January 1, 2011
MARIN COUNTY residents are fortunate to live in the coastal foothills near the bay and enjoy the Mediterranean climate. The effects of the nearby ocean, bay and local peaks and ridges as well as numerous plant communities provide us with have a plenitude of microclimates, ecotones
and niche habitats. This diverse canvas on which we create our gardens can offer numerous opportunities, but just as many constraints. Imagination and logistics are required if we are to bring out the best in our gardens.
Hillsides are, for the most part, an unavoidable attribute to most Marin landscapes. While views and geographic interest offer enticing layouts, the nuts and bolts of steps and terraces along with the concerns of erosion and drainage can make the typical Marin garden seem a daunting task.
There are some rules to keep in mind that can help you reap the benefits of hillside gardening. Grading is always work, but it is the foundation of the garden. Balancing cut and fill in the design can help reduce the amount of work. Proper drainage and compaction can help avoid catastrophe. The higher value of land has always promoted the use of retaining walls rather than simple slopes -- they take up less space. The value of topsoil and humus, along with the knowledge of the disastrous effects of silt and clay in our runoff, mandate that we slow, spread and sink rain water or collect it.
Early agriculture can be looked at as a guide, the contours of the land dictating the form of the garden. Four-foot walls in Marin need the stamp of an engineer. Permitting for smaller walls varies by jurisdiction, but offers gardeners the opportunity to make the most of their space. Multiple walls must be planned as a whole because, if they are close enough, one may be load bearing on another.
Steps are like smaller terraces. They help avoid erosion on steep paths. While steps in the house can have smaller treads, outdoor steps should be bigger. There are ratios and equations to ponder but 6- to 7-inch risers and 12- to 14-inch treads are comfortable and aesthetic. Risers should not vary by more than an inch. Uniformity means safety.
Landings are important for comfort. Three or more steps without a 3-foot landing may necessitate a hand rail. Likewise, a 30-inch drop beside a path or staircase may bring a guard rail into consideration. The two are not the same. While a hand rail only has to withstand a lateral force of 200 pounds and satisfy some dimensional constraints, a guard rail must restrict all but a 4-inch sphere from passing through. Designing steps and terraces without requiring many railings and guard rails would more aesthetic and cost-efficient endeavor.
Materials used, from simple timbers to mortared flagstone, and their ultimate use, from accessing a rustic hillside bench to a pool house, also determine how form should follow function. Because steps result in about 4,000 deaths in the United States each year, their design and the rules applied deserve attention.
There is a certain magic in 2 percent. This µ-inch per foot grade is a minimum when grading most paths, steps and terraces, as well as a minimum when draining water in a pipe. Anything less and water tends to puddle or lose sediment. As grades increase, water velocity does as well, as does the threat of erosion. Spreading and sinking water on site rather
than concentrating and draining it off should be embraced wherever possible. Never leave soil bare. Undisturbed native vegetation rarely erodes and runoff is minimized.
Look to your local plant communities for ideas and vegetative goals. Mulch and plant. Avoid burying layers of organic matter -- these decompose and can promote a future slide. Sometimes it is necessary to take additional measures: jute netting, waddles and erosion blankets are often used while revegetation is in progress. Catch basins, channel drains and culverts may also be warranted.
While an above-ground, vegetated infiltration system is always best, limited space and the need to design for our winter rains and saturated clay soils is practical and necessary.
It's satisfying to take advantage of a hillside's views, the added sunny spot in a garden or the adventurous garden path loop whether to stroll with others or find a meditative moment alone.