Monday, June 20, 2011

Green Roofs and Native Plants

One of the first interesting observations on a newly installed green roof is the plant preference of insects and pollinators.

"Biodiversity: Higher Taxa Variations, Green Roofs and Native Plants"
  A guest post by KEVIN SONGER

We always use many native Florida species on our green roof plantings.  We also use non-native ‘Florida Friendly’ plants, non-invasive landscape plants, and food plants.  Our green roof designs are centered around four or five main native plant families, Asteraceae, Poaceae, Fabaceae, Lamiaceae, and Solanaceae, if the green roof has a food garden or ethnobotanical component (though I love our native Solanaceae weeds too).

Of course invasive and exotic species with aggressive growth tendencies need be avoided as plants growing on a roof are situated high in the air, their seeds and plant DNA subject to being carried by the wind, picked up and redeposited elsewhere by birds, collected and transported downstream by stormwater A green roof serves as a platform from where species are provided with a remarkable advantage for spreading seed, rootable stems and leaves to areas far across the landscape and watershed below.

But as soon as plants are installed on the green roof, wildlife arrives.  First a scout or two then within a couple weeks many, many more as the ‘food is here’ songs are sung and dances choreographed in perfect geometric patterns.

Yucca filamentosa on the roof

Native pollinator species head directly for the native wildflowers and shrubs.  The exotic pollinators, such as European Paper Wasp, Poistes dominula, head straight to the non-native, cultivated flowers.  The imported Asian ladybugs fly straight to the food crops in search of aphids.  Bees, moth and butterflies seek out the native Asteraceae and Lamiaceae while the dragonfly and damselfly congregate around the Poaceae and wildflowers.  Ruby-throated Hummingbird, Archilochus colubris, are immediately attracted to the red tubular flowers of Lonicera sempervirens (Caprifoliaceae) and white flower spikes of Yucca filamentosa (Agavoideae).

Adding different species of the same genera seems to increase the quantity of specific pollinators.  But when plants of varying genera and families are added both the quantity and varieties of insects, birds and wildlife jump exponentially.  Since varied types of pollinators are required for food production, it behooves us as humans to do what we can to maintain a healthy level of pollinating insects by designing our landscapes and green roofs with native wildflowers and plants

But what is the optimal blend of higher taxa to species variance?  This is a question I’ve considered for many years.  It has been my observation the greater the plant variety on the green roof and the landscape, the more varied the wildlife occurrences.  But how much variety is best?

Is a green roof with many species of Asteraceae the target, or is it better for wildlife to design a green roof and landscape with a few species each of many genera?

The issue is commonly referred to as ‘determination of biodiversity through higher taxa’.

Published literature about biodiversity as represented by species or by higher taxa  (families) is scant.   The best I’ve done to satisfy my curiosity concerning biodiversity, species, genera and families is to observe nature, observe landscape designs and track the plants preferred by varying types of wildlife on green roofs.

I have read persuasive articles, some passionate but all having varying opinions on the matter.  Some say biodiversity is best supported by strong variance in higher taxa though others point to examples of strong biodiversity where the biota is limited among only a few families.

Could nature be best equipped to show us what to design into our green roof and landscapes?  At least this is the theory behind biomimicry.

The following chart categorizes many of the native plant families growing in Florida.  Certainly there are more and I recognize the data is quite generalized. 

As a side note, two of the most important design factors for green roofs are light and wind impacts.  Wind, I contend, is the biggest threat to green roof survival, for a two meter per second desiccating breeze can overwhelm the vascular system’s ability to move water from roots to replace lost leaf moisture.

 Reviewing the above chart I realized despite my best intentions old habitats die hard - the bright colored, multi-head flowers of the Asteraceae remained at the center of my green roof native plant design pallet.  I was drawn to the familiar, the reliable, comfortable species, those readily available across the state. Historically I used a half dozen species from each of two families to represent the native plant component of my designs.  Right there with Gaillardia, Rudbeckia and Helianthus of the Asteraceae were the Poaceae, the proven purple muhly grass, sand cordgrass, Fakahatchee and blue-green Andropogons.

Asteraceae  (Aster family) and Poaceae (true grasses, including cereal crops)


Between the two families there are about five hundred different plants I could choose from for green roof native plant design and still retain a comfort level for success.   However, even though the Asteraceae and Poaceae provided a wide variety of genera and species selection I was still working with just two families.

Targeting the 10-20-30 Rule for Biodiversity is a goal on every green roof and ground level landscape project we are involved in; no more than 10% of the design any one species, 20% genera and 30% family.  But where did the 10-20-30 rule come from and what was the reasoning behind the rule?

I know from years of field work that most ecosystems across Florida contain many species, genera and plant families.  Tagging along recently, I was fortunate to spend a day with Florida Department of Environmental Protection (FDEP) biologists assessing and completing Uniform Mitigation Assesment Method (UMAM )habitat surveys on a overgrown silviculture site.  The planted pine and hardwood treed site was typical for most of Florida and the diversity of higher taxa extensive. These types sites, though common looking, are rich with a wide variety of higher taxa diversity including many plant families, many genera and countless species.

Feeling overwhelmed and knowing I could never match the extensive diversity found in  a typical silvicultural site, I thought maybe I’d justify my past landscape design practices by finding a native Florida ecosystem low in higher taxa occurrences.  Maybe an ecosystem with little soil and insignificant, if any native hydrology, would contain only limited biodiversity; maybe an ecosystem such as upland glade.

Glancing through a Florida Natural Areas Inventory (FNAI) description of upland glades the notion of low higher taxa diversity in a harsh environment was quickly dispelled.  FNAI listed twenty five different plant families and I am sure there are many more.  Not surprisingly, Asteraceae and Poaceae both were well represented along with unexpectedly, Cyperaceae.  We’ll talk more on Cyperaceae in a future article.


Having little luck with finding a low higher taxa native plant community, I wondered if urban sites would maybe contain fewer genera and plant families.  Recalling the Breaking Ground Contracting project in Jacksonville where we installed an interesting green roof and native plant landscape (for more information see the project website located http://breakinggroundgreenroof.com/ ), I pulled up the existing plant survey conducted during construction activities.

The Breaking Ground Contracting floristic site survey conducted during significant ground disturbance and construction activity, revealed at least thirteen different plant families and more than twenty different genera, hardly an example of low higher taxa diversity. Feeling more distanced from my landscape designs where I specify Gaillardia, Rudbeckia and muhly grass I remembered my favorite place to photograph and study living wall plants, Castillo de San Marco in St. Augustine.


The old Spanish fort was built from coquina shell and limestone rock in 1672.  Her walls are constantly buffeted with strong, salty winds off the Atlantic Ocean and hot blazing sun, a harsh environment for any living organism.  Yet the fort walls are covered in plants. According to floristic surveys of the site, Castillo de San Marcos contains at least 61 families, 129 genera and 153 species, with again Asteraceae, Poaceae, Lamiaceae, & Fabaceae ranking as the most frequently observed plant families.


So if a construction site with concrete trucks, heavy machinery and nowhere for plants to grow has a minimum of thirteen different plant families, a harsh habitat such as an upland glade has at least twenty five different plant families and a rock wall many more, then maybe proper design for biodiversity support should contain a significant variety in higher plant taxa.  Maybe all natural plant ecosystems, no matter where they are located, are truly a complex and intricate representation of biodiversity.  Maybe our landscape designs should mimic our natural environment’s intricate diversity of life forms.

Wikipedia offers an enlightening definition of the term ‘ecosystem," describing the intricacies as “a web, community or network of individuals that arrange into a self-sustaining and complex hierarchy of a pattern and process” capable of creating biophysical feedback between both living and non-living components “sustaining bio-diversity” and integrating into “complex and regenerative spatial arrangement of types, forms and interactions”.

In reality, native plant ecosystems are complex.  They are, as described above, a web, a community arranged into a self-sustaining and complex hierarchy of a pattern and process. To create the complex web a native plant community needs representation from more than two plant families. Each individual plant family offers unique and important contributions for supporting nature’s complex hierarchy.  Ultimately, more plant families included in the landscape and green roof, will encourage increased biodiversity.

Understanding how biodiversity benefits from higher taxa variations opens many doors to green roof and landscape design.   I still love my Asteraceae, Poaceae, Lamiaceae and Solanaceae,  I am also now discovering many new plant families, excited about an enhanced understanding of what nature can teach and enthusiastic with newfound opportunities to support native wildlife biodiversity.

Kevin Songer, Juris Doctor
ISA Arborist,  Municipal Specialist,
Leed BG+C

1 comment:

faux boxwood said...

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