Hydrozoning a desert yard
How to divide a Phoenix yard into water zones so no plant is drowned keeping its neighbour alive, and how little rain the yard actually gets to work with.
Short answer
Hydrozoning means sorting the plants in a yard by how much water they need and running a separate irrigation line for each group, so that a desert shrub and a vegetable bed are never on the same clock. Done properly it turns the irrigation timer from something you fight into something you set.
A desert yard is not short of plants that will grow in it. It is short of water, and the arrangement of the water is the whole of the design problem. Hydrozoning is the name for getting that arrangement right: group the plants by how much water they need, give each group its own valve and its own schedule, and stop asking one irrigation clock to serve a desert shrub and a lawn at the same time.
The reason it matters here rather than somewhere wet is visible in a single number. Phoenix gets 7.22 inches of rain in a normal year. Everything else that a planted yard needs comes out of a hose, and it arrives on a timer that somebody set once and has not looked at since.
What the yard actually gets from the sky
Normal monthly rainfall at Phoenix Sky Harbor. The dashed line is 0.60 inches, what an even share of the annual 7.22 inches would be in one month. Six months of the year come in below it, and two come in close to nothing.
Normal monthly rainfall, NOAA NCEI U.S. Climate Normals 1991–2020, Phoenix Sky Harbor (USW00023183). The twelve months sum to 7.22 inches, and the dashed line is one twelfth of that. June is the driest month of the year at 0.02 inches; August is the wettest at 0.93. A yard planted for the desert still needs irrigation in every month, and needs most of it in the two months that are driest and hottest at the same time.
Step one: work out what the site already gives you
Hydrozoning starts with a survey rather than a plant list, and the survey has three parts.
Water from the sky. The chart above is the starting budget, and it is the same for the whole Valley to within a tenth of an inch or so. Seven inches a year, delivered in a blunt way: almost all of it in the two monsoon months and in December to March, and almost none of it in April, May and June, which are also the months the yard needs it most.
Water from the hose. This is the part you control, and it is worth knowing what it costs. AMWUA, the association of the Valley’s ten municipal water providers, puts outdoor use at up to 65% of residential water consumption, which is why the irrigation clock is the single most consequential setting in the yard.
Sun, and which way the walls face. Sun drives water use as much as the plant does. A plant against a west wall in July is in a hotter place than the same plant ten feet away under a tree, and it will use more water for the same size. Sorting the yard by sun before sorting it by plants saves a zone later.
Step two: sort the plants into zones
The useful number of zones is three or four. More than that adds plumbing without changing the answer, because the plants at either end of the scale do not need anything between them.
| Zone | Water need | What belongs in it | How it is watered |
|---|---|---|---|
| Dry | Rainfall, once established | Native desert trees, desert shrubs, cacti and succulents | Its own valve, and often none at all after the first two years |
| Low | Occasional, deep | Desert-adapted trees and shrubs, most perennials | Its own valve, on longer intervals |
| Moderate | Regular | Flowering perennials, herbs, small citrus, some groundcovers | Its own valve on the main schedule |
| High | Frequent | Turf, vegetable beds, shade trees from wetter climates | Its own valve, and the one worth reducing first |
The sorting question is not “how much water does this plant like”. It is “how often does this plant want to be watered”, because that is what a valve schedules. A native tree and a lawn may both want a lot of water in a single watering; the difference is that the tree wants it a fortnight apart and the lawn wants it every other day. Put them on the same line and either the tree is drowned or the lawn is dead.
Step three: give each zone its own valve
This is the physical work, and in an existing yard it is where the cost sits. The principle is simple: one valve, one schedule. A valve serving two zones cannot be optimised for either, and the clock will always be set to the thirstiest thing on the line.
In practice, most yards are already divided into valves by the original installer, usually by geography rather than by water need, which is the mistake. Front beds on one valve, back beds on another, and the two are watered identically regardless of what is planted in them. The rework is to move plants between valves rather than to add irrigation: replant the thirstiest things together, and let the valves that remain serve groups that agree.
The Hydrozone Yard Planner does the counting part of this (how many plants are in each zone and what that means for the clock) and the Desert Plant Picker is where the water need of a named plant comes from in the first place.
Step four: match each zone to the sun it sits in
A zone should run the same way along its whole length, and the sun is what breaks that. A valve that serves a strip of full western sun and a strip of afternoon shade will be right for one and wrong for the other.
The fix is to divide by sun as well as by water: put the west-facing, full-sun plants on one line and the shaded plants on another, and let each run to its own schedule. Where the yard does not allow it, the honest fallback is to plant the whole line for the harshest part of it, which means the shaded end will be slightly overwatered, and that is a tolerable answer where adding a valve is not.
Step five: change the schedule with the season
An irrigation clock that is set in April and left alone will be wrong by June and wrong again by November. Water use in the Valley roughly follows the temperature, and the temperature swings forty degrees between the coolest and hottest months of the year.
The practical shape of it is two big adjustments a year (one going into the summer, one coming out of it) and small changes either side. Spring, from March into May, is the ramp: the days lengthen, the temperature climbs and the yard starts drawing water faster than it was a month ago. Autumn is the reverse, and it is the adjustment people forget, which is why so many yards are still being watered on a July schedule in December.
The two mistakes worth naming
The first is zoning by geography instead of by plant, which is what most existing yards already do and what makes the clock so frustrating. The second is treating hydrozoning as a planting scheme rather than a plumbing one: it is not about where the plants sit on the plan, it is about which valve they sit on.
Get those two right and the rest follows, because everything else in the yard (how deep to water, how often, how to change it through the year) is answered by the zones rather than by the plant list. How often to water desert plants takes the schedule from here, and desert lawns, water and what they ask for deals with the one zone that usually needs rethinking rather than reshuffling.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, monthly summaries, Phoenix Sky Harbor (USW00023183)
The monthly rainfall normals in the chart, and the counts of days with measurable rain.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, annual and seasonal summaries, Phoenix Sky Harbor (USW00023183)
The annual rainfall of 7.22 inches and the 33.4 days a year with a trace of rain or more.
- University of Arizona Cooperative Extension
The principle of grouping plants by water use on separate irrigation valves, and the guidance to water established desert plants deeply and infrequently rather than lightly and often.
- Arizona Municipal Water Users Association
The association's figure that outdoor use is up to 65% of residential water consumption in the Valley, and its guidance to adjust an irrigation clock with the season rather than leaving it fixed.