Does Iron Sulphate Kill Moss?

Iron sulphate, also called ferrous sulphate, is one of the most popular DIY moss treatments for lawns. It’s cheap, readily available, and genuinely effective at killing moss.

But it’s not without drawbacks. And if you’re thinking of using it on your patio or paths, you might want to reconsider.

Here’s everything you need to know about using iron sulphate for moss control.

Does It Actually Work?

Yes. Iron sulphate kills moss effectively, particularly on lawns. It’s been used by gardeners and groundskeepers for decades.

The iron in ferrous sulphate is toxic to moss. When applied, it causes the moss to turn black and die within days. The effect is dramatic and unmistakable.

As a bonus, iron sulphate also provides iron to your lawn grass, which can improve colour and health. This is why many lawn feed products contain iron sulphate as an ingredient. You’re killing moss and feeding your grass at the same time.

So far, so good. But there are some important limitations.

The Staining Problem

Here’s the big issue with iron sulphate: it stains.

Iron oxidises (rusts) when exposed to air and moisture. If iron sulphate gets onto hard surfaces like concrete, patio slabs, or block paving, it leaves orange rust stains that are extremely difficult to remove.

These stains can appear from direct spillage, from granules bouncing off the lawn, or from runoff after rain. Even careful application often results in some staining around lawn edges.

The stains don’t fade quickly. They can persist for months or even years. Some homeowners have had to replace paving because they couldn’t remove iron sulphate staining.

This is why iron sulphate is really only suitable for lawns and grassed areas. For patios, paths, walls, and other hard surfaces, you need a different approach.

How to Use Iron Sulphate on Lawns

If you’re treating lawn moss, iron sulphate can work well when used correctly.

You can apply it as a liquid (dissolved in water) or as granules. Liquid application gives more even coverage but requires a sprayer or watering can. Granules are easier to spread but can bounce onto paths and patios.

The typical dilution rate for liquid application is around 20-40g of iron sulphate per litre of water, applied at roughly 5 litres per square metre. Check the product instructions as concentrations vary.

Apply on a dry day when rain isn’t forecast for at least 24 hours. The product needs time to work before being washed off.

Within two to three days, treated moss will turn black. This is normal and means the treatment is working.

After a week or two, rake out the dead moss. This is essential. If you leave blackened moss in place, it forms a mat that prevents grass from growing and creates conditions for new moss to establish.

The best time to treat is in spring or autumn when moss is actively growing but before it produces spores. Avoid treating in very hot, dry weather or when frost is expected.

Important Precautions

Iron sulphate requires careful handling. Here’s what to watch out for:

Protect hard surfaces. Cover or wet down any paving, decking, or paths near the treatment area. Even better, create a buffer zone where you don’t apply treatment close to edges.

Wear old clothes and gloves. Iron sulphate stains fabric permanently. The orange marks won’t wash out.

Keep pets off the lawn until the product has been watered in or rained on. Iron sulphate can irritate paws and isn’t good for animals to ingest.

Don’t overdo it. More is not better. Excessive iron sulphate can damage grass, particularly if applied in hot weather or on stressed lawns.

Store carefully. Iron sulphate absorbs moisture from the air and clumps together. Keep it in a sealed container in a dry place.

Why Moss Comes Back After Treatment

Iron sulphate kills moss, but it doesn’t address why the moss is growing in the first place.

Moss in lawns is usually a symptom of underlying conditions: shade, poor drainage, compacted soil, low soil fertility, or mowing too short. Kill the moss without fixing these issues and it will return.

After treating with iron sulphate, consider:

Aerating compacted soil to improve drainage and root growth. Improving light levels by trimming overhanging trees or shrubs. Feeding your lawn to help grass outcompete moss. Raising your mowing height to give grass more leaf area. Overseeding bare patches before moss can recolonise.

Iron sulphate is a treatment, not a cure. Long-term moss control requires addressing the conditions that favour moss over grass.

Iron Sulphate vs Other Moss Killers

How does iron sulphate compare to other options?

Against vinegar, iron sulphate is more effective and longer-lasting on lawns. Vinegar can damage grass, while iron sulphate (used correctly) actually benefits it.

Against bleach, iron sulphate is safer for grass and better for the environment. Bleach kills everything, including the lawn you’re trying to save.

Against dedicated moss killer products, iron sulphate is cheaper but messier and more limited in application. Purpose-made treatments can be used on hard surfaces without staining.

For lawns specifically, iron sulphate remains a solid choice if you’re willing to deal with the precautions. For anything other than lawns, there are better options.

When Not to Use Iron Sulphate

Avoid iron sulphate in these situations:

On or near patios, paths, driveways, or any hard surfaces you don’t want stained. Near light-coloured walls, render, or fencing that could be splashed. On decking, which can be permanently marked. Around water features or ponds, as iron can be harmful to aquatic life. On lawns that are already stressed from drought, disease, or recent treatment.

For these situations, choose a moss treatment that won’t leave rust stains. Our moss treatment options guide covers alternatives that work on hard surfaces.

The Verdict

Iron sulphate is an effective, affordable moss killer for lawns. It works, it’s been proven over decades, and it provides bonus iron for your grass.

But it’s fiddly to use, stains hard surfaces permanently, and requires careful application. It’s not a convenient spray-and-forget solution.

For lawns, it’s worth considering if you don’t mind the extra effort and precautions. For everything else, a dedicated moss killer that won’t stain is the smarter choice.

Want moss control without the staining risk? Our treatment works on lawns, patios, walls and more with no rust marks to worry about. Our Moss, Mould & Algae Killer is the easy alternative to iron sulphate.