by Tony Gurnoe, Director of Horticulture
Water is the lifeblood of any garden, and this is especially evident to horticulturists in Southern California. San Diego Botanic Garden frequently sees less than 10 inches of rain in a year, and nearly all of that comes between the months of November and March. Despite having such low annual rainfall, individual storms can still produce enough water to outpace the infiltration capacity of our Garden. Not only does this result in valuable rain running off-site rather than into the soil, it also increases the potential for polluting materials such as sediment or fertilizer to travel downstream to other habitats. To mitigate this, the landscape surrounding our new Dickinson Family Education Conservatory includes four distinct bioretention basins, specific soil amendments and state-of-the-art irrigation technologies.
Strategically situated vegetated depressions in the landscape effectively capture water that would otherwise become runoff. Instead, that water can percolate through the roots of these sunken gardens as both a filtration mechanism and to slowly recharge underground aquifers. Three of the swales are designed to receive rainwater coming off the Conservatory roof, the concrete amphitheater and the surrounding hardscape. The swale closest to the building, known as the rain garden, also captures any supplemental water not consumed by the plants inside of the Conservatory. This regular water allows for a lush tropical-looking section nearest the Conservatory. The parking lot adjacent to this project is constructed of permeable paving to further sequester valuable water and minimize damaging runoff.
If you visit the garden areas surrounding the Conservatory after a significant storm, you’ll notice that each of these bioswales becomes a temporary pond full of valuable water that would otherwise be burdening our stormwater system, contributing to waste and pollution. This also creates a distinct and challenging environment for the plants, consisting of long, dry periods punctuated with brief moments of being partially under water. Not all plants are so adaptable. Some will tolerate the dry times with no problem, but suffer when being inundated by rain. Others will flourish in a temporarily aquatic environment, but languish without irrigation assistance during the dry spells. Bioswales are often heavily planted in rushes, sedges and bunch grasses for these reasons. Many of them tolerate mild drought just as well as they grow with roots in water, but they only lend a certain type of utilitarian aesthetic. Rather than repeat this pattern and relegate a substantial portion of this new landscape to purely functional plantings, we aim to demonstrate that bioswales can be just as interesting and attractive as any other space in the garden. For this aspect, we partnered with the San Diego County Water Authority to pursue our shared goal of exhibiting sustainable, yet beautiful landscapes.
Making the most of the little rain that falls on the Garden is only one element of a sustainable landscape. Establishing a healthy and diverse soil food web is another critical factor to converting any captured rainfall into a thriving garden community. The best way to impart the water and nutrient holding benefits of living soil to a new landscape is by adding compost and organic mulch. We amended the gardens surrounding our Conservatory with 2 inches of compost followed by topdressing with 3 to 4 inches of raw wood chip mulch. The compost itself contributes a microscopic community of organisms that help to convert organic matter in the soil into the nutrients required by these plants. When topped with a thick layer of mulch, this system dramatically reduces evaporative loss of the moisture that is so precious to a California garden and provides a sustainable habitat for the worms, fungi, bacteria and other organisms that make up healthy, living soil.
Once the topography is established and the soil amended, the garden is ready for plants. Surrounding the Conservatory is a mix of rare and endangered species and horticultural varieties selected for the reliable beauty they bring to a low-water landscape. Near the upper edge of each swale, where plants receive less water, we have chosen very drought-tolerant specimens such as Encephalartos horridus. This endangered cycad comes from arid shrublands of South Africa with a climate very similar to San Diego. These plants had long suffered from being poached in the wild before recently becoming horticulturally available through San Diego Botanic Garden plant sales and other reputable nurseries. An unusual leafless species of bird of paradise known as Strelitzia juncea compliments the prickly Encephalartos by providing bright, long-standing flowers to this low-water garden. Grevillea ‘Noellii,’ Boronia ‘Lipstick,’ and other water-thrifty plants from Australia provide profuse, brilliantly colored flowers throughout much of the year, while lending a unique textural aesthetic. No landscape would be truly sustainable without including native plants, so we are fortunate there are more varieties of Salvia, Diplacus, Spheralcea, Ceanothus and other floriferous native plants available than ever before. Most of these native selections require minimal maintenance to provide bountiful displays of color.
Although these plants can survive periods of drought once established, they will grow more robustly and beautifully with some supplemental irrigation. Recent low-flow technologies have revolutionized how efficiently water can be supplied to plants in need. Weather stations, soil moisture sensors and various kinds of low-flow emitters have been implemented in the Conservatory landscape to put just the right amount of water where the plants need it. Each basin is equipped with tubing featuring built in drip emitters, just like the living wall inside of the Conservatory. Where spray sprinklers are more appropriate, we have used MP rotator nozzles. These nozzles apply water more slowly than traditional spray nozzles which results in less moisture lost to evaporation and better infiltration. Surprisingly small amounts of water can significantly enhance a garden when applied strategically. Not only does this supplemental water provide for the plants and soil biology, it also leads to longer growing and flowering seasons which provide more sustenance and enjoyment for pollinators, wildlife, and human visitors to the gardens surrounding our new Conservatory.
Promoting the sustainable use of natural resources is an important component of the mission at San Diego Botanic Garden. Considering how water interacts with, and is consumed by, our landscape is essential to establishing a beautiful, educational public space that acts to conserve plants and offer respite from our increasingly urban world. When you visit the Dickinson Family Education Conservatory take a moment before entering to appreciate the planning and water-saving features embodied in the surrounding landscape. Gardens are always an evolution and work in progress, but fundamental considerations like capturing rainwater, infusing the soil with rich biology and installing a well- configured, low-flow irrigation system provides the groundwork for a planted space that enhances our environment and can be sustained for future generations.