Mulch Problems: Fungus, Smell & How to Fix Them

Slime Mould on Mulch

If you’ve ever found bright yellow or orange blobs on your mulch, you’ve met slime mould — most commonly Fuligo septica, cheerfully nicknamed “dog vomit fungus.” It looks alarming but is completely harmless to plants, pets and people.

Slime mould appears when conditions are warm and damp, typically in late spring through autumn. It feeds on bacteria in decomposing organic matter — exactly what good mulch provides. Within a few days it dries out, turns grey or brown, and crumbles away on its own.

What to do: Nothing, ideally. If the appearance bothers you, scoop it off with a spade or break it up with a rake. It won’t spread to plants and doesn’t indicate any problem with your soil or mulch quality.

Sour Mulch: The Vinegar Smell Problem

Sour mulch is the one problem that genuinely can harm your garden. It happens when mulch decomposes without oxygen — typically in tightly packed bags or poorly stored piles. The anaerobic breakdown produces acetic acid (vinegar), methanol and hydrogen sulphide, giving off a sharp, unpleasant smell.

Applying sour mulch directly around plants can scorch foliage and damage roots. The acids lower soil pH dramatically in the immediate area, and the toxic byproducts can kill young plants outright.

How to spot it: Always smell your mulch before spreading. Fresh bark mulch should smell earthy and woody. If it smells sour, sharp, vinegary or like alcohol, it’s gone anaerobic.

How to fix it:

1. Spread the mulch out in a thin layer (no more than 5 cm) on a hard surface or spare patch of ground.
2. Turn it with a rake every few hours to expose it to air.
3. Within 24–48 hours, the sour smell should dissipate as the acids evaporate and aerobic decomposition resumes.
4. Once it smells earthy again, it’s safe to use around plants.

This is particularly common with bulk mulch deliveries that have been sitting in compressed piles. Always open bags on arrival and let them breathe before applying.

Mushrooms and Fungi in Mulch

Mushrooms popping up through mulch are one of the most common concerns gardeners have, and one of the least worrying. Fungi are essential decomposers — they’re what breaks wood chips and bark into nutrients your plants can use.

Common types you’ll see include ink caps, small brown toadstools, and the occasional puffball. The white thread-like network you might spot beneath the mulch surface is mycelium — the main body of the fungus — and it’s entirely beneficial.

When to act: If you have young children or pets who might eat mushrooms, simply knock them down with a rake as they appear. Removing them doesn’t harm the beneficial fungal network below. If mushrooms are appearing in large numbers, it often means the mulch is staying too wet — improve drainage or let the surface dry between waterings.

One exception worth knowing about is artillery fungus (Sphaerobolus stellatus). This tiny fungus shoots sticky black spores up to six metres, leaving nearly impossible-to-remove spots on light-coloured surfaces like fences, cars and house siding. If you notice small black dots appearing on nearby surfaces, switch from wood chip mulch to bark mulch or consider gravel in that area.

Water Repellent Mulch

Over time, especially during dry spells, mulch can develop a hydrophobic crust. Water sits on top and runs off rather than soaking through to the soil — exactly the opposite of what you want.

This happens because fungal hyphae bind the top layer of mulch particles together, creating a water-resistant mat. It’s more common with fine-textured mulches and in areas that don’t get regular rainfall.

How to fix it:

• Rake the surface to break up the crust every few weeks during dry periods.
• Water slowly — a gentle trickle from a soaker hose penetrates better than a blast from a sprinkler.
• Top up annually — fresh mulch on top prevents crusting. See our guide on how often to apply mulch for the right schedule.

Mulch Washing Away

Lightweight mulches like straw and fine bark can migrate in heavy rain, especially on slopes. This leaves bare patches where weeds quickly establish — and deposits mulch where you don’t want it.

Solutions:

• Use heavier materials on slopes. Bark nuggets or gravel stay put better than shredded bark or leaf mould.
• Install edging. A simple border of timber, stone or metal landscape edging contains mulch within beds.
• Apply in layers. Cardboard under mulch slows water flow at ground level and gives mulch something to grip against.
• Avoid over-mulching. On slopes, keep mulch to 5–8 cm rather than piling it thick. Use a mulch calculator to avoid buying too much.

Other Common Mulch Issues

Nitrogen robbery: Fresh wood chips and bark can temporarily tie up nitrogen in the soil as they decompose. This mostly affects the top few centimetres — deep plant roots are rarely impacted. If you notice yellowing in annual plants or vegetables, scatter a handful of blood, fish and bone meal over the mulch to compensate. This is less of an issue with well-rotted leaf mould or compost-based mulches.

Slugs and snails: Damp mulch can harbour slugs, particularly thick straw or leaf mulch in shaded areas. Keep mulch pulled back from vulnerable young plants and avoid mulching vegetable beds right up to hostas and lettuce unless you’re also using slug control.

Weed growth through mulch: If weeds are pushing through, the mulch layer is probably too thin. Most weeds need a minimum of 8–10 cm of mulch to be suppressed. For stubborn perennial weeds like bindweed, Japanese knotweed or ground elder, mulch alone won’t be enough — you’ll need to combine it with a strong weed killer or choose the best weed-suppressing mulch for your situation.

Termites and ants: Less of a concern in the UK than warmer climates, but ants can colonise dry mulch. If nests become a nuisance, rake the mulch to disturb them — they’ll usually relocate. Keeping mulch away from house foundations reduces the chance of ants finding their way indoors.

Frequently Asked Questions