What an irrigation controller actually does
An irrigation controller is a clock with one electrical output per zone. At the times you set, it closes a circuit that energises a solenoid valve; that valve opens and the water flows. When the programmed time is up, the circuit opens, the valve closes and the irrigation stops.
Everything else you can do with a controller — adjusting the duration, grouping days, changing the start time — is programming on that same circuit. That’s why the only requirement that really decides whether a controller is useful to you is how many zones it has and whether it can deliver enough flow. Everything else is convenience.
Four ways of opening and closing the water
| Type | How it works | What it’s for |
|---|---|---|
| Solenoid valve | 24 V coil that moves a piston | The standard option in a garden |
| Ball valve with actuator | Motor that turns the ball 90° | Zones with high flow or dirty water |
| Controller with pilot valves | Switches that swap between zones | Existing installations with wiring |
| Smart controller | Adds a rain or moisture sensor | Anyone who’d rather not calculate schedules |
The practical difference between the first two is flow and maintenance: the solenoid is cheap and enough for almost any domestic garden, while the motorised ball valve handles much higher flow and water with particles, at the cost of a more delicate installation.
The number of zones is the non-negotiable requirement
Choose the controller after deciding the zones, never the other way round. A controller with four outputs when you need six zones forces you to share an output between two zones: if the plants sharing a line need different water, you won’t be able to give it to them.
As a reference: for a balcony with ten pots, two or three zones; for a small garden with hedges, lawn and a vegetable plot, four or five. If the unit you’re after has fewer outputs than you need, get a different model.
Flow per zone overrides everything else
Each controller output must be able to move the flow of its zone. If a zone has six emitters of 2 L/h, you need 12 L/h with some margin. And the margin matters: if the pump or the supply runs short, the last pots on the line receive less and the symptom is “the controller doesn’t water properly”.
If you want to see the full order of the system before choosing, the home automatic irrigation guide explains how zones are laid out and why they’re the key decision.
Pressure, the requirement nobody reads
Two details from the datasheet worth checking before you buy:
- Maximum working pressure. If the water supply exceeds that value, the electromechanical assembly breaks down; that’s why you need a pressure regulator.
- Flow per output. Which is what we’ve just explained.
Software that lets you adjust the flow of each zone is useful, but it’s always better for the hydraulics to be properly resolved before you trust the software.
The features that are genuinely worth it
- Manual start per zone, to test one specific zone without waiting for the programme.
- Percentage adjustment per zone, to compensate for flow differences.
- Delay between zones, so the pump and the pressure can recover.
- Rain delay, if it accepts a sensor: it’s the feature that saves the most water.
- Programming by day of the week, so you don’t water on Sundays when it isn’t needed.
What adds nothing in a domestic garden: large screens, cloud connectivity or apps with history. If you want to record data, a sensor and a Home Assistant dashboard does that better.
How to programme properly from day one
- Start with one watering per zone, the shortest that seems reasonable to you.
- Check with a glass how much came out: multiply by the number of emitters and you’ll know whether the flow is the expected one.
- Adjust the duration until the soil is properly moist and water comes out of the holes.
- Weigh a pot before and after to know how much water you gave it; that figure is your reference for the remaining zones.
- Add the second watering only when the first one falls short.
Programming more often than necessary is the most common mistake and the hardest to detect: a short, frequent watering wets the surface and leaves the roots dry.
When you don’t need a controller
With two pots and manual watering: watering by hand for two minutes a week is cheaper and more reliable than any device. A controller only starts to make sense from five or six plants onwards, or when there’s a zone that needs an hour of water while you’re away.
Selection mistakes that cost dearly
- Buying based on the number of programmes shown on the screen instead of the number of zones.
- Choosing a model with no spare output for the future and having to rewire.
- Ignoring pressure and having the first valve fail.
- Installing the controller in a socket shared with the pump: if the controller loses power, the irrigation stops halfway through.
If you already have the system installed and it doesn’t fire, the probability-ordered diagnosis tells you where to start looking.
Sources and next step
Rain Bird’s installation guide documents the flow-per-zone criterion, the role of the filter and the relationship between line size and flow, which is what limits a controller. For sizing consumption, the water demand guide gives you the numbers to compare against the flow you’ll be moving.
External sources
- Rain Bird: sprinkler system installation guide (PDF) — flow per zone, filter and line sizing.
- MSU Extension: soil sensors for smart irrigation — why a sensor usually saves more water than fine-tuning the programme.