Most people assume that trees sequester carbon and so planting trees is a good thing and that in that the Western United States were water is a precious commodity using thirsty plants is can have negative environmental impacts. There is a lot of discussion about the problems surrounding pesticide and herbicide use. But when in come to sustainable landscape development there is little discussion of the amount of carbon that is released in the atmosphere in the creation of the garden. How much carbon is used in the maintenance of the landscape? How can the landscape reduce the amount of carbon released into the atmosphere in the operation and maintenance of the structures the landscapes surrounds?
Planting for temperature modulation in the built environment
Plants modulate temperature around structures, significantly reducing the energy load on heating and cooling systems. Deciduous trees (trees that lose their leaves, generally in winter) planted on the South and West sides of a structure, provide cooling shade in the warmer months; in winter, bare branches allow the sun to warm walls.
Low-embodied carbon material choices for hardscape
Hardscapes comprise critical landscape elements such as walkways, retaining walls, and garden beds. Using low-embodied carbon materials like re-used concrete, locally sourced sandstone and river rock, as well as cob and rammed earth, is essential in lowering the carbon footprint of landscape projects. Re-used, recycled, and locally sourced materials require far less energy to produce and transport to the building site.
Designing for rainwater and snow melt retention
Santa Barbara, and many other regions in Central and Southern California, rely on imported water as well as desalinated water to fulfill their municipal water supply. Both importing water and desalinization require pumps that use a lot of energy. Using local water saves energy. Permeable hardscape materials, along with land contouring to capture rainwater and snow melt, helps replenish aquifers and reduces the use of piped-in water for irrigation.
On this property, purple leaf plum trees are planted in the west facing front yard in front of the living room and master bedroom windows. Because they are deciduous (they lose their leaves in winter), sunlight can stream into the windows warming the house in the colder months. As the weather warms, the tree leaves grown in and shield the house from solar radiation—cooling the house naturally.
In this landscape, we created an overstory of low evapotranspiration (plants that drink less water). This heavy planting of low-water-need plants shields the earth below, creating a microclimate that blocks out the warning and drying effects of the sun and creates a protective thermal level that staves off frost. The end result is a lush oasis that stabilizes the temperature around a property allowing water-filled plants to be grown with less irrigation. Those full, broad-leaf plants also sequester lots of carbon.
The water-rich succulents in this garden cool the property located in the middle of Los Angeles. The lack of landscaped space as well as asphalt and concrete that comprise much of the city create a heat island. That heat island drives off rain, compounding issues of desiccation and bad air quality. By reducing hardscape areas and adding ample plantings to more landscapes across the city, the land can work to cool the city and reduce the need for air conditioning throughout the region. On hot days when you walk off the street in to this garden you can feel the temperature drop.
The outdoor kitchen and pizza oven are made out of cob. Cob is a natural building material that is created by mixing mud, sand, and straw. Here, the base of the structure was created with rubble collected on site as gradings and landscape preparations took place. Subsoil was taken form a local construction site where they were excavating for a foundation. The soil was then mixed with straw and sand and sculpted into this structure. Concrete pulled out of the old driveway on site formed the patio, and the arbor was made of old clothesline poles found on the property. This cozy hardscape—created with materials found with a 20-mile radius—minimized energy use in transportation and extractive mining processes.
Like plastic, concrete is an extremely useful material. Taking on many forms, it is strong and durable. Concrete has because ubiquitous in the built environment, and like plastic it also has detrimental environmental effects. Concrete production and installation accounts for 4-to-8% of the world’s CO2 emissions. The sand and limestone that goes into concrete is leading to the destructions of habitats including stripping valuable sand from the world’s beaches. New formulas for concrete production, which take less of a toll on the environment, are being created and experimented with. Another issue with concrete is that it creates an impenetrable crust that blocks the percolation of rainwater into the environment, causing detrimental effects to watersheds.
I aim to minimize the amount of new concrete being poured in my landscape design practice. But since concrete is such a versatile and durable material, I like to try to use old, broken concrete that is salvaged in the demolition process. In the backyard landscape featured above, we created a retaining wall with old concrete (sometimes referred to as urbanite) from broken-up pathways and patios throughout the property. The concrete patios and other hardscape were removed to make way from more-permeable hardscape that allowed more water to sink into the ground and out of our storm drains. The added benefit of re-using the broken concrete produced on site is that it saves on the emissions created from hauling this heavy material to the landfill.
The retaining walls in this central coast landscape are made of local sandstone. Using local building materials to create a garden hardscape is a great way of reducing the amount of CO2 that is created in the building of a landscape. Santa Barbara is rich in sandstone. Almost any excavation in the Santa Barbara area unearths sandstone boulders. Using local materials reduces the emissions caused by hauling materials across country—and the world in some cases. Another benefit of using a common and easily accessible stone such as local sandstone, is that it does not require damaging quarrying processes to harvest.
Here, rainwater from gutters and downspouts is directed into a constructed dry creek bed. The capacity of the dry-creek depression is equal to the volume of one inch of rain that comes off the rooflines that the downspouts serve. The equation is: the square-foot footprint of a given roofline times .62, equals the galIons coming off the roofline. The number of gallons from the roofline divided by 7.48 gives you the cubic feet that will occupy the depression below.
In this project an underground pipe conveys water from the downspouts in the back yard, meeting the downspout in the front yard. With the combined downspouts serving 2,000 square feet, there is 1,240 gallons running off that roof in one-inch of rain. Taking that 1,240 gallons of water and dividing in by 7.48, the cubic feet of water harvested off the roofline is approximately 165 cubic feet. Creating a six-inch deep swale with a footprint of 330 square feet, we can capture 1,240 gallons in most rain events. That is a fair amount of water that does not have to be mechanically pumped into the area, and is water that creates a lush, plant-filled garden which works to keep the structure and surrounding areas cooler.
Carbon sequestration recipe: place thin layer of compost on the soil surface; cover the compost layer with three-inches of wood chip mulch; add water—alchemy happens and carbon sequestration magic begins. Compost is rich in micro-organisms such as fungi, bacteria, protozoa and nematodes. When these micro-organisms (mainly fungi) come into contact with moisture (rain water directed in the swale) and food (carbohydrate-rich wood chips to feed on), theses little organisms can thrive—cycling nutrients, improving soil structure and sequestering carbon. Lots of carbon. Fungal bodies pull carbon out of the atmosphere and lock it into soil where plants then use the carbon to grow.
Allowing rainwater to soak into a property and not just sending it into drains that lead to the street has many positive environmental impacts. “Sinking” rainwater on a property creates opportunities for cooling and carbon-sequestering plants to thrive and replenish aquifers. Keeping rainwater on site helps keep rivers, streams, and the ocean clean as pollutants are captured were they can breakdown in the soil before they overwhelm waterways. Finally, capturing water on site takes pressure off storm drains and mitigates flooding. As climate change effects weather patterns, leading to unpredictable rain events, engineers find it challenging to properly size storm drains. Capturing the water on a property before it has a chance to get to the storm drains reduces the pressure on storm drains.
