Beware Toxic Mulch: How to Spot and Avoid It

The Hidden Danger in Bulk Mulch

Mulch benefits plants and soil, but improperly handled bulk mulch—especially shredded hardwood bark—can become toxic. Sawmills pile bark high, compacting it with heavy loaders. Without oxygen, anaerobic fermentation generates extreme heat and noxious gases. The result: mulch that burns plants on contact.

Real-World Damage

Toxic mulch releases gas that browns foliage within minutes. It can kill shrubs, trees, and even grass adjacent to the bed. One truckload ruined multiple landscapes in a single day. The gas—not the mulch itself—causes the damage.

Identifying Toxic Mulch

• Overpowering, acrid odor (distinct from normal mulch smell).
• May appear slightly darker than usual.
• Test: place a few shovelfuls from the center of the pile around inexpensive plants. If they’re unharmed after 24 hours, the mulch is safe.

Prevention

• Buy from suppliers who manage piles properly (lower, aerated).
• Avoid mulch that’s steaming or smells sharply of ammonia/sulfur.
• Spread mulch promptly; don’t let it sit in piles on your property.

Toxic mulch is rare but devastating. A quick test protects your investment and your plants.