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Seattle's Green Factor

By Buck Abbey, ASLA, Robert Reich School of Landscape Architecture, Louisiana State University, LASN Associate Editor for Technology






The Seattle Green Factor Landscape Code recognizes that increased tree canopy coverage absorbs carbon, produces oxygen, cleans the air, muffles urban noise and reduces storm water run off.


Greening the City

It is no surprise to readers of this column that community landscape codes and municipal tree laws have been greening cities as far back as the year 1700. However, contemporary green laws as we know them today date to the late 1960s. Land development codes, the third major type of green law used to keep nature in the city, date from the 1980s.

But new ideas about city greening are being developed in several places in the U.S. All of these new approaches to greening the city stem from one central idea: making nature do work, allowing nature to do what nature does for humanity.






The code encourages layering of vegetation in areas visible to the public and along streets adjacent to new development. Letting the ivy grow seems like an easy way to comply with that aspect of the code.


Greening the city is about allowing nature to provide eco-services to clean the air, purify the water, reduce storm water flows, clean away urban pollutants and reduce the use of energy and the so-called exceptional green house gases. When you think of new ideas about greening you have to think about green buildings, green roofs, green parking lots and more plant materials in the urban forest of the city.

One such new greening idea shows potential as written in a landscape code that is similar to a traditional code but innovative along the lines of sustainability mentioned in the June and July ordinance columns.






The green roof at the Seattle Justice Center (Karen Kiest Landscape Architects) is in keeping with the city's sustainable-inspired landscape code to make the urban environment greener and more livable.


The Green Factor

Seattle, Washington is one of the leading sustainable cities in the U.S. and the landscape codes reflect that leadership. In addition to a community landscape code and tree ordinance, (Directors Rule 8-2007, Landscape Standards, 8.16.07, Land Use Code, SMC Title 23, Environmental Policies and Procedures, SMC Chapter 25.05) this community has adopted sustainability design requirements for neighborhood business districts within the built up sections of the city.



" Green Design: A design, usually architectural, conforming to environmentally sound principles of building, material and energy use."--Seattle Department of Planning and Development



Known locally as the "Green Factor," the code requires that landscape plans for development or redevelopment in commercial areas meet new landscaping requirements based upon sustainability. This program, adopted in January 2007, requires landscape plans for neighborhood centers to address ecological function and aesthetic principles using point based criteria. These points are derived using a menu of green landscaping strategies that are included in a landscape calculator provided by the Seattle Department of Planning and Development. This point-based landscape code was modeled after a similar program in Berlin, Germany called the Biotope Area Factor (BAF) that mandates that new development install ecological landscapes within the city center. The Green Factor as adapted is designed to improve the "quality" of designed landscapes though sustainable practices, while allowing developers to meet "open space requirements" in the commercial parts of the city.

Compliance with this code recognizes and rewards good design in several ways. The design is rated by points if the landscape plan preserves trees, installs green roofs, green walls and irrigation systems that reduce the use of potable water. Extra bonus points are awarded for the use of drought tolerant plants in spite of Seattle's reputation as a rainy city. Bonus points are received for layering of plant materials across the property for increased visibility for pedestrians while encouraging the use of larger trees and taller shrubs.






The Seattle South Lake Union Discovery Center (Magnusson Klemencic Associates) is an example of the bioswales with native plantings being designed in Seattle.


Calculating Green

The Seattle code requires that all plans be carefully measured for a number of square feet or units each of which carry a factor between 0.2 and 0.7. The total calculation of these become the numerator and the total area of the site becomes the denominator. A total score of 0.30 is required. For instance, a 400,000 square foot building site with a landscaping numerator of 120,941 would have a green factor of 0.302 (N/D=0.302), just meeting the green factor standard.

The Seattle Planning Department provides designers with tools and reference materials that make the code easier to work with. The Green Factor website www.seattle.gov/dpd/permits/greenfactor provides several LEED-like worksheet calculators in which essential, quantifiable data such as tree canopy size, plant height, area of permeable pavement and depth of soil and water use is inserted to calculate point scores. By simply entering the number of plants or the area's square footage the calculator determines if the required points are obtained. Material is available on the website to supply the designer with ecological design back ground material that covers everything from salmon-friendly gardening to preparing landscape management plans and the essentials of green roof design.

But perhaps more importantly, the Green Factor Landscape Code recognizes and supports sustainability.The code recognizes that increased tree canopy coverage, supports the Cool Cities Program, absorbs carbon, produces oxygen, cleans the air, muffles urban noise and reduces storm water run off. Porous pavers, rain gardens and water harvesting are all recognized in this code. All of these design strategies rewarded with points are sustainable factors that will make a city more livable. The design of green roofs and green walls are used to shade buildings and reduce the use of energy and provide habitat for urban wildlife.






Porous pavers at the South Lake Union Discovery Center (Magnusson Klemencic Associates), rain gardens and water harvesting are all encouraged by Seattle's Green Factor landscape code. The code requires landscape plans for development or redevelopment in commercial areas meet new landscaping requirements based upon sustainability.


One of the interesting ideas behind this code is it reduces the cost of public streetscape improvements from the public side of the ledger and places those costs on the private sector. Street trees, pavements, street furniture, pedestrian amenities and connection to public utilities are now in part borne by the private developer or property owner, not the public. This reflects economic sustainability to the tax payer and city government.

The code is being evaluated by the Seattle Department of Planning and Development. Suggestions have been made to change the scoring of some items on the worksheet, as well as add new landscape elements. For the most part, the regulation is viewed positively, but there has been some criticism that the worksheet calculator is complex. Some properties find it hard to meet the minimum score due to the size of their lot or where they are situated on the block. For instance, less street frontage means there is less area to receive points for right of way landscape design.

As Seattle continues to update and refine the Green Factor regulation, it will serve as a model for other cities considering adopting a similar landscape code policy based upon quantitative design standards that can easily be calculated by city planners and landscape architects alike.

Liz Stenning, University of Washington, contributed to the Green Factor segment of this story.






D.G. "Buck" Abbey, Associate Professor of Landscape Architecture at Louisiana State University, is LASN's Associate Editor for Ordinances.




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November 22, 2019, 12:11 pm PDT

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