· Michael Blair
A dry patch in an irrigated lawn is usually telling you about distribution rather than duration. Running the zone longer overwaters everything else to reach the one spot that is short.
Head-to-head coverage
Sprinkler heads are designed so each throws all the way to the adjacent head. Anything less and the midpoint between them is underwatered, which shows as a dry band.
Systems installed with heads too far apart cannot be fixed by running longer, only by adding heads.
The catch-can test
Straight-sided tins spread across the zone, run for a set time, then compare the depths. Uniformity is what you are measuring, not volume.
Wide variation means a distribution problem — spacing, pressure, mixed head types or a blocked nozzle.
- Heads should throw to the adjacent head, not to the midpoint
- Never mix rotors and spray heads on one zone
- Check for blocked nozzles and heads that will not retract
- Grass growing over heads blocks the pattern
- Pressure too high atomises the spray and it drifts
Mixed head types
Rotors and fixed spray heads apply water at very different rates. On the same zone, running long enough for the rotors floods the spray areas.
They should be on separate zones. This is a common shortcut in older installations and it cannot be corrected by programming.
Pressure
Too low and rotors do not turn through their full arc. Too high and spray atomises into mist that drifts and evaporates rather than landing.
Both look like a coverage problem and both are fixed at the valve or with pressure-regulating heads rather than at the controller.