Everything You Need to Know About Iron Sulphate For Lawns & Gardens
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What is Iron Sulphate?
Iron sulphate, also known as ferrous sulphate, is a chemical compound that has many uses, including as a nutritional supplement for iron-deficient soils and plants, as a feed additive for livestock, and as a coagulant in wastewater treatment plants. It is also commonly used in gardening and agriculture as a moss killer, lawn greener, and soil pH adjuster.
In addition to its many uses, iron sulfate is also used in the production of other chemicals, such as iron oxide pigments, ferric sulphate, and iron chelates. It is an important industrial chemical and is produced in large quantities around the world.
As a Moss Killer
In the UK, iron sulphate is commonly used as a lawn moss killer and is available for purchase at garden centres and online retailers. However, when it’s marketed for lawn moss control, the product falls under the jurisdiction of the UK’s Health and Safety Executive and the product must undergo rigorous testing and must come with a safety sheet and instructions for use.
When iron sulphate isn’t marketed as a moss killer, but as a lawn tonic, hardener or greener, it doesn’t fall under the jurisdiction of the UK’s Health and Safety Executive and these products are usually much cheaper but as they aren’t marketed for moss control, they won’t come with instructions or a safety sheet.
Now every professional gardener I know buys the cheaper iron sulphate but that’s because they know how to use it safely as they have years of experience.
If you want to use iron sulphate in the UK and you want a product that’s been through the rigorous evaluation process, is licensed for moss control and comes with safety advice and instructions, I recommend Total Lawn Moss Killer:
My Experience With Iron Sulphate
Total Lawn Moss Killer contains 12% iron sulphate, and this is a standard concentration for killing moss in lawns, although I have used 20% concentrations before.
I’ve used 12% iron sulphate dozens of times to clear moss from lawns; here’s how I did it and the results I achieved:
1) I’ve found that the best time to apply iron sulphate is in the spring or autumn when rain is forecast; this dilutes the chemical into the soil, where it treats the moss.
2) The iron sulphate comes in fine granular form, and I’ve always applied it via a spreader for accuracy, but I’ve seen others do so with a gloved hand.
3) I’ve always applied 12% iron sulphate at a rate of 4-5g per square metre for moss control (diluted with water).
4) I watered it in if rain wasn’t forecast.
5) I’ve seen iron sulphate stain cloths, carpets and patios before, so I always keep off the lawn for a week after application.
Here’s what happened the last time I used 12% iron sulphate to treat lawn moss, the results were fairly typical:
After applying iron sulphate, I left it for a week, and after 4-6 days I could see the moss going brown.
After 10-14 days the moss went black, and I then raked it out with a wire rake.
Once the moss was removed, the lawn looked bare and sparse, so I scratched the lawn with a wire rake and then sprinkled grass seed in.
I used to work for a landscaping company years ago, and they always used iron sulphate to kill lawn moss, and it was always successful, I’ve never seen it fail to work if it was applied at the correct dose.
As a Lawn Greener, Tonic or Hardener
Iron sulphate is also widely sold as a lawn tonic, greener, hardener or conditioner. You’ll find these products online and in garden centres and they aren’t usually marketed for moss control and are fairly cheap.
Iron sulphate is a nutrient which can improve the health of the lawn if there are deficiencies in the soil.
I’ve used iron sulphate on lawns for this purpose and saw these results:
- The blades of grass became noticeably greener, often within a week.
- I’ve used it on yellowing grass and after a couple of weeks, the blades were a deep, lush green.
- It’s helped to improve the lawn’s ability to resist disease – but I only saw this when I used it on a lawn that was very poorly and had yellow grass and obvious nutrient deficiency.
Here are the doses I’ve used for greening up and lawn gardening; they are both less than what I use for moss control:
100 grams per 100 square metres (1g per sq mtr) – Perfect for greening up the lawn.
150-200 grams per 100 square metres (1.5g – 2g per sq mtr) – Ideal for lawn hardening and will also green up the lawn.
400-500 grams per 100 square metres (4g – 5g per sq metre) – This is the dose I use for moss control.
Above 500 grams per 100 square metres (over 5g per sq metre) – Repeat applications are likely to over-acidify the soil and may turn the grass yellow or brown.
- Dilute it into a watering can or sprinkle via a spreader and water it in well.
- Don’t use iron sulphate in the middle of summer or when it’s really hot.
- Be careful not to spill it as it will likely leave stains – I’ve seen it mark patios, even at lower doses.
This is the iron sulphate I use (sold as a lawn tonic)
As a Soil Adjuster
I’ve used iron sulphate as a soil adjuster on several occasions (using the cheaper product of course) for my acid-loving plants.
Iron sulphate has the potential to over-acidify the soil, so I’m always careful to apply it gradually rather than excessively. I’ve also used sulphur as an alternative as it’s cheaper, but I’ve found it takes longer to dissolve into the soil.
First I would check the pH level of the soil using a test strip such as this and then I would dig in the appropriate amount of iron sulphate to acidify the soil.
The test strips I use are cheap, and one pack comes with 100 strips.
Problems I’ve Encountered With Iron Sulphate
Years ago, I worked for a customer who spilt iron sulphate onto his patio when the ground was wet, and despite rinsing it off, it left a rustic stain on the surface that he couldn’t remove.
I’ve also seen a carpet damaged by it when a cat walked on the lawn and then came into the house.
Iron sulphate, even at low doses, has the potential to over-acidify the soil, and I saw one case where a gardener had miscalculated the dose and turned his grass yellow – I confirmed this by conducting a pH and nutrient test for him. While it was too late for him to do anything, the best approach would be to water the lawn heavily for a few hours to flush out the chemical.
Also, iron sulphate sold in the UK as a tonic, hardener, conditioner or greener won’t come with instructions on how to use it for moss control as the manufacturer isn’t licensed to sell it for this purpose. I think this could lead to confusion for some gardeners and probably explains why many gardeners are going to web forums to get advice.
Alternatives I’ve Used
There are several alternatives to iron sulphate, and I’ve used all of them in the past:
For moss control – I rarely use iron sulphate now as I prefer “no rake” products that contain bacteria that eat away at the moss. I’ve been using MO Bacter by Viano for several years and while I found it slow to act, I’ve never had to rake out the moss, it doesn’t turn the moss black, doesn’t stain patios and is safe for humans and pets, and more crucially, it doesn’t alter the pH of the soil as iron sulphate does. The bacteria only affects the moss and nothing else, it’s also endorsed by the Royal horticultural society.
To acidify soil – At my previous property, I had azaleas and a few other acid-loving plants in my pots and borders, and I had to occasionally test the pH soil to check the acidity level. When it was too alkaline, I would mulch with ericaceous compost and add either iron sulphate or sulphur.
I found that the sulphur worked best but was slow to act, while iron sulphate was more likely to poison the soil if too much was applied. The Royal Horticultural Society has an entire page dedicated to acidifying soil, but in summary, sulphur can be used as an alternative to iron sulphate.
To green up the grass – While I’ve found iron sulphate is the quickest and most effective way to make the grass turn a deep green colour, too much can damage the soil and cause problems. As an alternative, I suggest you apply a good quality organic seaweed fertiliser every three weeks during the growing season.
I used SeaFeed Xtra by Envii exclusively on my lawn in 2020, and I was delighted with the results. As a bonus, their product is fortified with added iron and amino acids, and the entire contents are certified organic and vegan-friendly. Here are a few photos I took:
- Apply a lawn weed killer in the spring just as the grass starts growing.
- Also consider a moss treatment either in the spring or autumn, if necessary.
- Scarify and sprinkle grass seed over the lawn in the spring.
- Feed and water regularly during the growing season.
- Apply an autumn lawn feed later in the year as part of autumn lawn care.
- Aerate the lawn once a year in the autumn with a hollow tine aerator and overseed if it’s looking patchy.
I hope you found this guide to iron sulphate insightful, and I’ll just leave you with a little tip; don’t overdo it, there’s no need to apply iron sulphate regularly as it will probably do more harm than good but as an occasional lawn treatment, it does work very well.
Our Experience: Backed by a Qualified Horticulturist
At DIY Gardening, we always try to create content that our readers will find helpful and where possible, we use photos to prove we’ve used the products or techniques we recommend.
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Our 5-step process is:
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2) The author creates the content based on their knowledge and experience of the subject. In this case, Daniel Woodley came up with this guide as he has previous experience with iron sulphate and alternative products.
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This guide was reviewed and fact-checked by qualified horticulturist Elizabeth Smith.
Explore: Elizabeth’s profile and qualifications.
Meet The Author: Daniel Woodley
Daniel has over 18 years of experience in the construction, home improvement, and landscape garden industries.
He previously worked as a project manager and has experience in managing teams of tradespeople and landscape gardeners on both small and medium sized projects.
Daniel is also a keen gardener and enjoys growing unusual plants and tending to his lawn.
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