Landscape Irrigation Design
Ornamental grasses and perennials in a low-water hydrozone near a Coast Live Oak tree in this Palo Alto yard.
Water is essential to life in a landscape.
And even though lately California has received an (over?)abundance of rain, our summer-dry climate means that what falls from the sky today won’t be enough to sustain the garden tomorrow.
So it’s imperative that the landscape architect creates not only a planting design that considers plants' water needs, but also an irrigation system that will fulfill those needs when Mother Nature can’t.
The first step in this process is called “hydrozoning,” which means grouping plants in the landscape according to their water requirements. This creates water-efficient landscapes that are also easy to irrigate.
We follow the water needs classifications defined by the University of California’s WUCOLS database. WUCOLS groups more than 3,500 species into “high,” “moderate,” “low,” and “very low” by climate region.
Hydrozoning ensures that all the plants in a given area share the same needs—all are “high,” or all are “moderate,” and so on. (We also consider other cultural factors such as sun, wind, and soil; an otherwise low-water plant in a particularly hot or windy location, or with particularly sandy soil, might need to receive moderate water.)
The secret to hydrozoning is to think ahead: what will be a reasonable size area to irrigate, and how will it be irrigated?
For example, a 3’ x 4’ hydrozone doesn’t make a lot of sense if you’ll be using spray heads that throw the water 10’ or more. Even if you’re using drip irrigation that can be applied much more discretely, such a small area probably isn’t worth the expense of the unique valve, piping, etc. it would require.
Designing a patchwork of small hydrozones throughout the landscape runs contrary to the intent of hydrozoning, which is to create efficiency.
MP Rotator sprinklers are a cost-efficient way to water a large area. Photo courtesy of Hunter Industries Incorporated.
Lawn presents particular challenges. Turfgrass is a high-water plant, usually in a hydrozone of its own because it has a unique watering schedule and so needs its own irrigation valve. But water always migrates beyond where it is applied, so planning a low-water hydrozone next to a high-water lawn is a bad idea that will mean death for those plants.
Once we have our hydrozones figured out, with some fancy calculations WUCOLS helps us know how many inches of water each hydrozone should receive in each month of the year. From there, we can consider what type of irrigation equipment is best to deliver that water.
Spray heads are the most efficient way to apply a lot of water to a large area of short plants, such as lawn. Adjustable nozzles and lower-flow designs such as Hunter’s MP Rotator provide significant water savings over old, fixed-nozzle designs.
Lawn can also be irrigated using inline drip tubing—low-volume emitters built into a flexible distribution tube—placed below the surface of the ground. Similarly, subsurface mats of synthetic fibers that wick water from a distribution tube throughout a wide area. Both methods eliminate the disadvantages of spray heads such as misting, overspray, and misdirection, but at a higher expense.
In the case of mixed planting beds, inline drip tubing can be spaced to quickly create a grid of emitters that waters an area evenly. This provides more efficient coverage than spray irrigation, especially in curving or irregularly shaped beds, and applies water at the root zone where it will not be blocked by branches or foliage as the plants grow.
However, inline drip has two main disadvantages: it is difficult to move the lines once installed, and impossible to change the rate at which they deliver water. And, because they tend to live below mulch if not below soil, it is very easy to sever the drip lines with a shovel or trowel when, say, adding a new plant.
This “button” drip emitter applies two gallons of water per hour. Photo courtesy of DIG Corporation.
The alternative to inline drip is “point-source” drip emitters. This system uses individual emitters that tap into a main distribution tube and are placed precisely relative to the root zone of each individual plant. As the plant grows, the emitters can be repositioned, or removed entirely in the case of very-low-water species that need no supplemental irrigation.
Although installation of the individual emitters and supply lines is more labor-intensive than with inline drip, and more components comprise the system, point-source irrigation can further increase irrigation efficiency, especially in sparsely planted areas where line-source drip would provide unnecessary water.
Finally, once the delivery method is known, we can schedule how long the irrigation needs to run to deliver the correct amount of water, and how many times a day or week, and program a timer-controller accordingly.
Irrigation controllers have become incredibly sophisticated, able to not just sense rain but even poll meteorological data to adjust the watering schedule based on current and predicted conditions. They can control just a handful of zones (or stations) on a small property, to hundreds on a park or campus. They can integrate with home-automation systems and apps. And they can be controlled remotely by a landscape contractor or maintenance company, for troubleshooting and fine-tuning after the new plants have established.
The most extraordinary planting design is worthless if those plants don’t receive the proper water throughout their lives. Investing in thoughtful irrigation design, expert installation, and ongoing maintenance will insure your investment in the landscape design itself.