Overlay is a word growers use when the surface becomes overly dense, matted, or unproductive relative to what they hoped to see.
The exact look can vary, which is part of why the term gets used loosely. Sometimes it describes heavy surface colonization that seems to stop progressing. Sometimes it refers to a thickened layer that no longer responds well to fruiting conditions. In practice, the important part is not the label alone but what that surface is telling you about the system.
Causes can include environmental mismatch, extended colonization, species-specific behavior, or a culture that simply expresses itself in a more aggressive surface pattern. That is why overlay should be interpreted carefully rather than treated like a one-size-fits-all diagnosis.
Why this matters
Overlay is useful to understand because it forces growers to think beyond simple success-or-failure language. It is a surface behavior, and surface behavior is often a clue about timing, environment, and culture expression working together.
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