By John Black, RLA, CPLD
From the June 2024 Issue
No matter how large or small the site, irrigation is essential to a landscape’s health and beauty. But too much water can be as bad for plants as too little; and giving any plant more water than it needs wastes resources and money. To create water-efficient landscapes that are also easy to irrigate, start with a strategy known as “hydrozoning.”
In simplest terms, hydrozoning means grouping plants according to their irrigation requirements. Most growers simply label their plants as “low,” “average,” or “regular” water users, or with vague instructions like “keep soil moist.” While these terms don’t tell us how many gallons to apply, they do provide a rudimentary basis for combining comparable plants into hydrozones. Grouping species with similar needs allows all the plants in a hydrozone to receive just the right amount of water.
Lavender, kangaroo paw, smoke tree, rockrose, and hopseed bush comprise this low-water hydrozone enveloping a lounge patio. (Photo: Verdance Landscape Architecture)
Defining The Zones
Typically, we refer to a hydrozone by the water requirement of its plants: “high” (like a lawn), “moderate” (most ornamentals), “low” (usually native and climate-adapted species), or “very low” (such as desert plants and succulents). You don’t want to irrigate any one plant at the expense of its neighbor, so don’t mix plants with different needs within a hydrozone.
In actual practice, many factors contribute to a plant’s water use. For instance, shrubs located against a west-facing wall may get more reflected light and heat, and so require more irrigation than east-facing counterparts. As a result, hydrozones must consider not only plant species, but also local cultural factors such as sun, wind, soil, adjacent conditions, even the mulch used.
Hydrozones must consider not only plant species, but also local cultural factors such as sun, wind, soil, adjacent conditions, even the mulch used.
Existing vegetation, especially trees, can also define hydrozones. A high-water birch, willow, or redwood will require consistent irrigation that’s incompatible with low-water understory species like lavender or succulents. Conversely, a low-water specimen such as a native oak would be jeopardized by the irrigation needed to sustain high-water groundcover within its root zone. If you have significant trees or shrubs in a landscape, learn their water needs and plan the hydrozones around them.
Lawns present special challenges. Turfgrass is almost always considered high-water, and usually gets a hydrozone of its own. But water always migrates beyond where it’s applied, especially in clay soils, so planning a high-water lawn next to a low-water oak could be a death sentence for the tree. A smarter approach would be to create a moderate-water buffer zone between the lawn and the oak; or better still, keep all plantings (and irrigation) outside of the oak’s root zone.
High-water vegetable beds are located well outside the native oaks’ low-water hydrozone. (Photo: Verdance Landscape Architecture)
Zone Sizes & Valves
To that end, start your hydrozoning by considering the shape and the size of your planting areas. Spray areas in particular should be designed with the heads’ throw distance and pattern in mind: if high-water sprinklers overspray onto a moderate-water border, that border will receive more than “moderate” water.
Even if you’re using drip irrigation that can be applied precisely, a 3’ x 4’ hydrozone probably isn’t worth the expense of the unique valve, pipe, and other equipment it would require. Designing a patchwork of small hydrozones throughout the landscape also runs contrary to the intent of hydrozoning, which is to create efficiency.
It’s convenient to think of hydrozones in terms of irrigation valves: each valve serves one zone, and its controller will be programmed according to the unique water requirements, sun exposure, soil, and other cultural factors of that hydrozone. (Note that a very large hydrozone might require two or more valves to ensure uniform pressure and coverage; all those zones would simply follow the same watering program.)
The type of irrigation also informs the hydrozone design. Since high-volume and low-volume systems shouldn’t be combined on one valve, if you plan to use spray heads for most of an area but desire to use drip for part of it, you’ll need two valves, two pro- grams, and therefore two hydrozones — even if both have the same water requirement.
Evapotranspiration
The demarcation is clear between the low-water hydrozone beneath the native oaks and the moderate-water lomandra, berberis, carpinus, and daylilies beyond. (Photo: Verdance Landscape Architecture)
So far, we’ve described hydrozones in purely relative terms, like “low,” “moderate,” and “high.” But irrigation happens in measurable amounts, like inches and gallons. How can we quantify how much water each hydrozone should receive?
One term you’ll hear in hydrozone design is evapotranspiration rate, or ET. This refers to how much water is lost from the soil over time, whether by evaporation directly into the air or by transpiration up into and through plant tissues.
State agricultural agencies, often university extensions, publish reference ET rates—called “ETo” for short — for various locales. The ETo describes how many inches of water evaporate or transpire per square foot from a high-water reference surface (usually a large grass field, maintained and measured by the agency) in an average month.
The ETo is important because it provides a baseline against which plants’ water needs can be measured. Instead of just accepting that all the plants in a hydrozone require “average” water, we can now calculate how much water to apply. A 2017 national standard (ANSI/ ASABE S263) provides guidelines for the amount of water needed by turf, flowers, woody plants, and desert plants. Expressed as fractions of the ETo, these coefficients are known as plant factors.
For instance, if we’re working in Charlotte, the NC Cooperative Extension tells us that the ETo for an average June is 6.05″ per square foot. Assuming we have plants with “average” water needs requiring 50% of ETo, we can program our irrigation to deliver about 3″ per square foot that month. But that’s still an assumption, based on incredibly general guidelines. On the other coast, the University of California has evaluated the water needs of more than 3,500 plant species and defined four categories or plant factors there:
High water needs plants require 70% to 90% of ETo;
Moderate plants consume 40% to 60% of ETo;
Low water users take 10% to 30% of ETo; and
Very Low species need less than 10% of ETo.
(Learn more CA-specific data.)
As in most states, California publishes ETo rates for specific locations. The ETo for Palo Alto, where we work, is 5.3″of water per square foot for an average June, so species found to have a “moderate” plant factor here should receive 40–60% of that, or between 2.12″and 3.18″, over the month. Once you know the location’s ETo, the area of each hydrozone, and the plant factor of each hydrozone, you can begin to determine exactly how much water the irrigation system should deliver to each hydrozone every day or week.
Moderate-water Acer palmatum ‘Butterfly’ defines this hydrozone with miscanthus, heuchera, anemone, berberis, festuca, and dymondia. (Photo: Jude Parkinson-Morgan)
Some additional adjustments are necessary to account for the type of irrigation in each hydrozone — drip is more efficient than spray — as well as for environmental variations such as wind or soil type. For instance, you may opt to assign an otherwise moderate-water hydrozone (50% of ETo) a plant factor of 0.6 because it’s in a windy location that increases evapotranspiration.
Exacting Efficiency
A stone planter of moderate-water plants like canna and hakonechloa is a different hydrozone than the low-water euphorbia, echium, and Lavandula beneath. (Photo: Verdance Landscape Architecture)
While such fine-tuning might seem like overkill, or intimidating in its complexity, the possibilities that hydrozoning offers the modern landscaper are actually quite exciting. Fortunately, you’re not alone in your journey toward efficient and responsible water management.
Irrigation controllers have become incredibly sophisticated, able to not just sense rain but even poll meteorological data to adjust the watering schedule. They can calculate run times based on any plant factor, root depth, and efficiency you specify. They can control just a handful of zones, or hundreds on a park or campus. And they can be managed remotely by you or a maintenance company, for trouble- shooting and fine-tuning long after the new plants have established.
Low Input, High Impact Landscapes
Hydrozoning is not merely a technical strategy; it’s an art that blends scientific precision with environmental stewardship. Plants that are irrigated properly look better and grow better, needing neither excessive pruning nor extra fertilizer. And hydrozoning can help you and your clients save significant money — not to mention a precious natural resource — by cutting water use and reducing waste. If you haven’t started bringing all the benefits of hydrozones to your projects yet, the time to start is now!
Black is the founder and Principal of Verdance Landscape Architecture in Palo Alto, CA. He is a licensed Landscape Architect, a member of the American Society of Landscape Architects (ASLA), and certified by the Association of Professional Landscape Designers (APLD). He uses hydrozoning to create “Califriendly” landscapes that celebrate life in the Golden State.
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