LEED V4: How does it affect trees, soils, and stormwater?

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.

Trees

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

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.

Soil triangle. Image: Clover's Garden Center.

Soil triangle. Image: Clover’s Garden Center.

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.

Stormwater

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.

Conclusion

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.

Top image: LEED certified condos in Hoboken, NJ. Did they earn any points from the landscape?
Flickr credit: Hoboken condos

4 comments

  1. Many landscapes fall into the category of the built environment. The term built environment refers to landscapes that are built to provide a setting for people to live, work, shop, move around and other activities. The built environment includes infrastructure like electricity, water for irrigation and drainage structure. This soil in the built environment is extensively modified by compaction to support human activities. Compacted soil in the built environment has a high bulk density and low infiltration rate. This modification is very different from soil modification for agriculture or gardens. Also in agriculture and gardens the soil is regularly tilled and organic amendments can be periodically added. Once installed a commercial landscape cannot be tilled or amended. Development such as parks with little infrastructure, hardscape or building may not need the soil compaction,
    Commercial landscapes are typically installed into compacted soil. This engineered process requires the removal of most organic matter and the crushing of air filled pores where water can enter, be stored for a while and then drain out. The elimination of air filled spaces is what increases the soil bulk density and results in a very low infiltration rate. Almost all commercial sites are mass graded to make efficient use of expensive land. Soil is compacted during site development as part of reshaping the landscape contours to provide for vehicle movement and parking. Cut and fill grading is very common. Also people in urban environments usually feel safer if they can see where they are going, so the soil has to be graded to provide for this pedestrian “flow”. Typically large, heavy equipment is used to speed the building process and reduce labor costs. Soil is also compacted on commercial landscapes to limit damage from soil settling and other soil movement. In most of California soil has to be compacted as part of protection from earthquake damage. Compaction eliminates voids or pores in the soil where water can collect. Water in soil can act as a lubricant allowing the soil to move.
    The result of this compaction is that it is only near the soil surface where gas exchange between soil and atmosphere is rapid enough to provide the roots with enough oxygen to support root function. Roots will also grow into any reasonably continuous space where there is air and water exchange. For example, we often find roots growing around valve boxes and along curbs where soil has pulled away.
    Landscape irrigation systems should be designed, maintained and scheduled for shallow rooted plants. Also since water penetration takes place very slowly into compacted soil run off has to be managed by low water application rates and multiple start times.
    It is difficult to uncompact the soil and make a better environment for root growth. The clay in Western soils much stickier and cohesive than clay in the geologically older rest of the country. Western clays have a higher electrostatic charge. An indirect measure of this attraction of one clay particle to another, is called cation ion exchange capacity. Western clays have a very high cation exchange capacity. The compacted soil has to be fractured to make spaces for roots, plants can be grown in mounds of loose soil placed over the compacted soil and constant addition of easy to degrade mulch will feed soil life that builds soil structure from the surface down. Soluble organic compounds from the degrading mulch bind to clay particles and reduce their stickiness. The organic compounds also coat tunnels made by soil organisms and keep them from closing up.
    Some common recommendations do not work. Ripped compacted soil usually re-compacts before roots can grow into it. Amendments are very difficult to blend into the sticky clay and they quickly break down. When the amendment breaks down the soil subsides into depressions. Irrigation water collects in the depressions saturating the soil. Roots do not grow in saturated soil. As discussed above, amendments can be regularly tilled into agriculture or garden soil to compensate for their breakdown
    It is also important to remember that the compacted soil may be part of the building and hardscape support. Significant reduction in soil compaction for better root growth near hardscape, pavement and structures may require the sign off of a structural engineer.
    Many native plants and “xeriscape” plants are not adapted to compacted soil and are therefore may not be appropriate plants for many commercial developments.

  2. Western clays and clayey soil
    A clayey soil can be molded in to a shape that holds together. If the clay in the soil is very active or “potent” very little clay in the soil can make a clayey soil. A clay soil is a soil texture definition based on the percentage of clay silt and sand. This definition says little about the characteristics of the soil serves no useful purpose. Furthermore, look at the shape of the sandy loam area. No product of the natural world will have a shape like that. Nevertheless, even though it provides essentially no useful purpose the USDA soil texture triangle is widely used
    Western clays are especially sticky because they are geologically new. Western clays typically have a high charge density. They have not been degraded by millions of years of weathering. As clays weather in the humid East they gradually lose their clean, electrically active edges, turn more gel like and become less sticky. Furthermore, Western clays are residual clays, which means they are found where they are formed. Clay in the Eastern half of the country tends to be transported clay, also known as sedimentary clay. This is clay that has been moved from the place of origin by erosion and deposited in a new place. When the clay is moved it is more easily weathered and new clay is mixed with weathered clay and other fine materials.
    Any anecdotes or engineering studies from the East may not apply to the West. Because Eastern clayey soils are less sticky they are easier to cultivate. Amendments are typically easier to mix into an Eastern clayey soil than a Western clayey soil. It is frequently impossible to blend amendment into a Western clayey soil. Often claims for additives that improve clayey soil are from cultivation, not the additive.
    Clay minerals are highly reactive, and they are colloids. A colloid is a finely divided material that is dispersed through another medium. It shares many properties with a solution. For example, colloidal particles introduced into water tend not to settle out, and they can pass through very fine filters. In a soil, the colloidal clay particles can be carried by water percolating through the soil. Clay will tend to fill voids until the clay clumps together by wetting and drying, complexing with divalent cations like calcium and magnesium and by the action of soil organisms. It is important to note that most Western soils have plenty of Calcium and Magnesium to complex clay. The common recommendation to add gypsum, a common form of calcium sulfate, is not based on experimental evidence that gypsum has any benefit.
    Planter mix
    Specifications concerning amount of clay or fines in a planter mix need to take into account the properties of western clays. Very little western clay results in cohesive slowly draining mix.

  3. Maybe this was a change after the time you looked at it, but soils are definitely all over the SS category in LEED v4.

    SS Prereq: consturction activity pollution prevention required calls for an erosion and sedimentation plan
    SS Credit: Site Assessment requires a survey and assessment of soils
    SS Credit: Site Development – Protect or Restore Habitat Option 1 gives points for restoring all disturbed or compacted soils

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