roof with 5' to 6' agaves may weigh three or four times the original weight.

Wind resistant can be another important factor with large plants on a rooftop, especially in hurricane impacted areas.

When you increase a roof's live load fourfold you may cause structural failure issues.

Treat some succulents as potential large shrubs or small trees.  Remember, just because a small succulent looks just right for your green roof doesn't mean it will stay that way.

Green Roof Plants, Rain-Root Zone and Root Architecture

One of our favorite Florida #Greenroof plant Genus is the Allium genus.  We've also been working with Resurrection fern also - and I love this plant.  Resurrection fern,  Polypodium polypodioides, was the first fern in space - going up on a 1997 Space Shuttle Mission to see if the roots would absorb water in a space capsule.

Green Roof Plant Root Architecture - Florida Extensive Green Roofs - MetroVerde

Both of these plants, the Allium and R. fern can have unique root characteristics, depending on how they are raised by the grower.

We call plant root structure by the name - 'Root Architecture'.

Green roof design has unique root structure and root architecture requirements.

Unless you have an unlimited potable water or well water source and are going to pump all that water up on a roof to keep plants up there watered, then your green roof plants need to be somewhat drought tolerant.

Certain root architecture patterns support plant acclimation to drought conditions better than others.

Remember, Florida's rainfalls usually are short, afternoon events of 1/2" or less and because rain generally occurs between the hotter months of the year - June - September, there is a tendency for it to evaporate quickly.

Except for hurricanes and tropical storms, rain events in Florida are usually over relatively quickly.

Meaning green roof plants have to scramble to grab the rain water.

Also recall, most green roof plants do not like wet roots (wet feet) so the soil must be well drained.

Proper green roof plant root architecture is crucial for providing a Florida extensive green roof plant with the advantages needed to survive a Florida vegetated roof.

Examine the diagram below showing the root architecture of a green roof plant raised in a one gallon standard nursery container and then a green roof plant raised in a built-in-place green root extensive planting bed.

The plant raised on the built-in-place green root extensive planting bed possesses 8 times the amount of Root-Rain surface contact area as the same size plant grown in a nursery container.

So when the afternoon 1/2" rainfall (13mm) event occurs and every drop is important - the green roof plant with the appropriate root architecture will sequester the most water.

More stormwater is captured, runoff is reduced, plants acquire necessary water volumes, plants have less of a tendency to uproot in high winds, and more.

Green Roof Plant Root Architecture is important to the success of a green roof.

Coastal Green Roof Plant, Railroad Vine, Ipomoea pes-caprae Creating a Living Roof

Here is another photo of a living, green roof created by Mother Nature's amazing coastal, salt-tolerant beach plant, Railroad Vine, Ipomoea pes-caprae.

Florida Green Roof Plants - Ipomoea pes-caprae, Railroad Vine

This plant is potentially a great addition to any coastal green roof design.  With its ability to grow into the salt water, withstand extreme heat, drought tolerant and inorganic soil media loving characteristics, Railroad Vine is a potentially great choice for many tropical, coastal green roof projects.

Here, in the above photo, the vine has colonized a parking cone, growing up into the top and outwards, creating a 'mini-green, living roof', providing habitat, attracting pollinators and shading the cone.

Nature can teach us so much about living walls and green roof design.

Coastal green roof plant, Railroad Vine, Ipomoea pes-caprae

Coastal green roof plant, Railroad Vine, Ipomoea pes-caprae

Florida Green Roof Images, Herbs and Roses on the Roof

Herbs on the Florida Green Roof.  Parsley, sage rosemary and thyme.  Yesterday Breaking Ground Contracting #GreenRoof photos.

Green Roofs in Florida, rosemary and garlic chives, herbs on the roof

Green Roofs in Florida, Louis Philippe antique rose and Florida native yucca filamentosa

Green Roofs in Florida, aloe and yarrow growing on the living roof

Green Roofs in Florida, aloe is such a great rooftop medicinal plant, drought tolerant & hardy

Green Roofs in Florida, aloe in bloom on the rooftop

Green Roofs in Florida, Louis Philippe rose buds, such a lovely rooftop plant

Coastal Green Roof Plants and Cross Section Design for Florida and the Tropics

Designing any green roof involves integration of many site physical, meteorological and biological variables, the actual creation of an ecosystem so to speak.  Coastal green roofs, those with salt impacts, are especially complicated.

Green roofs for coastal structures can present complicated issues to overcome +MetroVerde

Among the biggest variables to be considered during design are wind, salt and fire.  Salt spray will kill most landscape plants.  Around the clock steady to cyclone-strength winds can strip soil media right off and out of a green roof, rendering the system uninhabitable and possibly sandblasting an adjacent structure.  Accumulated biomass can fuel a lighting started wildfire across adjacent lots, wind carried sparks igniting a dry living roof and burning through a roof deck.

So we look for those plants that will grow despite salt, continual desiccation, fire potential and the geological components that successfully support them when designing a coastal green roof.

Coastal Green Roof succulent, seashore elder, Iva imbricata +MetroVerde 

Not only do I enjoy walking along the dunes because of the sense of beauty, the salty smell, the warm sun and sense of relaxation, but the seashore is rich in examples of vertical green, nature created green roofs and native living walls.

Coastal Green Roof plant, seashore elder, Iva imbricata +MetroVerde #greenroofs

Surprisingly, here in Florida there are some really good plants for a coastal green roof.  A walk along the dunes relaxes and teaches.  I recommend a good handbook, one such as The Smithsonian Guide to Seaside Plants of the Gulf and Atlantic Coasts or Florida's Living Beaches.

Native salt tolerant living walls by mother nature +MetroVerde , Florida living walls, Dune Daisy

Mother nature also does an awesome job creating spectacular salt and drought tolerant living walls along the shoreline.  Better than any design guide, beach strolls with camera, pencil and paper will show the designer what works and what does not.

Native salt tolerant living walls by mother nature +MetroVerde , Florida living walls, purslane

The following is a partial list I usually see growing well along the frontal portion of the dunes, in the harshest and most exposed areas.  I would use most any of these on a coastal green roof in the tropics or along the Gulf and Atlantic shoreline.

• Seashore paspalum, Paspalum vaginatum
• Seashore cordgrass, Spartina patens
• Panic grass, Panicum amarum
• Dune daisy, Helianthus debilis
• Beach morning glory, Ipomea imperati
• Rail road vine, Ipomea pes-caprae
• Seashore elder, Iva imbricata
• and Spanish daggers, Yucca aloifolia

But there are many more waiting for the designer to discover themselves!

Of course wind is a big challenge.  Wind can kill those plants not possessing protected photosynthesis pathways.  It is always good to review the differences in C3, C4 and CAM pathways when designing any green roof, but absolutely crucial for a coastal green roof.

Coastal Green Roof Cross Section - Typical Design +MetroVerde copyright MetroVerde 2014

Above is a general sketch of a design approach we may take in creating a coastal free roof subject to cyclone winds.  Of course the roofing assembly will need to be fire rated Class One.  Your roofing contractor working with the roofing supplier can specify the appropriate fire rated materials.

If you use all succulents on the coastal green roof you may not need to design in buffers, yet even the juiciest succulents can burn under the right conditions.

Be sure to follow ANSI, IBC and local standards and codes when designing a coastal green roof.

Seaside green roofs can be very complicated to design, but the very best teacher is always available.  Just hop in your car and drive down to the beach.  Mother nature's creations are phenomenal!

Florida Living Walls by Mother Nature

Really there are not adequate words to use here with the photos of living walls created by mother nature in Daytona Beach, Florida.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

 Amazingly, these lush living walls have no added soil nor do they boast an irrigation system.  They are buffeted by hard desiccating coastal winds and beat upon with hot sun's rays.  Yet they rival the most beautiful living walls made by us humans.

Nature is the ultimate instructor for green roofs and living walls

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Natural living walls on Florida coquina rock walls comprised of ferns and other plants.

Florida Green Roof Plant Root Architecture - Think Lateral!

Florida Green Roof plant root architecture and the converse relationship to irrigation is an important design function for the Green Roof professional to understand.

We've been working with Resurrection fern as a Green Roof plant for Florida and the tropics - and I love this plant.  Resurrection fern,  Polypodium polypodioides, was the first fern we know of in space - going up on a 1997 Space Shuttle Mission to see if the roots would absorb water in a space capsule.

Both of these plants, the Allium and Resurrection fern have unique root characteristics.

We call plant root structure by the name - 'Root Architecture'.

Green roof design has unique root structure and root architecture requirements.

Unless you have a huge potable water or well water source and are going to pump all that water up on a roof to keep plants up there watered, then your green roof plants need to be somewhat drought tolerant.

Certain root architecture patterns support plant acclimation to drought conditions better than others.

Remember, Florida's rainfalls usually are short, afternoon events of 1/2" or less and because rain generally occurs between the hotter months of the year - June - September, there is a tendency for it to evaporate quickly.

Except for hurricanes and tropical storms, rain events in Florida are usually over relatively quickly.

Meaning green roof plants have to scramble to grab the rain water.

Also recall, most green roof plants do not like wet roots (wet feet) so the soil must be well drained.

Proper green roof plant root architecture is crucial for providing a Florida extensive green roof plant with the advantages needed to survive a Florida vegetated roof.

Examine the diagram below showing the root architecture of a green roof plant raised in a one gallon standard nursery container and then a green roof plant raised on a green root mat.

The plant raised on the mat possesses 8 times the amount of Root-Rain surface contact area as the same size plant grown in a nursery container.

Green Roof Plant Root Architecture - Florida Extensive Green Roofs - MetroVerde

So when the afternoon 1/2" rainfall (13mm) event occurs and every drop is important - the green roof plant with the appropriate root architecture will sequester the most water.

More stormwater is captured, runoff is reduced, plants acquire necessary water volumes, plants have less of a tendency to uproot in high winds, and more.

So think lateral.  Think horizontal.  Experiment with green roof planting designs that encourage outwards rather than downward root growth.

Green Roof Plant Root Architecture is important to the success of a green roof.

Green Roof Plant Selection for Tropical and Florida Green Roofs

Best Florida Green Roof Plant?

We are always working to find suitable plants for Green Roofs in Florida.  For years Northeast Florida has been experiencing a severe drought.  Finding the toughest and most visually appealing is only part of the process.  Habitat value, invasiveness, wind and drought tolerance are other issues we consider.

Green roofs in Florida are harsh places – remember the 7 (or more) H’s:

• High Humidity
• Hot, hot heat
• High desiccating winds (killer)
• Hurricanes (not the football team)
• Hard Freezes
• Horrible temperature swings
• Hurtful droughts
• Harmful floods

And we all are cautious about irrigating a green roof (I speak as a lawyer – not a botanist here) – our litigious society has already bred a number of legal articles on green building and tort.  Imagine – the issues of:

• Mold
• Water damage to interiors
• Collapse from weight (water is heavy)
• Bacterial breeding
• and who knows what else…

So if we choose to acknowledge Florida’s water shortage problem and build a green roof with micro-irrigation or no irrigation at all, then we need to look to plants that:

• Can survive the many H’s
• Are visually acceptable by the community
• May be cost-effective
• Are preferably native species (or non-invasive species)
• Do not present a fire hazard or contribute too much dry leaf litter
• Are low maintenance
• Can survive long periods of drought
• Can survive twenty inch downpours
• Resist fungal infestations
• and much more

Five of the most outstanding plants that almost begin to come close to the above requirements are:

• Frog Fruit, Lippia nodiflora
• Wild Garlic, Allium canadense
• Adam’s Needle, Yucca filamentosa
• Lemongrass, Cymbopogon spp.
• Purple Muhly Grass, Muhlenbergia capillaris

What works for you?

Green Roof Plant Dwarfed 2 Year Old Allium Canadense

Green Roof Plants, Frost Damage to Certain CAM Plants

In earlier posts, we examined the damage frost can do to the mesophyll cells of C3 plants. Mesophyll cells are those cells across the surface of the leaf. C3 plants usually possess lots of stomata and C3 photosynthesis is a quick and rapid process, allowing C3 plants to grow rapidly, filling in a green roof.

In C4 and CAM plants photosynthesis occurs in a different manner with different cells. Today we will briefly examine one aspect of CAM plants and look at damage incurred by freezing temperatures.

Many CAM Plants, such as some sedum may die back to their roots upon experiencing a frost, others die completely.  There are some CAM plants however, in the genus Yucca and similar genus who can survive mild freezes.

CAM stands for Crassulacean acid metabolism. CAM plants generally have stomata adapted to open at night and not during the day, preserving water during hot periods or drought. Many plants living in arid ecosystems are CAM plants and can be recognized by smaller leaves with a higher leaf volume ratio (fat, fleshy leaves where water is stored in vacuoles), waxy leaf surfaces, sunken stomata areas. 

Examples of CAM plants popular in Florida include;

• Pineapple
• Jade Plant
• Euphorbias
• Sansevierias
• Aloes and others

Green Roof CAM Plant, Graptopetalum spp.

 The photo to the left is of a Mexican Ghost plant, many of us call a sedum, however it is really a not really a sedum but a Graptopetalum.

For an informative article with many reference links about CAM and Graptopetalum, see the blog post linked here.

Many succulents such as the Mexican Ghost plant and others can endure a little frost, though some of the more tropical varieties are tender to any type of frost damage.

However unlike C3 and C4 plants, many CAM plants store water in cells called vacuoles.  I call vacuoles 'smart blobs'.  Vacuoles are storage cells used for a number of plant physiological purposes.  In CAM plants vacuoles are used to store water and other photosynthesis related substances like the carbon dioxide.

Green Roof Plant, CAM Type After Hard Dreeze, Graptopetalum spp.

As mentioned, most CAM plants have thicker leaves - because of the presence of the stored water.  We have collected data over the years showing how the Mexican Ghost plant and other similar succulents survive minor frosts yet because of the expansion of water as it freezes, their cells rupture and the plant usually dies during a hard freeze.

Water stored in the vacuoles and other parts of the succulent's leaves expands as the temperature drops into the lower 20'S F (-6 to -8C).  As the stored fluid freezes and expands in size the vacuole wall and other cells rupture, injuring or killing the plants.

Though this phenomena can be observed across the green roof, it is especially prevalent in plants along the edges of the green roof.

CAM type succulents planted along green roof edges are exposed to unbroken cold, dry winds.  However within the interior of the green roof plantings we observe less freezing damage, probably due to stored heat in the planting media and the break from constant winds.

What lessons can we learn from the Mexican Ghost plant and freezing temperatures?  Many, including plant placement on a green roof and inter-relations between C3, C4 and CAM type plants.   We will discuss some of the hardier CAM plants in a future blog.

Green Roofs and Biomimicry's Lessons

An understanding of proper plant use for Urban Greening projects, including green roofs and urban permaculture gardens can come from many sources including, research, books, libraries, on-line information sharing and hands-on experience.

Florida Green Roof Plants - Frog Fruit crows across harsh, hot coquina sea wall boulders

One of my favorite ways to learn about what species of plants to use and how to grow them across cityscapes is to study successful plant habit in harsh ecosystems.

Those plants growing well under the influence of extreme heat, high humidity, strong desiccating winds, intense light or shade levels and other environmental factors will  usually survive on a city roof, against a wall, on a patio or balcony and in a windowsill.

Too many times designers try and use those plants that need to be gently cared for in cityscapes.  Ultimately the plants die and the urban greening project is shelved for another with less perceived maintenance requirements.

Florida Green Roof Plants - Nature's Examples are the Best Way to Learn About Urban Greening

However, there are many places to find the results of nature's selections and choices.

The beach and her sand dunes; hot, dry roadsides; Urban rooftops; gutters on buildings, vacant lots are just a few of many mini-biomes presenting opportunities for learning about plant growth habits, root architecture, soil, water and nutrient requirements and more.

This week I was walking along a coquina rock seawall.  I cannot go far with my dissected aorta and many times, walking ten feet or so is enough to tire me to the point of sitting down to rest.  The slowness has advantages though and stopping more often allows for opportunities to examine small outcrops of plants in detail.

The coquina boulders in front me, though they were surrounded by salt water, buffeted with strong winds and unrelenting sunlight, supported an amazing array of lovely plants and wildflowers.

Those plants I saw on the boulders, the Frog Fruit, Hydrocotyle, native succulents, many of these we have successfully used across green roofs in North Florida.

Nature can teach us many things.  We just need to stop and look.

Green Roof Plants - choosing between those plants that produce seed and those propagated vegetatively

When choosing plants for green roofs one is always faced with a decision to specify those species with a tendency to bloom and flower then set seed, or on the other hand go with those plants who normally are propagated primarily by vegetated cuttings.

Green Roof plants setting seed in Jacksonville, Florida

 Yet the decision is not a simple one and carries far reaching consequences.  Arguments for and against highlight the many benefits and disadvantages for selecting either approach.  I prefer to integrate in so many plants who reliably produce viable seed for the following reasons discussed below.

November seeds sprouting on a Florida Green Roof

Seed provide variability.  Like a roll of the dice, the many combinations of predictable but also unpredictable color, patterns and texture always provide a surprise waiting once plants set seed and those seed germinate into a new generation.  Because of inherent variability, seeds are generically difficult to remove from the open source system and patent.  Plants propagated via cuttings or asexual reproduction are much easier to protect under intellectual property rights laws and remove from public domain.  So by adding plants who reproduce by seed to your green roof design, you ensure for the time an ongoing, royalty-free plant source season after season.

Green Roof Salvia lyrata and green roof seeds sprouting

Longing for seasonal variability including color hue shifts, texture changes and other plant responses is a fundamental part of humankind's side by side relationship with the plant kingdom.  Plants who do  not rise and fall with nature's cycles seem almost 'plastic' to me.  Granted I use these 'plastic' plants to create central design structure and they serve faithfully providing green and form month in and month out.  But these structure plants are only a backbone.  The skins are the many hundreds of species, genus and families of flowering ephemeral and persistent native wildflowers, grasses and food plants.

Once established, seed producing green roof plants are an economic & ecological benefit

Familiarity with the signs of plant growth teases the senses like aroma from fresh baked bread.  The Salvia lyrata in the above photo for instance is preparing to cover certain areas of the green roof with her wonderful lavender spring flowers, one of the first sights across the roof as winter recedes.

Life is a complexity of changing interactions.  Green roof designers can tap into the interest of nature's spectacular array of surprising variability by using lots of plants who seed and reseed.

Florida Extensive Green Roof Systems, Green Roof Depth Considerations and Root Architecture

I ran across an interesting article in the American Journal of Botany entitled 

Root deployment and shoot growth for two desert species in response to soil rockiness by Gretchen B. North, Edward G. Bobich and Park S. Nobel.

The article confirmed interesting data we've been seeing with respect to root formation in coarse and shallow green roof soils.

MetroVerde Green Roof system, shallow, extensive and well-drained

Shallow green roof soils work well for us  we believe, for a number of reasons, including;

1. Except for the rare lengthy, monsoon-like downpours that occur annually and deliver the false impression Florida is more rain forest like than desert, water penetration into green roof soil media is minimal and lengthy periods of drought are frequent.  Therefore growing systems that force shallow root growth also promote stronger rainwater contact for those light drizzles we may experience.

2. Shallow growing systems combined with appropriate soil media enhance root-oxygen contact.  Root-oxygen contact importance has been especially shown as important through agrinomy studies in hydroponics.  Shallow root systems on extensive green roofs allow for increased oxygen uptake leading to significantly more biomass production.

Agaves and Yuccas are some of our standard green roof plants for use here in Florida.  There are two agave species native to Florida (and the Southeastern U.S.) - they are false sisal (Agave decipiens) and wild century plant (Agave neglecta).  Native Yuccas include:  Spanish bayonet (Yucca aloifolia), Adam's needle (Yucca filamentosa), and moundlily yucca (Yucca gloriosa), and curve-leaf yucca, Yucca recurvifolia.

Though the paper looks at Agave deserti and bunchgrass, Pleuraphis rigida, the concepts and conclusions are important.

Observations included:

1.  Water potential was higher in sandy soils than in rocky soils.

2. Agave deserti preferred rocky soils and was absent in pure sandy soils.

3. Root growth was greatest in sandy soils 

4. Agave deserti had twice the biomass and root surface area in sandy soils over rocky soils (example of environmentally induced acclimination)

5. Agave deserti leaf's water potential was the same at rocky and sandy sites but the transpiration rate was twice as high in rocky soils.

Conclusions and assumptions Green Roof Designers can use are:

Certain agaves (and yuccas) growing in well drained soils (rocky instead of sandy) exhibit acclimation characteristics of slowed growth rates and biomass production.  Slow growing plants on the green roof may be  more likely to survive long periods of drought.

Agave root architecture will develop slower and more shallow in rocky soils than in sandy soils.  Again, on an extensive green roof - one with relatively shallow soils - a plant with a shallow root system does better than a plant requiring a deep root system.  Agaves can acclimate to the coarse soils. Again, a shallow root system plant may be preferable over the deep rot system species.  A deep root species may have the tendency to assert agressive root barrier or roofing membrane attack.

Agaves do not want wet feet.  Where there is an over-abundance of moisture (very sandy soils) agaves may not be present.  Keep the green roof well drained.  This concept is in general disagreement with the school of thought that green roofs should serve as stormwater retention volumes or basins in conjunction with cisterns.  We've made the suggestion many times that due to Florida's water crisis, an irrigated green roof may not be considered sustainable - as one third to one half of the year it will end up irrigated with potable (during times of drought).  Within rainy season periods one will being irrigating the roof more frequently to empty the ever filling cistern - thus exposing the agaves, or any slow growing and green roof tolerant plant to 'wet feet' conditions.  Keep your green roof plants in conditions that do not generally comprise long term saturation.

Rocky soils may have a tendency to absorb and capture more dew than sandy soils.  This assumption/conclusion has many important implications for green roof plants water needs.  Because air can flow more easier through rocky soils, the dew has a tendency to penetrate to rockier soils.  Sandy soils may block moisture laden air.

I like to compare the ability of rocky or coarse soils to capture dew to the summer morning wet grass in your front yard.  There will always be more visible dew on the high surface area grass blades than say, on flat concrete.

Capturing all the water  from dew, fog and slight drizzles is important for our approach to green roof design based on non-potable irrigation.

Nature is always the best teacher.