Florida Green Roof Plant Root Structure, Horizontal Root Architecture

Green roof plant architecture is an important biomechanical component of all green roof design, construction, installation and maintenance activities.  In the end a green roof is first, foremost and all about the plants.

Florida green roof plant root architecture - beautiful horizontal root structure

Without the happy, thriving plants, there is no green roof.  Possibly a brown roof but not a green roof.

I think the Florida extensive green roof root structure depicted here in these photos is simply beautiful.

Green roof plant root architecture -  horizontal root structure growing into anchor

An understanding of green roof plant root architecture is one of those fundamental design talents that every green roof professional should possess.  Some of you will have learned about green roof plant root architecture from years of observation, hands-on planting and study of how green roof species grow, others through educational programs.  I examine root structure across green roofs every chance I get.  

My preference is shallow soil media and unimpeded horizontal growing space for green roof plant roots.  I do not like sectional barriers that may limit horizontal root growth and ultimately cause root circling-root bound growth patterns.

These grasses possess good green roof plant root architecture - not too aggressive but sturdy

Another reason I prefer unimpeded horizontal root growth opportunity on a green roof is because I believe green roof plants will over time, relocate themselves or their offspring to the best place on the roof for their particular species survival.  Yes, plants do move through root biomechanic mechanisms.  It is poor planning to restrict green roof plant root architecture any more than necessary.  Just think of how many times you may have pulled a plant from a nursery tray or pot with twisted and circling roots that have practically strangled the plant.

An anchor system may keep plants on a roof during tropical storms

In addition to catering to the green roof plant through design of open space for root growth to occur we like to provide the roots an anchor to grow into.  There are many different approaches one can take when providing an anchor, including cables, mesh, netting or fabric.  By permanently attaching the anchor material to the roof you create a green roof plant growing system that may be resilient to tropical storms or cyclone winds.

Unimpeded root growth prevents strangulation of green roof plants

Using this approach we have created green roofs that have stayed in place when blown with 130+ MPH winds.

The grasses in these photos show this principle in practice.  Using a small, mock up green roof growing system these plants have embedded their roots into a nylon fabric, creating an impressive anchoring form of root architecture and growing in a well defined, horizontal fashion.

A good green roof plant architecture will create a monolithic growing mat with plants anchoring each other

While some prefer deep plantings with roots reaching down vertically, we find horizontal root structure strategically places roots in an optimal position to absorb those frequent one half to two inch afternoon rainfalls here in Florida.  Rain water usually stays in the top inch or so of the green roof soil media.  With a horizontal growth pattern, green roof plants can take advantage of this rainfall where deep roots may have less rain reach down into deeper soil horizons.

If you are wondering, the photographed soil media contains less than 5% organic material.  The bark-looking chips are actually ground recycled rubber tires.  This is an experimental soil media, one we do not use on actual green roofs due to fire ratings.

Know your green roof plant architecture.  Remember, in the end a green roof is first, foremost and all about designing a growing system that keeps the green roof plants happy, healthy and surviving in the long term.

Root Architecture and Green Roofs SimRoot Model by PennState

SimRoot has been out for a couple years now.  Developed by Penn State, PSU is a leader in understanding root architecture.

In the SimRoot video shown below, Nitrogen uptake is modeled.  Note how the root hairs closest to the surface take in the most available nitrogen.

When designing nature irrigate, native plant green roofs it is essential to capture as much rainwater and water vapor as possible, for irrigation and for scavenging dissolved nutrients.

Check out the following SimRoot Video by PennState!

Florida Green Roof Plants - Root Architecture Summary

We've been discussing the optimal green roof plant root structure.

There are many adequate types of Florida green roof plant root structures.   The variables different structures become a part of include;

1. Hurricane, tropical storm and wind resistance,
2. Rainfall and water absorption,
3. Plant support,
4. System cohesiveness, and
5. other factors.

One of the major advantages of using a mat with a weave, or trays with an inserted weave for roots to grow into is the overall imparted strength of the system.

 Laterally growing roots will provide greater anchoring during high winds.  This can be an important factor when considering placing a green roof here in Florida -  or anywhere with potential for tropical storms and high winds.

Florida Green Roof Plant - Good Root Architecture - Lateral Growth

 Finally, the most important aspect of installing plants on a Florida Green Roof is - don't go tot he nursery and purchase plants that have been grown in a production nursery, pumped full of fertilizers, used to timely irrigation and replete with tender new growth. Instead, look for those plants with a lateral root system that are hardened off and acclimated to the tough environment you are installing them into.

Roofs are harsh places for plants to survive!  Remember the 5 H's - High desiccating winds, High heat, Hard Freezes, HOT-HOT-HOT!, High humidity, Hurricanes,  and more...

Lateral root growth contributes towards support, enhanced water absorption area, anchoring and a host of other success factors.

Happy Green Roofing!

As always, e-mail me with your questions.  Kevin.

Green Roof Plants - Root Architecture - Nitrogen Uptake

The SimRoot root architecture 3D model is fascinating and informative.

Though the analysis results are common sense, seeing available nitrogen removed from the uppermost soil levels is important to the capabilities of green roof plants and their capability to clean stormwater.

Watch the following youtube video of SimRoot from Penn State.

You will see the time lapse simulation of the root growth and note the concentrations of available nitrogen decreasing rapidly in the surface area.

More soon!

Happy Green Roofing - Kevin

Green Roof Plant Root Architecture - Photos and Comments - Florida Green Roof Plants

Yesterday we looked at green roof plant root architecture and the benefits of green roof plants possessing a broad, shallow root structure rather than a narrow, deep root structure.  Click here to review the blog post information.

The diagram is posted for review here.

Green Roof Plant Root Architecture Comparison - MetroVerde

Real life Green Roof Plant photos of the two types of root architecture are posted below:

Root Architecture - Poor Green Roof Formation 

The photo above typifies the normal nursery container plant root structure - narrow and deep.  This is not a good root formation or architecture to begin a green roof with.  Note the following photos of a plant grown on a mat.  I've cut the mat out of a green roof that had a 2" engineered soil layer.  The plant had grown on the roof for approximately 3 years or longer.  The green roof system was non-irrigated.

Green Roof Plant - Root Architecture 1 - MetroVerde

Green Roof Plant - Root Architecture 2 - MetroVerde

Green Roof Plant - Root Architecture 3 - MetroVerde

Green Roof Plant - Root Architecture 4 - MetroVerde

When I removed the mat section I cut out about twice the diameter of the plant crown or habit (form diameter) thinking I'd capture all the root mass.  Upon further examination the plant had extended its roots over triple the area I'd cut.

Note how the roots have grown and woven themselves into the mat - providing an anchor against high winds and allowing for use vertically.

The root architecture of plants grown in mats exemplifies the benefits of growing green roof plants in mats.  Roots do not like to be bound up with limiting walls.  It you do not use mats, then use large tray systems.

With time, green roof plants with access to a large, monolithic and non-sectioned growing space will eventually 'move' to the best spot for their particular needs - on the roof.

The same basic principle applies to living walls.  A monolithic fabric with no limiting sectional structure will allow the plant to choose the best spot for successful, long tern growth.

More on Green Roof Plant Root Architecture Soon!

Happy Green Roofing.

Kevin

Plants for Florida Green Roofs - Root Architecture - Rain-Root Zone

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 R. 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.

Tomorrow - Actual photos of green roof plant root architecture styles.

Email me with questions by clicking here.

Happy Green Roofing!  Kevin