Mold in mulch
What Is Mold in Mulch?
Wood chips and bark nuggets used as landscape mulch can develop a condition commonly known as "sour mulch" when wood byproducts are stockpiled before use.
This problem arises from storage conditions that affect the quality of the mulch, and while UMass Extension describes it as significant, it is also noted as rare.
How Sour Mulch Harms Plants
UMass Extension documents a case where mulch was spread around shrubs, perennials, and spring bulbs, and within half an hour many plants began to wilt.
By the following day, the foliage of tulips, Phlox, Astilbe, and violets had turned white or light tan, appearing as though bleach had been applied, and adjacent turf was also affected.
Where Mulch Comes From and Why Storage Matters
Many landscape contractors source wood mulch from sawmills and pulping operations, where wood byproducts are primarily intended for fuel but also used for landscaping.
According to UMass Extension, wood mulch storage techniques significantly affect product quality, making how and how long mulch is stockpiled a key factor in whether sour conditions develop.
The Benefits of Mulch and Using It Safely
Despite the risk of sour mulch, organic wood mulches offer well-documented landscape benefits including soil stabilization, moderated soil temperature, reduced weed development, decreased evaporation of soil moisture, and improved aesthetics.
Before applying mulch to your landscape, calculating the right amount can help avoid over-application issues — use the mulch calculator at / to estimate how much you need.
Key takeaways
- Sour mulch can develop when wood chips or bark nuggets are stockpiled under poor storage conditions, and spreading affected mulch can rapidly damage or kill nearby plants.
- Organic mulches provide genuine landscape benefits, but sourcing fresh, properly stored material and applying the correct volume are important steps for safe use.
Related tools
Estimate materials with our mulch calculator on MulchCalcs.