The Living Landscape Plant Function Calculator

Origin and Development

Christopher S. Sohnly

September 2017



  • Dr. Doug Tallamy; Professor of Entomology and Wildlife Ecology

Department of Entomology and Wildlife Ecology
University of Delaware; Newark, DE

Adjunct Professor of Biological Sciences


  • Rick Darke; BS, MS

Principal; Rick Darke LLC

Landenberg, Pennsylvania


  • Sam Droege; BS, MS

US Geological Survey; Patuxent Wildlife Research Center
BARC_EAST; Beltsville, MD


  • A. Miles Arnott; BS, MS

Executive Director, Bowman Hill Wildflower Preserve

New Hope, PA


  • Tom Fischer

Senior Acquisitions Editor

Timber Press; Portland, OR


Executive Summary


The central goal is to design a Plant Function Calculator based on the format of charts presented in the back of The Living Landscape by Richard Darke and Douglas Tallamy. The calculator would ideally be freely available and similar in function to the Plant Stewardship Index but would have numerical scores in several categories.





If plantings in designed landscapes are going to sustain the life around them, serve a variety of ecological functions and survive environmental stresses, they need to be more then ‘parsley round a pig.’ Planting design in a post-wild world cannot view plants as individual components to be arranged in a sea of mulch. Anyone doing landscape design, whether landscape architects, horticulturists, or landscapers, must recognize that in the current era of climate change, urban pressure and fragmentation of natural areas, our landscapes need to do more than ever before. They must become functional entities that enhance local ecosystems rather than spaces that degrade them.

“Generally, ecologists refer to the group of plants that live together as a ‘plant community.’ A plant community and the animals occupying the same habitat compose what is called the “biotic community;” thus, the biotic community represents the living part of an ecosystem. The well-known scientist Edward O. Wilson more precisely defines community as “all organisms-plants, animals and microorganisms-that live in a particular habitat and affect one another as part of the food web or through their various influences on the physical environment.” (Collins, Anderson; Pg73)

Landscape plantings, whether a residential garden or a corporate campus, need to emulate plant communities if they are to produce ecological services. As Prof. Darrell Morrison put it; “All of our ecological restorations need to be good designs and all of our designs need to be ecological restorations.” (March 2015)



Dr. Douglas Tallamy emphasizes that we have reached the point where designed landscapes have to accomplish certain things. Primarily they need to:

  1. Maintain food webs

  2. Sequester carbon (in plants and soil)

  3. Manage the watershed

  4. Support both generalist and specialist pollinators



In order for our designed landscapes to fulfill these functions we must consider far more than a plant’s aesthetic qualities. We must have some understanding of what environment is best suited for a particular plant, what other plants do best in association with it, and what insects, birds, microorganisms etc., will likely interact with it.

For the average homeowner, landscaper or even trained landscape designer who wants to design and install diverse native plantings, choosing and finding the appropriate plants to fulfill these functions can be a daunting task.

From the gardener wanting to create residential habitat to the landscape designer adding ecological value to commercial plantings there is a passionate desire to understand how to create gardens that enhance biodiversity while also being aesthetically pleasing. Along with a growing interest in nature-inspired native plantings there is also a vast increase in information available about the functions of native plants. There is also an ever-growing body of research on topics from carbon sequestration to pollinator preferences and beyond. We are beginning to understand some of the complex interactions which comprise natural communities that we can foster in our home gardens, but often this information is difficult to find, understand and apply to plantings.



Project Overview


How can we chose wisely from the wide range of plants available to enable plantings to create ecosystem services and be appropriate for local ecosystem and soil type? A recently launched tool is the Native Plant Finder offered by the National Wildlife Federation. Anyone entering their zip code can find native plants in their area, ranked by the number of butterflies and moths they support as larval host plants. One of the other easily accessible guides was the Plant Stewardship Index (PSI), now expanded as the Universal Floristic Quality Assessment (FQA) Calculator. The FQA Calculator allows anyone to submit a list of plants and immediately get a value based on floristic values. (


PSI Methodology

“Bowman’s Hill Wildflower Preserve offers a free online PSI calculator to groups and individuals interested in monitoring and documenting the landscape.


“Once a list of plants on a given site (or portion of a site) are recorded, the online calculator computes several measures of naturalness and disturbance that can be used to track the site’s progress over time. Site reports can be printed from the website to be included in progress reports and grant applications. (Website)


“The Bowman Hill Preserve’s Plant Stewardship Index (PSI) was designed for use in the land conservation community. As stated on the PSI section of the Preserve’s website, PSI is a "thermometer" reading of the ecological quality of open land by seeing what plants live there. PSI is designed to assist in answering two questions:

- What is the naturalness of any site?

- How have land management practices (or their absence) affected that naturalness over time?


“Those plants, and plant communities, with exacting habitat requirements score highest, and are also the plants of greatest concern because we are losing them as the landscape is fragmented and developed. An index of conservatism to rank species has been used in a dozen states and has been found to correlate well with wildlife diversity and other environmental indicators.


“Given how and why the PSI was developed and the methodology behind the determination of the coefficient of conservatism numbers (i.e., what is the naturalness of any site?), it is not appropriate to use it to score designed landscapes, although many people do. This, in my opinion, is an inappropriate use of the tool. It can, however, be used to monitor restoration efforts over time to determine the effectiveness of management techniques, including the addition of new native species to a natural area.” (Arnott, 2016)



My interest in understanding the use of the PSI comes from studying evaluations in the Case Study Investigation series sponsored by the Landscape Architecture Foundation. Where possible, the LAF uses the Plant Stewardship Index in the Northeast in their Case Study Investigation series to evaluate designed plantings. While the PSI may be the best evaluation tool currently available, another more broadly based calculation system might be useful.




My goal is to design a Plant Function Calculator based on the format of charts presented in the back of The Living Landscape by Rick Darke and Douglas Tallamy. The calculator would be similar in spirit to the Plant Stewardship Index but would have numerical scores in several categories.

The Living Landscape offers “…strategies for making and maintaining a diverse, layered landscape—one that offers beauty on many levels, provides outdoor rooms and turf areas for children and pets, incorporates fragrance and edible plants, and provides cover, shelter, and sustenance for wildlife.” (Timber Press 2016)

To support their ideas, Darke and Tallamy have laid out a comprehensive list of plants in charts in the second half of The Living Landscape. Those charts inspired my original idea. I have followed Dr. Douglas Tallamy’s research since the publishing of Bringing Nature Home and began corresponding with him on this idea of developing a calculation system in February of 2016.

Rick Darke and Douglas Tallamy have given permission to use the data they compiled for the mid-Atlantic region chart in The Living Landscape. Thomas Fischer, editor of The Living Landscape at Timber Press has also allowed the use of the Excel tables which were compiled for the book. Preliminary work has begun on developing data tables that would provide the basis for the function calculator.

Drawing on the methodology of the PSI and the layout of the charts in The Living Landscape the development process would create a calculator which would put the various functions into a decimal rating system with values between 1.0 - 10.0. With the initial categories, eleven for Ecological Function and ten for Landscape Function, an individual plant could be considered on its collective services as well as comparing plants across a broad spectrum of qualities.

Similar to the Plant Stewardship Index, once a list of plants on a given site is compiled and uploaded to the Living Landscape Function Calculator (LLFC) website, the online calculator will compute several measures of ecological and landscape function. Reports can be printed from the website to be included in design documents or site analysis.

Additionally, the LLFC will have another level of analysis that will offer additional scores for plant communities. Plant communities, not individual plants, are best able to accomplish the four core qualities stated earlier; Maintain Food Webs, Sequester Carbon (in plants and soil), Manage the Watershed, and Support Pollinators. As the diversity and abundance of plants in a particular community increases, the score will go up. The website will offer prompts to aid in the fulfillment of plant community populations, offering suggestions of plants that should be included.

One benefit of this calculator would be to bridge the gap between current research and those who are designing and installing landscape plantings. There is an ever-growing, diverse body of research which is not readily available or easily translatable to the individuals working in the office or the field. Putting this research into a data table with decimal values allows anyone to access the information in a meaningful way.

The Function Calculator could be used to evaluate existing designed landscapes and aid in evaluating planned landscapes for a variety of ecosystem and landscape services. This would allow a science-based analysis of an existing designed landscape as well as provide a measurable way for a designer to communicate to a client the value of a wide range of services that a well-researched planting could provide. In addition to allowing ready access to a large detailed plant database it would also allow the designer to balance ecosystem function with landscape function in different areas of design.




To move from the symbolic entries in the Living Landscape tables to viable decimal values will require several steps of development. All of these steps will have to be documented, particularly for the source of the ideas or data. The documentation will also be essential for all the later steps required.

At this writing several fundamental choices remain to be clarified.


Step one, define the plant list.

The list complied for the Living landscape contains 336 plants for the Mid-Atlantic region. What other species should be included?

What introduced species should we include? If we are facilitating the comparison of existing and proposed landscape plantings then we need to include a robust list of common introduced landscape plants.

Do the original 21 categories need to be modified based on the data available?


Step two, obtain the necessary data.

The chart created by Tallamy and Darke was based on their research and experience but was never intended to be a scientific tool per se. All the ecological and landscape functions need to be based on quantitative or consensus driven qualitative data.

The development process will be to discern what data are available that can be quantified to create the values in the data table. The original table had just symbols for Pollen Source and Nectar Source, but is it possible to quantify that designation into a value between 1.0 -10.0?


In discussing this particular category via email with Sam Droege, a prominent Wildlife Biologist with the United State Geographical Survey, his comments made clear how difficult it may be to simplify these characteristics to a decimal value. A major goal is to sustain as many species of native pollinators as possible, so making a rating based on the number of specialists a plant genus supports is a great first step. The generalist bees will use the plants that support specialists.


“…it is difficult to really say how important a plant is to pollinators other than being a host to specialists since there are many variables involved:

When it blooms changes its value

How many species find the nectar acceptable

How many species find the pollen acceptable

Nutrition content of both

Length of seasonal bloom


How many flowers they produce

How favored it is by native/introduced bees

What species of bees it favors...etc.” (Droege 2016)


There is also the challenge of quantifying such qualities as fall color, bloom time or value providing shade and cooling. For example a value for bloom time should reflect the importance of continuous bloom in a landscape which is highly desirable from a pollinator perspective

It will require a through exploration of the data available and discussion with researchers who are familiar with each subject. We will need the cooperation of many individuals from many different disciplines, who will hopefully be willing to contribute, while being given all due credit.

The challenge will be to find the data to fill all 21 columns and convert those data to a meaningful 1.0 – 10.0 scale. Both steps of this part of the process need to be documented thoroughly for later revision and review and to allow further development to follow the same process. Eventually, the data, the conversion process, and the final results will need to be peer-reviewed to have scientific integrity.


Step Three, the data table will need to be transitioned to a searchable database which can be linked to a website.

Collaboration with a website developer will be needed to convert the Excel data tables into a searchable database, likely converted to JavaScript. For example, the PSI allows for calculating Native mean or Total mean. The Living Landscape Function Calculator (LLFC) could also allow for higher level calculations, such as added value using associated plants or for plant communities.


Step Four, once the alpha version is workable, it will need testing and peer-review.

Once the first level of functionality is reached, the data will need to reviewed, as well as the process of conversion to decimal values. There should be a thorough process of contribution and consensus on the data themselves. This might entail horticulturalists, botanists or other researchers.

The functionality of the calculator will also need to be tested to refine how it is used by practitioners. This might require the involvement of landscape designers, horticulturalists, landscape architects, landscapers and others.


Step Five, corrections will need to be made to create a launchable beta version.

Whatever feedback was obtained in step four will need to integrated and updated into the LLFC before its official release to the public.


Step Six, once released there will be a need to have a how-to-guide and hands on training to promote the use of the LLFC.

The Living Landscape Function Calculator, as envisioned currently, will reside on a website and have as its primary use a place to upload a plant list and get a variety of scores. It can be used to rate either existing or proposed plantings, and calculate a score for ecological and landscape function.

Ideally, there would be other ways to use the database. There are 20 columns of information, from Pollen Source to Fall Color, so it will be possible to search for plants that have a high value in any one, or combination, of characteristics. To make the fullest use of the calculator may require some online or in-person help and training.


Step Seven, the Mid-Atlantic LLFC will need to be documented, continually revised and curated to ensure its usefulness.


Step Eight, five additional LLFCs can be created to cover other regions of the country.

Tallamy and Darke created only the Mid-Atlantic chart, the other five were created by others hired by Timber press. With this step we can overcome one of the principle drawbacks of the PSI, the fact that it is regional. Once created and launched the availability of six LLFCs to cover the entire United States will allow anyone, anywhere, to create meaningful, science based plant lists by ecological and landscape function.




My estimate is that the first six steps will require at least 2,000 hours of research and compilation by a dedicated primary researcher. 2,000 hours x $35.00 per hour = $70,000


There will also be a need to have a dedicated website developer or programmer to create the actual interface for the Living Landscape Function Calculator. I estimate that to require 300-500 hours. 500 hours x $25.00 per hour = $30,000


Other materials and expenses will be involved, transportation, computer and storage devices, printing costs and programs. $2,000


Total for first six steps $102,000





The guiding purpose for the development of the Plant Function Calculator is to help all individuals who are concerned with creating and supporting a healthy environment a way to better understand the range of ecosystem services that individual plants and plant communities must fulfill in designed landscapes. It is an effort to bridge the gap between the exciting and vital research that becomes daily available and practical application in the field.

It serves the broadest spectrum of the industry for a tool like this to be openly available, as is the PSI. This would encourage individuals from all aspects of environmental studies, horticulture and landscape design to contribute data to the current and future development of the calculator. No one person or group will have all the information needed to develop this tool, so the best practice would be to encourage an atmosphere of collaboration and consensus. Currently we are seeking development funding, a range of data inputs and guidance from all aspects of industry and academia.

Performance metrics are vital to establishing the links between academic research and how built projects are actually performing. The Living Landscape Function Calculator would be

a powerful tool to empower landscape designers with science based metrics to show the value of specific native plants, and further, the essential significance of plant communities.


Works Cited
Arnott, Miles. Email communication September 26, 2016
Collins, Beryl Robichaud, Anderson, Karl H. Plant Communities of New Jersey. Rutgers University Press. 2005
Droege, Sam. Email communication. October 10, 2016
Morrison, Darrell. Lecture, “Landscape Design as Ecological Art.” Temple University, Ambler Campus. March 2015
Tallamy, Douglas. Personal communication. University of Delaware. September 7, 2016
Timber Press Website, Accessed 10/20/2016