What Is That Black Soot on My Plants? (Sooty Mould Explained)
- Tammy Johnson
- 9 hours ago
- 4 min read
Written and edited by Tammy 1st February 2026
One of the very first plant issues I learnt to identify and help customers with, back in my early days, was sooty mould. I remember feeling genuinely excited (and pretty proud of myself) when an unlucky customer would come in holding a freezer bag sample and ask, “What is that black soot on my plants?”
I used to pounce on the opportunity to practise my newly acquired skills. “Sure, I can help you with that…” and away I’d go.
Fast forward to now, and humbly, my repertoire has grown.
Sooty mould can look a little alarming, especially when a plant resembles the sole survivor of Ash Wednesday, but once you understand that it’s the consequence, not the cause, you’ll start to see it differently. The good news is, it’s very fixable once you understand what’s really going on.
Let’s walk through exactly what sooty mould is, why it appears, and how to clean it up properly.
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What Is That Black Soot on My Plants?
Sooty mould is a black fungal growth that we're familiar with seeing on the surface of leaves and stems; but here's the thing....it doesn't actually infect the plant tissue itself.
It’s there as a consequence of something else, a sugary, sticky substance called honeydew.
Think of it like this: If you spill something sugary on your kitchen bench and forget to wipe it up, it eventually goes mouldy. The mould isn't there because of the kitchen bench, it's there because of the sticky mess that was left there on it.
The mould is simply taking advantage of a free banquet.
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So, Who's the Culprit?
Sooty mould appears when honeydew-producing insects are feeding on your plant.
The common culprits are:
Aphids
Scale
Mealybugs
Other sap-sucking insects
Even small insect populations can produce enough sugary residue for sooty mould to develop.
You'll likely notice:
Sticky leaves
Ant activity (ants LOVE sugar and often “farm” these insects)
Black residue on leaves and stems.
As bad as it looks, the mould isn’t really the problem; it’s a symptom of a bigger problem-the insects.
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Is Sooty Mould Harmful?
Most of the time, sooty mould is cosmetic, but that doesn’t mean it should be ignored.
Heavy build-up can:
Block sunlight from reaching the leaf
Reduce photosynthesis
Slow plant growth
Add stress over time
Left unmanaged, the extra stress can weaken a plant over time, especially if the insect activity continues.
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How to Clean Up Sooty Mould
The clean-up approach really depends on how heavy the build-up is and the size of the plant too. There’s a big difference between wiping down a pot plant and dealing with a six-foot shrub or lemon tree. (Let’s be realistic… no one’s wiping every leaf on that!)
Light to Moderate Coating
Hose leaves with plain water
Gently wipe with a soft cloth or sponge
Use mild soapy water if needed
Often, this is enough to improve airflow and light absorption and help the plant bounce back.
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Heavy or Long-Term Build-Up
Wipe leaves gently (avoid scrubbing)
Prune badly affected leaves or branches
Accept that some leaves just won't return to their pristine condition
Cleaning the mould off helps the plant breathe and photosynthesise again, but unless the insects are dealt with, the mould will return.
For larger shrubs or trees, a prune can help in more ways than one. Removing the worst-affected growth improves airflow and light and encourages a fresh flush of healthy new growth, allowing the messiest parts to drop off naturally.
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The Long-Term Fix: Treat the Cause, Not the Mould
This is where calm, sensible Integrated Pest Management (IPM) comes in.
Instead of reaching straight for harsh sprays:
Monitor plants regularly
Manage ants (they protect honeydew-producing insects)
Encourage beneficial insects
Use targeted natural treatments where necessary to treat the guilty culprits!
Sooty mould is a classic example of how our gardens often shows us the symptom first, not the cause. Once you know how to read those clues, everything gets a lot less stressful.
The Takeaway
Sooty mould looks dramatic, but it’s really just our plant or garden trying to show us something else is going on.
Once we understand that, the solution becomes much clearer,
clean up what we can, support the plant, and manage the insects that started the mess in the first place.
Happy Gardening 😘🪴
If you’re curious to learn more, I'd love to have you join me in The Backyard. Members have access to my 26 page, in-depth guide, 'The Gardener’s Handy ‘Doctor’s Bag' full of practical, step-by-step ways to diagnose what’s wrong with your plants, and identify some of the culprits along the way!
A peek at what you’ll find inside....
My 26 page guide to becoming a 'Plant Doctor'. Learn to identify and deal with all the common pests, diseases and disorders.
A 20 page detailed look at what hydrophobic soil is, how to fix it and prevent it ever happening again.
A lovely little 16 page guide to water saving practices and plant recommendations for a drying climate.
