Stewardship of mature campus landscapes, like many found in the Northeast, requires the balancing of competing pressures on heavily-used greenswards, while maintaining healthy soil and plant communities. Water-efficient irrigation systems are a good way to both improve plant health and increase water efficiency for a modest investment. The most efficient irrigation systems are those that combine local weather data- or soil moisture-based controls, efficient sprinkler heads, and adequate zoning to deliver water appropriate to microclimate-level soil types and plant communities.
The benefits of providing adequate irrigation without over-watering plants are multiple. Water-efficient irrigation can:
• Conserve a valuable resource and save money on water bills
• Provide appropriate water for plant health encouraging formation of a healthy root system
• Foster healthier plants that require less treatment for pests and disease, reducing maintenance costs and lower applications of chemicals
• Prevent runoff and non-point pollution
When a calendar dictates the irrigation schedule, over-irrigation may result. Timer-based controls that water at regular intervals often keep only the top few inches of soil wet, encouraging root hairs (fine roots that take up water) to concentrate in the top 2” of soil. That same top 2” of soil is the first to dry out in an irrigation interruption. When a line breaks or the irrigation regime changes (such as during mandatory municipal water rationing), plant health suffers.
Conversely, when adequate water is supplied to the entire root zone of plants, rather than only the top few inches, plants develop extensive root systems. Healthy, plentiful roots make for robust plants that are less susceptible to drought, fungus and insect diseases, and other stresses. Proper irrigation brings the additional benefit of leaching salts and other toxins from plant root zones. (See Carleton, Bruce, Special Districts Department, City of Moreno Valle, CA. “Wednesday is NOT a Good Reason to Irrigate”. From the EPA WaterSense website: http://epa.gov/watersense/index.htm.)
Radcliffe Central Yard Project
As campuses grow, tree-lined quadrangles and lawns are increasingly repositories for underground infrastructure, including “green” technologies such as geothermal wells or stormwater storage tanks. While necessary and often laudable, installation of these structures requires drilling and excavating that can lead to compacted and disturbed soils. Similarly, construction staging for building renovations sometimes spills over onto lawns. The same green spaces continue to support high-traffic activities like commencements and reunions. When severely compacted, these iconic central yards may exhibit muddy soils, thin lawns, and stressed or dying canopy trees.
The leafy central campus of the Radcliffe Institute for Advanced Studies, at Harvard University, is highly compacted from recent construction activities on silty soils. A comprehensive Landscape Master Plan for the Radcliffe campus was created by Stephen Stimson Associates in 2007. Multiple, phased projects were proposed to reinvigorate quad and garden landscapes. The focus of this article is the Radcliffe Central Yard project. Now in design development, proposals include plans to:
(1) create a tree survey and care plan by certified arborists;
(2) re-grade and amend or replace Yard soils to improve drainage and mitigate recent construction disturbance (including installation of stormwater storage chambers and geothermal wells);
(3) simplify the perimeter path system; and
(4) retrofit the irrigation system to improve lawn and tree health and reduce water usage.
Various retrofits to the existing irrigation system were considered. Rainwater and grey-water storage tanks were studied and rejected because they were too costly and inefficient, and required pumped-in water to supplement these options. Existing irrigation wells are increasingly saline and may require an eventual switch to city water supply.
As proposed, soil amendments to improve drainage and mitigate compaction in the Yard will create a complex horizon of “new” and “old” soils around mature trees. Approximately 9 inches of compacted silt/clay soils will be removed from open lawn areas, underdrainage will be installed, and compacted soils will be replaced by engineered, free-draining sandy soils to support an athletic-performance turf that can handle high foot traffic. To preserve tree root zones, large beds of soil will be retained around mature trees. Arborists will use an airspade to find the limits of healthy tree roots, and soil will be replaced outside established root zones. The resulting soil interface will require careful irrigation zoning for tree health.
EPA “WaterSense” Guidelines and Smart Controller Systems
Irrigation Consulting, Inc. is designing the irrigation system at Radcliffe to meet forthcoming EPA “WaterSense” guidelines for efficiency. (See EPA WaterSense Guidelines at: http://epa.gov/watersense/index.htm.)
The cornerstone of a water-efficient irrigation system is a “smart” controller, in which irrigation frequency and duration would be set in response to local weather or soil conditions. According to the EPA website, “weather- or sensor-based irrigation control technology uses local weather and landscape conditions to tailor irrigation schedules to either actual conditions on the site or historical weather data. Instead of irrigating according to a preset schedule, advanced irrigation controllers allow irrigation to more closely match the water requirements of plants.” (See http://epa.gov/watersense/pp/controltech_overview.htm)
As a rule of thumb, smart controllers realize a 20% water savings over timer-based systems by using a combination of a weather-data controller, more efficient sprinkler heads, and more zones. For most campus applications, weather-based systems that download local weather data are more dependable than soil moisture monitoring, which tends to require frequent re-calibration and oversight. In addition to rainfall measurements (collected in a low-tech rain gauge), weather-based controllers collect local solar radiation, wind, humidity, and temperature data every day. At Radcliffe, the proposed system would replace the old timer model with one or more weather-data controllers (depending on the number of zones), an increased number of zones (especially in the interface between lawn and tree soils), and more efficient sprinkler heads. More control over the overall system would allow a more efficient delivery of water, and healthier plants. According to Brian Vinchesi, Irrigation Consulting, Inc., smart controllers offer a quick payback: they cost about $800 each (approximately three times that of a standard controller) and require a modest monthly fee for a daily weather download.
Though still under study, a smart control irrigation system at Radcliffe should lower water use while tailoring water supply to trees and lawn, reinvigorating the leafy heart of the campus.
Perspective renderings of Radcliffe Yard by Glenn Valentine of Stephen
Image courtesy of President and Fellows of Harvard University.
Laura L. Tenny, ASLA, is Capital Project Manager, Harvard Real Estate Services, Harvard University and can be reached at: firstname.lastname@example.org. Glen Valentine is an Associate at Stephen Stimson Associates in Falmouth, Massachusetts and can be reached at: email@example.com.