Green Roof Root Architecture, Understanding Water Adsorption

Today brought the monthly watering of a 1/4" H2O for the Alliums.  Within a matter of hours they were bright green.

We've 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.

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.

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

Green Roof Plant Sleuthing! Understanding why some plants grow better on roofs.

I call the process - 'Green Roof Plant Sleuthing'!

Simply put, Mother Nature is the best teacher for what grows well on a wall or on a roof in hot, dry, arid and monsoon-like climates.

Nature created green roofs cool buildings and provide habitat

When students of Green Roof technology ask me for recommendations concerning plants and green roofs, the first task I suggest is always to do a little Urban Sleuthing.  Walk around downtown, watching your step and keeping an eye out for traffic yet looking up to the city roofs, gutters and tops  of building structures.

Most are amazed to see just how many plants actually grow out of cracks, with minimal soil and survive the typical harsh conditions of a rooftop.

Green Roof Sleuthing is fun & educational!

Excitingly, these naturalized plants can produce a full and flush growth habit, offering an almost tropical look to the roof.

Walking through St. Augustine the other day I ran across many nature inspired green roofs, all quite beautiful and providing habitat, cleaning stormwater, cooling the buildings and pumping fresh oxygen back into the air.

Hidden among the lush resurrection fern were also many interesting smaller ferns, vines and other plant species.

MetroVerde finds many new Green Roof Plants through Florida Green Roof Sleuthing

So next time you wonder about the type of plants to use on your next green roof project, be sure to consult with your local nurseries and growers but also do not forget to do some Green Roof sleuthing yourself.   Take a walk through the downtown and look up.

Chances are you may never use those plants you see growing out of the buildings but then again, knowledge is power, especially when working with plants on roofs!

Green Roof Plant Design - Heliotropoism, Diaheliotropoism and Paraheliotroposim

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, and other notes

Yet nature is complicated and does not limit herself in resisting environmental stressor conditions to just the above three types of photosynthesis processes.

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 Intense Sunlight Leaves Fold to Prevent Desiccation

Under optimal Sunlight C3 Leaves Open Widely

Under Optimal Sunlight Conditions C3 Leaves Open Widely

Heliotropoism 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.

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

Paraheliotroposim denotes a plants actions similar to the above photographs.  During parahelioproposim, 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 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.