Dead Spots Around Trees: Allelopathy, Competition, or Mowing Artifacts?

A homeowner has 12-inch dead circles around three trees despite irrigation and Scott’s 4-step program.

Possible Causes

1. Allelopathy: Some trees (walnut, cedar, eucalyptus) release chemicals inhibiting grass. Species ID needed.
2. Root competition: Tree roots outcompete grass for water/nutrients, especially on higher ground near trunks.
3. Mowing pattern: Circles match typical mower turning radius—could be scalping or compaction from repeated turns.
4. Fungal signs: Check for mushrooms, white mycelium, or musty smell.

Diagnostic Steps

• Identify tree species.
• Check soil moisture at depth in dead vs. green areas.
• Look for fungal fruiting bodies.
• Test soil pH and nutrients in affected zones.

Management

• Mulch under trees instead of fighting grass.
• If grass desired, use shade-tolerant species, irrigate separately, and avoid scalping.
• For allelopathic trees, consider raised beds with imported soil for planting beds.

The circular pattern strongly suggests mower damage or root competition. Adjust mowing and consider mulch rings.