How Irrigation Zones Should Be Laid Out

What is an irrigation zone, and why not water everything at once?

A zone is the group of heads or emitters served by one valve. Our irrigation systems page compares zones to electrical circuits: your supply can only push a limited number of heads at once, and a sunny front lawn needs different watering from a shaded flowerbed, so each zone gets the water its plants need. Two limits set how zones are drawn: the hydraulic limit (how much flow and pressure the supply can give) and the plant limit (what is planted, where, and in what soil and exposure).

How much water can one zone use?

Start with the supply. Rain Bird's homeowner guidance describes two simple tests: a pressure gauge on the faucet nearest the water meter with nothing else running, and a bucket test, timing how long a 5-gallon bucket takes to fill and dividing 300 by the seconds to get gallons per minute. For well or pump systems, Rain Bird says pressure and flow capacity should come from the well and pump dealer or the pump's manual. Then add up the flow of every head on a proposed zone at the working pressure. Rain Bird's example: a rotor using 3.11 gallons per minute at 35 psi means a home with 10 gallons per minute available could run three on one zone.

We design each zone to use comfortably less than the measured supply, so pressure stays up at the last head and the house still has water while the system runs. On wells that margin matters even more, as our well water irrigation guide explains.

Why should each zone have only one type of head?

Because different heads apply water at different rates. Colorado State University Extension says spray heads apply 1 to 2 1/2 inches per hour, rotors roughly a quarter to three quarters of an inch, and it states the rule directly: do not mix sprinkler types in a zone. A zone with both has no correct run time. Drip is measured in gallons per hour rather than per minute, so it always gets its own valve. Our sprinkler head types guide explains each option.

How should zones follow plant water needs?

By grouping plants that drink alike, often called hydrozoning. EPA WaterSense's watering tips say to group plants with similar water needs and sun or shade exposure into the same zones. In a typical western Montana yard that means separate zones for lawn, for shrub and perennial beds on drip, for trees, and for any vegetable garden. Montana State University Extension's Yard and Garden Water Management MontGuide adds that species like poplar, aspen, willow, maple and mountain ash need more water than lilac, cotoneaster, Douglas-fir and pines, which are sensitive to excess water, so a bed of pines and lilacs should not share a zone with a thirsty lawn.

Trees deserve their own thinking. MSU recommends soaking the soil about 30 inches deep across the whole root area, which can spread as much as three times the spread of the limbs. A lawn zone that wets the top few inches every few days does not do that, which is why we often add drip or a separate deep-watering zone for established trees. See watering trees in drought.

Should sun and shade be on different zones?

Yes, when the difference is large. A south-facing lawn strip against a hot driveway can use far more water than the shaded north side of the same house. EPA's guidance to group by sun or shade exposure is the reason: put them on one valve and the shady side stays soggy while the sunny side browns. On windy, exposed Montana lots, we treat wind exposure the same way and favor low-angle, wind-tolerant heads on the exposed zones; our windy yard landscaping guide covers the planting side.

How do slopes change zone layout?

Two ways. First, elevation changes pressure: CSU says a grade change of more than eight vertical feet on a single zone produces significantly higher pressure at the low end and distribution problems, and Utah State University Extension notes pressure changes by 0.433 psi per foot of elevation. Zones on hillside lots are drawn across the slope, not up and down it. Second, slopes shed water: EPA recommends cycle-and-soak, shorter runs with breaks, to let water soak in on clay soils and slopes, and CSU notes that rotary nozzles and rotors, with their lower precipitation rates, suit slopes. Our landscaping slopes and hillsides guide covers the rest.

How big can a drip zone be?

CSU's drip guide suggests no more than about 200 feet of mainline per zone, roughly 200 gallons per hour on a half-inch mainline and up to 480 gph on three-quarter-inch, with only 75 percent of the source flow treated as usable. In practice one drip zone can often water every bed on one side of the house, but trees and vegetable gardens usually deserve their own because their schedules differ. Our drip irrigation guide covers emitter sizing.

What are the options when an old system zones badly?

There are three, in rising order of work. Renozzling and swapping heads within a zone, for example converting a mixed zone to all rotary nozzles, fixes the run-time conflict without trenching. Splitting a zone by adding a valve fixes zones that are hydraulically overloaded or mix lawn and beds. Converting beds to drip removes spray heads from planting areas entirely and often frees enough flow to fix the lawn zones too. A full redesign is rarely needed; most systems we see need one or two of these, which is part of our sprinkler repair work.

What zoning mistakes do we see most often?

The same handful, over and over. Beds left on the lawn zone, so shrubs get shallow daily water and the lawn gets less than it needs. A spray head added to a rotor zone to cover a gap. A long zone that runs uphill, so the top heads barely rise. Trees with no water of their own, relying on lawn heads that never wet deep enough. A parking strip on the same zone as the back lawn, even though the strip sits in reflected heat off the pavement. And zones that grew one addition at a time until they ask for more water than the supply can give.

None of these is rare or exotic, and none needs a new system. Each is fixed by moving heads, changing nozzles, adding a valve or converting to drip. The point of good layout is to avoid creating them in the first place, which is far cheaper than fixing them after the lawn is established.

What should you ask whoever designs your system?

Ask how they measured your flow and pressure, and what each zone is designed to use. Ask whether any zone mixes head types. Ask how lawn, beds, trees and the garden are separated, and how sun, shade and slope are handled. Ask where the backflow preventer goes and what assembly your water provider requires. Ask whether the controller can be upgraded to a weather-based model later. Ask for a zone map. A designer who can answer those plainly has thought about how the system will run, not just how it will be installed.

How does zoning fit the rest of a landscape plan?

Zones follow the planting plan, which is why we lay out irrigation alongside the design rather than after it. Our landscaping design service handles both, and sequencing matters: irrigation goes in after grading and hardscape and before planting, so lines do not have to be cut later. If your yard also has a ditch or district water, read the ditch and water rights guide before designing around it.

Ready to lay out or fix your zones?

We start with a property and water-pressure assessment, map what is planted and how the ground behaves, and design zones you can schedule simply. For an existing system, we run every zone and tell you which ones are worth changing. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate. We work across Missoula and Ravalli counties, from Missoula and Lolo to Florence, Stevensville, Corvallis, Victor, Hamilton and Darby.

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