Sour Mulch: How to Avoid It, or Fix the Problem

large pile of mulch — *Healthy mulch should have an earthy, pleasant aroma. If it smells sour, vinegary, or like alcohol, it’s a sign that the mulch has turned sour.*

by James Aldwin

Homeowners, beware. There’s an invisible threat that can undermine your hard work and leave your plants gasping for air – sour mulch. Though it may sound like a harmless term, sour mulch can cause serious damage to your plants and soil.

What is Sour Mulch?

Sour mulch is a result of improper decomposition of organic materials in mulch piles. It’s sometimes also called wood alcohol syndrome. When mulch is stored in large, tightly packed piles with minimal oxygen, anaerobic bacteria can take over, producing toxic byproducts such as alcohol, acetic acid, and other volatile organic compounds. These toxic substances can harm or kill your plants when the mulch is spread in your garden.

Symptoms of Sour Mulch, and the Plants Most Susceptible

Sour mulch tends to have the most devastating impact on young, herbaceous plants. These fragile, tender plants can show signs of damage shortly after being exposed to a hardwood bark mulch affected by souring. The symptoms may resemble those caused by excessive fertilizer, pesticide burns, or severe water stress. Often, the injury to plants manifests within hours or a day of applying the sour mulch, making it crucial for gardeners to act swiftly to minimize damage.

The most common symptoms include yellowing or blackening of foliage, leaf drop, and an overall wilted appearance. Fortunately, many plants have the resilience to recover from sour mulch exposure, particularly if gardeners take prompt action. It’s essential to provide thorough watering, particularly during hot and dry weather, to alleviate stress on the affected plants. Avoid applying fertilizer for several weeks to plants injured by sour mulch, as it could exacerbate the damage.

How to Avoid Sour Mulch

Buy from reputable suppliers: Ensure that you purchase mulch from a reliable source that properly composts and aerates their mulch to avoid anaerobic decomposition.
Inspect before you buy: If possible, examine and smell the mulch before purchasing. Healthy mulch should have an earthy, pleasant aroma. If it smells sour, vinegary, or like alcohol, it’s a sign that the mulch has turned sour.
Proper storage: Store mulch in a well-ventilated area, and avoid stacking it too high. Allow air to circulate to promote proper decomposition.

What to Do if You’ve Already Spread Sour Mulch

Remove the mulch if you can: If you suspect that you’ve spread sour mulch, remove as much of it as possible from your garden. Be thorough, as the toxic substances can linger and continue to cause harm.
Water it in: Thoroughly watering the affected area can help dilute and flush out the toxic substances produced by sour mulch. If you can’t take out the mulch, generously water it to help disperse any remaining harmful compounds and dilute them.
Apply compost: Spread a layer of well-decomposed compost over the affected area to reintroduce beneficial microorganisms and nutrients to the soil. Compost is well known as an effective method of soil bioremediation.

Sour mulch can wreak havoc on your plants and soil if you spread it in your yard. By understanding its causes, learning how to avoid it, and knowing how to remedy it when it strikes, you’ll be well-equipped to maintain a healthy, thriving garden. Keep these tips in mind, and you’ll be one step ahead of sour mulch and the yard damage it can cause.