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.

Harvesting Fog and Dew for Drinking Water and Vegetable Garden Irrigation

With natural water supplies drying up and disappearing and areas affected by drought increasing in acreage every day, learning how to harvest all available atomized water from the air is important.

Alternative forms of irrigation for green roofs and living walls can sometimes be the primary irrigation source for rooftop plants.  We use air conditioning condensate and dew catchers on many of the green roofs we design.

This morning our lanai screen was covered in water droplets though there was not a cloud anywhere to be seen in the sky, a reminder of the available irrigation potential.

Dew available for Green Roof Irrigation

Additionally, a quick early morning walk through the garden offered up the opportunity to see dew droplets all across the vegetable leaves and other plants.

Dew available for irrigation in the garden

Dew can be an important source of 'free' irrigation.  Learning how to harvest the condensed water vapor is easy, as humans have been tapping this resource for ages.

Check out the TreeHugger article here for an informative look at how dew catchers provide drinking and irrigation water to some of Peru's underdeveloped areas.

Then check out an amazing YouTube video showing the construction of a DIY dew catcher!

Always consider dew as an irrigation source when designing vertical green in the Urban Core.

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!

Nature Irrigated Green Roofs - how much water do you need!

Nature is amazing.  It has not rained for several days here and though we did recently have a little rain we are still far below our normal.  In fact Jacksonville is still classified as officially experiencing severe drought conditions.

National Weather Service Drought Index

Yet nature provides ample water, even in times of drought here in the form of water vapor.  Good green roof design takes advantage of all sources of water, including morning dew.

Nature's irrigation - morning water vapor

I am constantly amazed at just how much water rests on the green roof plants some mornings.

As though an irrigation sprinkler has been turned on across the roof, opportunities exist for the plant designer to use those species capable of condensing (those with fine leaves, hairs or waxy surfaces) to capture the water then direct the droplets to the green roof planting media below.

Working with an international hotel chain to improve their landscape vegetation I was asked to diagnose why a large collection of very big planters were doing so poorly.  A large roof overhang directly above prevented morning water vapor from reaching the plants.  However, only a few feet over and out from under the overhang other planters looked lush.

Fog and morning dew can contribute an important source of nature supplied irrigation.

I make it a regular habit to walk green roofs early, middle of the day and late.  Surprises abound always.  This morning I was amazed at just how much water was on the plants.  Who needs a sprinkler?

Nature's Irrigation for Green Roofs

Nature's designs are always the most successful.

She knows when droughts come and rainfall is deficient, providing water vapor in the form of dew and fog for her plants.

Water vapor this am, no rain for days, Nature-based Irrigation

Designing green roofs and landscapes to avoid irrigation from potable water sources is important in today's world.

Success of a nature irrigated green roof depends heavily on sourcing a steady supply of water through rainfall, fog, dew and even frost. Using biomimicry based on Nature's plant designs design allows for important air water vapor collection.

Additionally, understanding the principles behind Nature's plant designs of water capture successes lie 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 construction

Air 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.

Understanding the value of water vapor in the forms of dew and fog can greatly increase your green roof's survival chances during a drought.  Potable irrigation is not the answer a smart green roof designer looks to for water.

Green Roof with sprinklers on top to keep plants from wilting during drought - giving our industry a bad name..

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.

Green Roof Irrigation Opportunities, Exploring Water Vapor

Fortunately, there are many ways green roof plants can be provided irrigation water.  First we should utilize native rainfall amounts, second water vapor in the form of dew, fog and air humidity.   Air conditioning condensate can provide a significant amount of irrigation as well.  Finally, rainfall captured in rain barrels or underground storage and cistern systems may be utilized to support plant water requirements.  Grey water too may be used if local or state regulations permit.

I've always been amazed at how much water the air holds.

Several of our past blog posts have addressed dew, fog and water vapor (biomimicry).

This morning as I inspected a recent green roof project, I was awestruck at just how much water was rolling off the solar panels, dripping to the single ply roofing, running across the roof to the drip edge and into the gutters.  Alot of water!

Dew Stream Dripping from Solar Panels

So when we think of green roof design we need to visualize the advantages elevated vegetated plantings have over ground level plants from water laden air exposure.

Nature has provided many plants with fine hairs, such as one of my favorite green roof plants, Yucca filamentosa, a physiological attribute allowing for water collection.

Before we immediately specify irrigation systems with potable water for either primary or backup supply,  lets seriously look at what nature provides.

We may be surprised at just how much water nature is making available.

Green Roof for Landscape and Parking Lots - cleaning and storing stormwater

Green roof concepts are not just limited to the top of a structure.  They can be adapted almost anywhere plants, water and a platform can be integrated.  

Referring to the term 'volumetric green' in the Urban Core - green roof technology can be adapted to provide habitat, clean stormwater, sequester carbon, produce oxygen and create beauty across the cityscape.

Ultimately, to be cost-effective, a best management practice must bring cost effectiveness to the table.

Maximizing site development potential by combining stormwater, green space and other site functions allows for green practices to be more readily implemented.

Below is a diagram of what we've done with the lessons learned from building lightweight green roofs however the 'roof' is actually a landscape cover for stormwater storage and recycling.

"Green Roof" over Rainwater storage, cleaning and recycling system

One of the aspects about plants we have come to realize is plants are efficient with uptaking nutrients - nutrients otherwise contributing to algae blooms and dead waterbodies. 

As site development becomes more constrained with respect to available space, stormwater and wastewater treatment can turn to green roof inspired, plant based systems as a method of sequestering nitrogen, phosphorus, zinc, copper and other contaminants. 

Once sequestered, these pollutants may be removed through biomass harvesting and composted on-site for reuse in the landscape. 

Removing nutrients from the water cycle is the most efficient way of cleaning our water supply. 

Green roof technology applied to stormwater storage and recycling has many benefits.  Stored rainwater may be used for irrigation, flushing of toilets and more.

Combining landscape and rainwater recycling through green roof technology

Green Roof Technology applied to Stormwater Capture and Storage

Green Roof Xericscape Primary Design Variables & Rooftop Polygons

Interestingly, according to the USEPA, Americans use approximately 1.5 billion gallons of water every day on landscaping.  Contrast this over-looked wealth and waste to the realization two hundred million hours each day are spent by families across less developed nations without adequate water infrastructure in securing daily domestic water supplies, some carrying heavy jugs of muddy water on their backs great distances.

Here in the U.S. we use twice as much water for landscaping than the number of gallons of gasoline we burn in our automobiles daily.  With the present U.S. population estimated to be about 311,000,000 persons, landscape water use is on the average about 5 gallons per person per day, or about 19 liters per person per day.  Yet fortunately many governmental agencies are presently encouraging use of native species and wildflowers acclimated to reduced watering or nature based irrigation.

So, if sustainable development practices call for conservation of water in the landscape then irrigation in green roofs should be no hidden exception.  

Because roof ecosystems are subject to significantly harsher biophysical conditions than most ground level landscapes, industry response sometimes has typically been one of adding irrigation and fertilizers to hopefully mitigate additional heat, dryness and desiccating wind stressors typically impacting green roof plants.  Moreover, because the green roof industry here in the US is still relatively young there is a lack of detailed design data to assist in planning and installing nature irrigated green roofs.

Fortunately water conservation practices in green roof design can be simple and cost-effective.   Though one can delve deeply into design theory, effective nature irrigated green roof design theory can truly be best understood with spending time outdoors in and around the project site, looking up and paying attention to what is already there.  We shall see that though we can model design variables in an attempt to analytically predict ‘what works best on a green roof’ sometimes a walk through the town, looking up to see what plants grow naturally in gutters, in the cracks of mortar and across roofs, provides the most useful design information.

To fully understand the design criteria needed for a nature irrigated, native species and biodiversity focused green roof we must first divide the rooftop area into simple polygons that are representative of existing environmental factors.  We want to know where on the roof areas are exposed to harsh, desiccating winds and we want to know where on the rooftop the sunlight becomes either lacking or unbearable.

Understanding these rooftop design variables is made easier through the use of rooftop polygons and there are two abiotic categories of green rood design variables to be used in calculating roof polygons;

• Primary Variables, and
• Secondary Variables.

Primary Variables include those variables that may vary based on individual roof polygon, including;

• Light, and
• Wind.

Secondary (yet important) variables include those design inputs remaining generally consistent across the roof and not considered in the roof polygon calculation.  They are;

• Heat Zones
• Cold Zones
• Precipitation and Water Vapor Profiles
• Smog
• Allopathic Plants Nearby, and
• other variables.

Though not used to determine the roof polygon boundaries, the secondary variables play an important role within a design model considerations and we will discuss those interactions in detail in future sections. 

Importantly, certain light wavelengths are required for photosynthesis.  Without light photosynthesis does not occur and plants do not grow.  Light is the first primary design variable to be considered when creating roof ecosystem polygons.  Understanding where on the roof photosynthetically reactive radiation and light volume measured by Daily Light Intervals or DLI is critical to good green roof design.

Wind is the second of the primary design factors used to determine roof ecosystem polygons.  

Though ambient outside air temperatures, precipitation, air quality and other design variables are generally consistent across the roof, light availability and wind effects can change depending upon where one stands on the same roof.  Both light and wind are critical factors in designing a green roof for dry and arid climates.  Without adequate light plants will not live and consistently buffeted by desiccating winds plants may cease to transpire and quickly die.

Over the next couple of weeks we will be exploring xeric green roof design variables.  First, as an introduction to developing rooftop polygons an Youtube powerpoint presentation is included here for review.

The presentation was used as an introduction for native plants on green roofs but has a solid explanation of rooftop polygon development included.  After reviewing the powerpoint you will have an understanding of rooftop polygon development, and how the two primary design variables can be incorporated into the green roof model.

Further discussion of why light and wind are the two primary green roof design variables will be included in subsequent articles.  Enjoy the powerpoint worksheet.  TOmorrow's post will discuss in length why light and wind are te two primary design variables for green roof plantings.

Green Roof Fertilizer, Irrigation with Potable Water, Invasive Species and Mark Madoff

Mark Madoff's death is a tragedy no doubt, but no more so than any of the other countless lives lost to the end result of another  Ponzi Scheme - the use of Persistent Organic Pollutants (POPs) - and the associated denial of silent environmental and health impacts.

Having read the article posted by @psustentavel on Twitter - click here for the article about Persistent Organic Pollutants - the imagery of 'inorganic chemical 'fertilizers across countless green roofs,  the use of drinking water for irrigation - even backup irrigation - and the reckless abandon with which we use non-native species for landscapes, including those on rooftops, made me think - in our rush to install green roofs atop buildings, are we too creating a Ponzi Scheme someone else will have to pay for?

The value of green roofs is unquestionable.  There is solid scientific data to back up claimed benefits of habitat creation, carbon sequestration, oxygen production. greenhouse gas reduction, heat island effect tempering, cleaning of stormwater, integrated pest management and so much more.

Florida Extensive Green Roof Nature Irrigated

There is no doubt about the value properly designed and maintained green roofs can bring to the Urban Core.

Green roofs can be used to support Urban Permaculture - small scale, individualized rooftop farming operations providing food, fiber and medicine to city residents.

And many urban areas around the world are moving ahead with plans to increase urban agriculture.  Singapore, for example, produces 25% of the entire city's vegetable crop within the city limits. For a fascinating look at the importance of urban permaculture  read foodsecurity.org's white paper entitled Urban Agriculture and Community Food Security in the United States; Farming From the City Center to the Urban Fringe.


Rooftops are the new frontier of real estate in many urban areas.  Cities with limited available ground level real estate have acres of open rooftops.

Vegetated and green roofs are increasing in popularity, showing an impressive growth rate trend over the past decade.  According to Greenroofs.com, public awareness of and interest in green roofs is steadily growing.

Yet we must learn from the Madoff venture's collapse, the immense loss incurred by investors and also those innocent persons affected worldwide by careless and widespread use of persistent organic pollutants commonly associated with some inorganic fertilizers.

In our noble rush to restore volumetric green to the urban core we hopefully will recognize the potential environmental costs associated with mass production of green roofs.  The 'One Size Fits All' approach to green rooftops will not work for the worlds vastly different ecosystems.  Sadly and reminiscent of mega-agriculture's damaging past practices, we as an industry turn to potent chemicals to fertilize, control pests and then use much needed drinking water to either irrigate or serve as backup irrigation.

Independent and craftsman-like green roof design and construction could disappear.


As with the POPs causing health issues in Pakistan and other places, our indiscriminate use of hardy invasive species, potentially toxic chemical fertilizers, pesticides and herbicides on green roofs is a Ponzi Scheme our children too will pay for.


Importantly, we as an industry have already moved far in the direction of ecologically inappropriate and environmentally dangerous green roof design.  Florida's FDEP green roof in Pensacola, Florida possesses a large drip irrigation system, backed up with potable drinking water and planted with slow-release fertilizer compounds and soil amendments.


As in Ponzi Schemes, denial of reality is the basis of success.  Though the 'green' benefits of the FDEP green roof are often touted in the media - the fact that chemical fertilizers are applied and potentially damaging to Escambia Bay's ecosystem are hidden.


Yet FDEP is fully aware of the environmental problems resulting from POPs, fertilization and pesticide use on green roofs.  In the proposed, new State of Florida Stormwater Applicant's Handbook,   FDEP allows green roof credit for volume retention of stormwater only (no water quality credit),  and recognizes fertilizers and pesticides will be used on green roofs (why no water quality credit is given), an approach we must reject.  Moreover, FDEP is requiring the purchase and use of a 'pollution control media' made from ground up automobile tires under the green roof soil media to keep the fertilizer's nutrients on the roof. 

Over time and as water evaporates, added pesticides, herbicides and fertilizers will concentrate back into the surface waters and become an issue for our children.

In the rush to add volumetric green to the Urban Core, the Ponzi Scheme could easily go unnoticed.

Temper the hurry to install a green roof with the knowledge of a local native species botanist and a soils expert.  Rather than ordering large amounts of hothouse pampered plant material, find a local nursery with individual plants acclimated to the climate.

Green roofs can be crafted to add artistic, educational, ecological and ethnobotanical value.

But we must first reject the Ponzi Scheme that green roofs must be fertilized, irrigated with drinking water and doused in pesticides.

As always, email your questions here!

Happy Green Roofing...

Kevin

Plants for Green Roofs - The new MetroVerde Carbon Pirate Variety

First off, even though we are a registered nursery in the State of Florida we do not generally sell green roof plants.  The plants here and the way they were raised are for our own green roof projects.  We are sharing the information of both our successes and failures in hopes we will someday have a greener world.  Our commitment is to restore volumetric green to the urban core.

We've mentioned before in the blog the practice of hardening off drought tolerant and succulent plants before installing them on green roofs, but today MetroVerde officially rolls out its Carbon Pirate (trademarked) line of green roof plants.

What is so special about the MetroVerde Carbon Pirate plant line is the time and preparation put into the plants in anticipation of roof duty.

Realize we focus on the Southeast US and especially Florida, so our approach may not be the best approach for other regional climates.  But here in Florida - green roofs face the big 5 H's, Heat, High Humidity, Hard Freezes, Hard Dessicating Winds, Hurricanes, Heavy Downpours, Hot Periods of Long Drought (Looks like 7 and I could add a Host more) - the point is that once you step up onto a Florida roof, no matter what time of the year, your breath can be taken away.  And you understand why you cannot just plant a nursery grown specimen on the roof.  May look good for a few months but after that - well just look at most of the previous attempts here across the state (except for a select few).

Drought tolerant plants possess specialized physiological processes that respond to changes in nature such as rainfall patterns and temperature.  They have developed these functions over the eons and rely on these adaptations for survival in areas where water supply is unpredictable.  Many have very shallow radial root architectural structure with diameters that exceed many times the plants actual form.  These roots want to grow laterally rather than downward in response to the infrequent rainfalls typical of their native habitat.  During a light rainfall event the stormwater tends to barely penetrate soil surface (depending on soil types).

As the rain ends and the water either evaporates or infiltrates, the drought tolerant plant broad surface area of roots responds quickly, storing the liquid for later use.

Additionally the drought tolerant have developed an array of mechanisms for dealing with drought such as CAM - or Crassulacean Acid Metabolism, and we will discuss these in upcoming posts, but lets get back to root structure.

For example, a two foot tall cholla plant often has a thirty foot architectural root diameter.  The fundamental distinction of any drought tolerant plant is the broad lateral root architecture.  Note the difference between some plants that do well in dry areas yet long, vertical systems that tap into the ground water supply.  Examples of these would include; Acacia, Parsley, Burdock, and Mesquite (whose roots have been know to grow down vertically over 200 feet!).

In choosing a green roof plant you first want to select one with roots that want to grow radially, not vertically.  But there are logistical limitations with plants that have radial root systems - they do not like pots or containers.  They need room for their roots to grow.

Recall the times you have gone to a nursery and purchased a plant and returned home to find the plant's roots had circled and circled the inside of the pot - we call this being 'root bound'.  Not good for the plant.

However, practically speaking nurseries are not going to grow one plant in a large tray because of the economics of space, real estate and cost-effective efficiency (I couldn't afford to either).

Hence the dilemma.  A drought tolerant plant grown in a two inch deep container that is three inches by three inches wide will not survive over term on a green roof.  The available functioning root area will only be approximately four and one half cubic inches.  Now as long as the plant is in the green house with irrigation applied (strange to think of applying irrigation to a drought tolerant plants) the plant will probably survive.   However, if raised in a small pot and irrigated, the green roof plant when planted on a roof most likely won't make it.

Drought tolerant plants for the most part have water intake processes that work gradually.  They are used to taking in required water volumes at a lower rate per root surface area using a large root diameter rather than absorbing large volumes of water with a limited root surface area.

The change from greenhouse container with irrigation to the roof is usually just too much and the plants will not function as expected without intense maintenance, watering and fertilization.

Though a topic for another blog session, my firm belief is because here in Florida we have a severe water crisis and nutrient pollution crisis, designing a green roof with irrigation is a fundamentally not environmentally sound.  I know, the cistern argument - hold the rainwater and irrigate with rainwater, but anyone with a cistern in Florida will tell you it is empty most of the time and then you have to use potable makeup water.....  I'll reiterate - my firm belief is because here in Florida we have a severe water crisis and nutrient pollution crisis, designing a green roof with irrigation is a fundamentally not environmentally sound.

Working with some of our green roof species we've 'trained' (this almost sounds cruel) certain plants to bridge the gap between being raised in containers and roof plantings.  Using drought tolerant plants (we will discuss in future blogs) - and let me note here there is a difference between drought tolerant plants and the succulents and cacti - we are not referring here now to succulents or cacti - using drought tolerant plants grown in standard nursery containers, we apply restricted watering schedules over a long period of time - usually over a year, to harden off the plants and prepare them for roof service.

These plants are grown in hot greenhouses with background humidity levels and irrigation amounts of less than 1/2 inch per month.

Once installed on a green roof with a mat based foundation, their roots quickly grow out laterally and with cumulative input from humidity and natural rainfall, they survive and begin to fill in the roof area.  In contrast to the irrigated, happy plants that are fatally shocked once installed on a roof, these plants are relieved to find their home on a roof.  We call these plants MetroVerde Carbon Pirates (trademarked).  They sequester C through respiration, produce oxygen, clean stormwater, provide habitat and create a much need sense of place.

There is so much to talk about here - tray systems versus mat  based systems, whether or not to irrigate, cost, weight, fire hazards, maintenance, volunteer species, disease - but we have plenty of more blog discussions coming!  In the meantime happy green roofing and as always you can e-mail Kevin with your questions - kevin@metroverde.com.