Residential Landscape Diversity Study

1.0: Background & Purpose

78% of residential land in Los Angeles is zoned for single-family housing, representing the single largest land use category. The aim of the study is to understand the effect of ground cover on soil physical, chemical and hydrological characteristics within single-family residential properties, and to understand homeowners motivations behind choosing one form of ground cover over another. This is the starting point towards developing better socio-ecological approaches for improving soil development. As ground cover is often the choice of the homeowner, this is an actionable opportunity area to improve soil properties across Los Angeles County, leading to improved stormwater capture, increased resilience, reduced extreme heat, and other benefits.

2.0: Methods & Data Collection

2.1: Soil Analysis Methods

Sampling was performed on patches, defined as discrete sections of soil separated from other parts of the property by either impermeable surfaces or other clear delineations. For example, a front yard may be considered one patch, or subdivided into multiple patches. At each site, patches were chosen to best reflect the overall composition of different available ground covers in order to best compare differences in soil characteristics within the same site. Each patch was also separated into an edge component (first 0.5 meters from the border) and center component. Patches narrower than 1 meter did not contain a center component. The purpose of this method is to determine if ‘edge effects‘ (such as increased foot or vehicle traffic) play a significant role in affecting soil physical, chemical and hydrologic properties.

Three primary ground cover types were investigated:

• Traditional/Lawn: A typical grass lawn with at least 30% of surface covered by grass. Trees may be present within patches.

• Modernist/Non-Biological: A non-living cover such as gravel, bare soil, or artificial turf. Presence of succulents & other low-metabolism plants permitted in this category

• Ecological ​Densely vegetated patches with native or non-native vegetation and interstitial space between plants covered by mulch or leaf litter.

25 representative sites were sampled across LA county. In-situ measurements were performed, and 232 bagged samples were analyzed at the USFS Pacific Southwest Research Station and at the Texas Tech University Bayer Plant Sciences Laboratory. All testing was performed on the top 5″ of the mineral soil within the patch.

The following in-situ measurements were taken: Unsaturated Hydraulic Conductivity, Surface Bearing Strength, Sunlight Exposure​, Electro-conductivity​, Soil Temperature​, Bulk Density

The following laboratory tests were performed: pH​, Texture​, Organic Matter, Microbial Respiration​, Soil Moisture Content

2.2: Social Analysis Methods

Interviews were conducted for each participant to better understand their motivations & decision-making process with regard to their landscape choices. 26 interviews were conducted in total. Interviews were transcribed and coded. Participants are believed to be more sustainability-motivated than the average LA resident due to their connection to TreePeople and willingness to participate in this study.

3.0: Key Findings

3.2: Social Analysis Findings

• The interviews found that residential landscape managers in our study are not only motivated by the aesthetic and recreational aspects of gardening, but also engage in gardening as a means of contributing to broader environmental goals.
• Awareness of soil’s potential as a climate mitigation tool remains relatively low among not only study participants, but presumably the general public.
• Residential landscape modifications seem to occur in clusters, with local gardening “steward-influencers” igniting change most successfully proximally in their community, suggesting a strategic entry point for cultivating soil knowledge.

• Preference for low-cost, low-maintenance landscapes that do not use much water. Some participants who park outdoors were concerned about plants impeding their ability to park or keep the vehicle clean.

• Most participants found that their landscaping choices had an impact on social interactions, with results equally split between increased and decreased interaction. Most likely, the decreased interaction came from the use privacy barriers such as tall plants, walls, fences or hedges.

4.0: Synthesis

One of the most important questions we sought to answer is: to what extent does ground cover – which is the choice of the property owner – affect soil health?

We found that overall, soil quality metrics were significantly influenced by landscape type, ground cover type, patch size and perimeter as well as patch eccentricity. Ecological landscapes and natural ground covers such as mulch, leaf litter, and pine needles were associated with higher soil organic matter, greater moisture retention, better infiltration rates, and lower electrical conductivity. Our findings highlight that larger, less eccentric, vegetated patches with natural ground covers (e.g.,

mulch, litter) and within ecological landscape types have the most positive impact on soil health indicators.

Further, due to low awareness of soil’s potential as a climate mitigation tool, there is an opportunity to promote better soil management by emphasizing existing low maintenance and water conservation aspects of native, ecological landscapes when discussing these landscapes as an option for homeowners.