Keys to Sustainable Gardens

by Kate Easton, CPH

Where garden design principles support sustainable ecosystems, specific low impact development techniques and choices for managing water, soil, and nutrition are the keys to minimizing the impact of humans on the larger ecosystem and enhancing your garden ecology. These design choices and practices that maximize aesthetics and minimize disturbance include: storm water management through controlled infiltration for a more hydrologically functional landscape; protection of native vegetation and restoration of native species through design and maintenance; integrated pest management and other low impact controls; a deliberate and planned soil building and maintenance program; and a planned system of garden maintenance. This article focuses on managing water and plant protection techniques.

Managing Water
Puget Sound lowlands have a geological history shaped by glacier action, native forest growth, and a climate strongly influenced by the moderating affects of the marine environment. With water everywhere, it is easy to take it for granted because much of what happens is underground. However, the encroachment of cities and towns on the native forests have also affected how much and when water recharges aquifers that supply water to communities, and the discharging of water to streams and lakes. These changes in turn affect the plant and animal communities which also impacts the ability of some businesses to be profitable.

Research shows that half "the annual rainfall [of wet coniferous forest] is intercepted by foliage and evaporated during the rainy season" (Hinman 2005) – this is evapotranspiration. In addition, native soils (highly organic matter chemically and physically bound to mineral particles creating stable substrates 2 to 4 feet deep) move water slowly down slope over many hours, days or weeks. Native soils absorb 20 to 30 percent of the precipitation (infiltration) and can move 10 to 40 percent of annual precipitation and move it into deeper groundwater (interflow). The typical storm event of gentle rainfall on forests allows water to move down slope below the surface and slowly displaces subsurface water near streams, lakes and wetlands. As a result of evapotranspiration, interflow and infiltration, surface runoff in native forests is less than 1 percent of the precipitation delivered in the average storm event.

In contrast, the typical developed community surface runoff from an average storm event accounts for 20 to 30 percent of total precipitation, evapotranspiration for 20 to 30 percent and interflow for 0 to 30 percent. The causes of these changes in water absorption and flow are directly related to the amount of impervious surface, changes in soil structure and reduced levels of plant material. Impervious surfaces include roads, parking lots, side walks, and roofs. Changes in soil structure is largely a result of the practice of clearing and leveling land for development below the two to foot limit of native soils. The result is sand, silt, and clay (e.g., minerals) with no to very limited bio-organisms and little, if any, organic material to feed them. Plant material levels are reduced because part of the ground that was forest is now covered by a structure and the landscapes installed by housing developers are usually the minimum to cover the ground for aesthetic value. Many newly installed landscapes fail because there are no or minimal nutrients in the soil to support plant growth or persistence for the long term. Where development causes changes to wetlands by either increasing or reducing water levels, additional plant material changes through succession occurs and results in changes in wildlife that can live in that environment. Also, the natural filtering capabilities of wetlands is altered so that pollutants may be released into water bodies as overflows occur or redirected to other soil that can not filter pollutants.

As a result of these factors, watershed conditions and streams are impacted by the increased amount of surface water. Storm flow volumes and increased large storm event frequency cause increased erosion of streams, increased sedimentation, increased water pollutant load (hydrocarbons, heavy metals, etc.), and increased blockage of streams (sedimentation, loosened rock). Stream and river channels become unstable and bank habitat is lost. As banks loose stability and sedimentation increases, water quality decreases and habitat of water creatures and those that prey on them is lost. More water goes directly into streams without the natural filtration of infiltration and interflow processes. As a consequence, excessive nutrients end up in the water and feed excessive aquatic plant growth resulting in excessive daily oxygen fluctuations that can cause or increase the death of aquatic dwellers. Also, with increased flows directly into water bodies, less water is infiltrated into underground aquifers, while demands on those aquifers increases as communities expand.

"Heavy metal concentrations of less than 1 percent can alter spawning and migration behavior of salmon and trout. Eighteen of forty-five endocrine disrupters are found in Puget Sound fish tissue. Water for human consumption is held to higher quality standards than stream outflows. N of N1 streams in Kitsap County fail to meet Washington State’s water quality standards."

Low impact development as defined by "Low Impact Development Technical Guidance Manual for Puget Sound" addresses the affect of community development on the native forest environment with the intent of restoring natural infiltration and interflow processes to the extent possible so that communities will continue to have fresh, potable water while also supporting native or restored forests, streams, lakes and wetlands.

Low Impact Development Defined
Low Impact Development (LID) is a storm water management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions. – Chris Hinman, Low Impact Development Technical Guidance Manual for Puget Sound

Gardeners and homeowners can apply LID techniques to their garden and property to contribute to the overall improvement of interception, infiltration and interflow of water on their property. LID techniques such as grading and clearing practices, use of pervious surfaces, cisterns (or other water collection methods), rain gardens, roof gardens, protecting and restoring vegetation levels, building good soil and a regular maintenance program that emphasizes monitoring and matching controls to the level of the problem.

Impervious materials for patios and walkways can be replaced with pavers or flagstone laid in coarse sand or gravel. In order to increase the amount of interflow in this area, additional preparation steps are needed.

"We’re always talking about biodiversity, but that’s an abstract term. We’re not saving the panda because of biodiversity. We’re saving it because it arouses our emotion. And the emotional component is extraordinarily important to get the public behind conservation." – George Schaller, National Geographic, October 2006