Cause and symptoms
The hydrophobic (non-wettable) nature of many Western Australian soils, particularly the sandier types, means that once they dry out, they are very difficult to re-wet thoroughly. During the hot months, waxy substances that emerge from the soil’s organic matter form a coating on the surface and this can prevent water from penetrating to the root zone.
The result is that dry patch develops on the lawn. Water droplets falling onto it simply run off to the side, and the water begins to establish a preferred pathway — so bordering areas of the lawn receive more of a drink and become greener just as the dry patch becomes browner. Too often, dry patch is blamed on African black beetles when few or none are present.
Remedying the water repellence of the surface is not quite as simple as applying a soil wetting agent, because the wetting agent will immediately run off in the same way the water does.
Before application aerate the surface, either with a mechanical or manual aerator or with a garden fork. Then soak the area with water and then apply the wetting agent. Read the directions on the label and follow up with another application after the recommended interval to prevent repellence building up again.
In the meantime it is worth checking to see whether faulty reticulation or a broken sprinkler was responsible for the dry patch forming in the first place. Even if all the sprinklers are working, a strong easterly wind may be diverting the spray, especially if it is in the form of a fine mist.