LEED version 4 (V4) is the fourth and latest iteration from LEED (Leadership Environment, Energy & Design), a rating system developed by the U.S. Green Building Council and independently certified by the Green Building Certification Institute. The primary mission of LEED is to encourage the marketplace to provide sustainable buildings, sites, and neighborhood development using a completely voluntary points and classification system. V4 is an improvement on LEED’s last iteration, Version 2009, because it has moved further and further away from a prescriptive model and toward a performance-based model of sustainability. But how does V4 affect the treatment of trees, soil, and stormwater in the landscape?
First, a few basics. LEED provides an incentive for project owners to build or rehabilitate projects for an audience that wants to live, work, and play healthfully by evaluating the sustainability of construction projects. As the primary neutral, third-party gold standard, LEED loosens market forces – an ingenious way to create, and drive a market that was always there, but never served. Designers can become LEED accredited and buildings/sites can become LEED certified.
There are five certification designations for buildings: Building Design + Construction (BD + C); Operations and Maintenance (O+M); Interior Design and Construction (ID + C); HOMES; and LEED Neighborhood Development (ND). Building Design and Construction (BD + C), was formerly known as “New Construction” – this category, which addresses new projects and represents by far the largest number of LEED certified buildings and projects, is the beating heart of LEED. Trees, soils, and stormwater fall within the “Sustainable Sites” and “Water Efficiency” subcategories of BD + C.
So, how thorough and ambitious are LEED V4’s guidelines in regard to trees, soils, and stormwater? Not very.
There is some good news for trees in LEED V4. When the new category of Neighborhood Development (ND) joined the fold of LEED Specialties in v2009, a credit was added for “Tree lined and shaded streets.” This credit remains in V4.
Within the Sustainable Sites subcategory of LEED 2009, there was a single (1) credit for mitigating heat island effect using non-roof methods. While trees were not specifically mentioned as a solution, reading between the lines, it was hard to imagine using something else besides trees to provide shade to surface parking lots and buildings. Other than this oblique reference, there was no further mention of trees, except for a “don’t cut them down” directive. LEED V4 has refined this credit so that trees are specifically addressed as an acceptable tool, as are vine-covered structures.
Soils are not seen as belonging in the world of LEED, and V4 is no different from previous ones in this regard! The preservation, amendment, and protection of soils are not mentioned – not even as a material reuse item. This is an extraordinary and obvious oversight, particularly since soils are often the biggest resource that can be reused at a site. Sunlight, water, and soil are the three critical ingredients needed to grow all terrestrial plants. If any one of those three (soil, water, sunlight) is missing, there is no vegetation.
Another aspect of soils that is not addressed is quality. Abused soils are the most serious hindrance to the growth and development of plants. Compacted soils, usually a result of the use of heavy equipment (especially if they are worked while wet) always result in unhealthy and stunted vegetation, particularly trees. Professor Kim Coder, PhD, a famous tree researcher at the University of Georgia – Athens, ranks soil compaction and insufficient soil volume on an equal footing with lack of water in limiting tree growth.
Soil is not rock, soil is not dirt, soil is not asphalt, concrete, and building debris mixed together. Soil is a 50:25:25 mix of minerals, oxygen-rich air, water and millions of tiny animals. The 3 mineral components are sand, silt, and clay – the difference between those three materials is primarily a matter of size. If a sand particle is the size of a 50 gallon barrel, then a silt particle is the size of a dinner plate, and a clay particle is the size of a US dime. The gold standard for a good soil is loam – a ratio of sand, silt and clay particles adhered together, like glue, by organic compounds (compost, etc.). Think of a nice black or dark brown soil, the best vegetable garden soil you ever saw. Us soil geeks say that it feels ‘good,’ smells fresh (lots of oxygen), and holds together into good sized chunks (about the size of an eyeball), naturally aggregating together. Within a tablespoon of loam soil there are literally a billion microscopic animals: springtails, bacteria, nematodes, and fungi, all of which help support a healthy soilfoodweb.
Soils are composed of horizons and ought to be more than six inches deep, the standard specified re-spread depth, on almost all construction projects in North America. Heavily compacted mineral soils can never grow vigorous vegetation. Adding nitrogen, fertilizers, micro-nutrients, compost tea, or any of a dozen other tonics for trees, shrubs, or perennials will not revive them. If soil is not healthy, these additives are a waste of money and resources. Large volumes of un-compacted soil, a.k.a. loam, are the solution to over 90 percent of plant problems on projects. We must see this soil problem corrected with the next version of LEED.
The previous version of LEED, v2009, already did pretty well with its treatment of stormwater. LEED V4 retains the same standards of the 2009 version and doesn’t add much extra.
Two subcategories, Water Efficiency (WE) and Sustainable Sites (SS), address stormwater quality, quantity, harvesting, and re-use – they can earn sites up to two (2) credits. The Water Efficiency sub-category also has a two (2) credit placeholder for water efficient landscaping. Water-efficient management, both rainwater and potable, are addressed with four (4) total credits, the primary driver for this being regulatory authority of the Clean Water Act of 1972.
Here is the TL;DR (too long; didn’t read) summary of LEED V4 as it pertains to the treatment of trees, soils and stormwater:
- Water: water-efficient landscaping, rainwater harvesting and its re-use, stormwater quality, quantity, and rate control are well handled
- Plants: conditions to improve tree numbers are improving slightly, other planted vegetation – not at all
- Soil: health, re-use, volume and quality have all been completely missed. As I said, we have to see this changed in the next Version of LEED.
LEED’s weakest categories have always been the sustainability of sites, landscapes, and vegetation. This weakness is what gave rise to the Sustainable Sites Initiative (organized by ASLA, Lady Bird Johnson Wildflower Foundation, and the US National Botanic Garden). While green infrastructure remains a relatively new field, there is more than enough science supporting it to justify a more integrated and rigorous approach in the LEED system. While the current version has made some improvements to the treatment of trees, I’m disappointed that it doesn’t reflect what we already know about how to sustainably integrate soils and stormwater, together with trees, in to the built environment. I hope the next version will.
L. Peter MacDonagh, ASLA, LEED Green Associate, is the Director of Science + Design at The Kestrel Design Group.