Iron Sulphate on Lawns and Moss: Is It really a Good Idea?

Moss is a challenge in lawns around the country. It can turn your beautiful green grass into a yellowing, spongy mess!

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Iron sulphate is often cited as a great way to control your moss problem. But is that really true? And if it is, how do you use it to get the best results?

We’re going to take a look at everything you need to know about iron sulphate on lawns and moss. And we’ll give you all the facts and tips to eradicate moss and achieve the lawn of your dreams.

Step this way to find out more …

What is iron sulphate?

What is iron sulphate
The Salutation Gardens

Iron sulphate is also known as ferrous sulphate or sulphate of iron. They’re all exactly the same thing.

In chemical terms, iron sulphate is a salt, and it has a wide range of uses. It’s used in diet supplements to treat iron deficiency in humans – but it also has a number of industrial applications. To gardeners, it will be familiar as a fertiliser and well-known moss killer.

Pure iron sulphate looks like a white powder, and you can buy it in large bags. It’s often far less expensive than branded lawn treatments.

Fertiliser, moss killer or both?

There are very different views on whether iron sulphate is suitable for use on lawns in its pure form. That’s partly because of its somewhat peculiar legal status.

Products need to be registered as pesticides before they can be used for that purpose in the UK. And if you’re buying something to kill moss, you’ll be using it as a pesticide. But iron sulphate isn’t registered as for that, so you can’t legally use it to remove moss.

Fertilisers, on the other hand, don’t need to be registered – and iron sulphate is also a fertiliser. That means you can buy it for the purpose of fertilising your lawn, but not as a moss killer.

The Royal Horticultural Society cautions against using iron sulphate in its pure form. It notes that it can blacken grass if you apply too much.

Most branded moss killers will have iron sulphate as their active ingredient, but it will be mixed with other substances. As long as you apply it in line with the manufacturer’s instructions, you’re unlikely to have that problem.

That all means that you can use iron sulphate to fertilise your lawn. And if you’ve got a moss problem, it will have the added bonus of getting rid of it.

You can save money if you buy it in its pure form. But you will need to prepare it for application yourself. And you’ll need to take care to apply it at the correct rate.

How do you use iron sulphate?

We’ve heard of some gardeners applying iron sulphate in its powder form. That means minimal preparation is required. The downside is that it’s very difficult to get even distribution. That increases the risk of ending up with blackened grass, moss that’s still alive and kicking – or both.

Mix it with water and it will be far easier to achieve a uniform application. How much iron sulphate and water you need to use depends on several factors. The purpose for which you’re using it, the size of your lawn, and the method of application will all affect that calculation.

Working out the area of your lawn

Working out the area of your lawn
The Salutation Gardens

Begin by calculating the surface area of your lawn. If your lawn is square or rectangular, that’s a simple case of multiplying the width in metres by the length in metres. The result will give you the surface area in square metres.

If your lawn is an irregular shape, measure the widest and narrowest, longest and shortest points. Then draw the shape of your lawn to scale on a grid or piece of graph paper. Make each square of the grid or graph paper equal to one square metre. Don’t worry – you don’t need to be completely precise!

Now count up all the squares that are completely filled by lawn. Add in squares that are more than half-filled by the shape. Ignore those that are less than half-filled. The total will give you a fair estimate of the whole surface area.

Now decide how you’re going to apply the mixture to your lawn. Your choices here are between a watering can and a sprayer attached to a rucksack.

Applying iron sulphate with a watering can

Applying iron sulphate with a watering can

A watering can is an inexpensive option, but it will take longer. Depending on the size of your lawn, you’ll probably need more than one can’s worth.

You’ll also need more water for the same amount of iron sulphate than with a rucksack sprayer. That’s because the holes in the rose will be larger than the fine mist from the sprayer nozzle.

It’s a good idea to start by mixing up the total amount of iron sulphate and water for your lawn. For larger lawns, a vessel like a clean dustbin works well. You can then fill up your watering can as many times as you need to.

You’ll need to use one litre or water per square metre of lawn. That’s the same amount, whether you want to fertilise the grass or kill moss.

If you want to use iron sulphate as a fertiliser, use 1 to 2 grams of powder per litre of water. If your lawn has an area of 12 square metres, that means you’ll need 12 litres of water. Measure that out, then mix in between 12 and 24 grams of iron sulphate. The mixture will green up your grass beautifully.

If your aim is to get rid of moss, the concentration needs to be higher. Use between 4 and 5 grams per litre. For that 12 square metre lawn, you’d still need 12 litres of water, but with between 48 and 60 grams of iron sulphate mixed in.

Make sure you know the capacity of your watering can. That will allow you to work out how much of your lawn you should cover with the contents of each can. Remember, it’s one litre of mixture for every square metre of lawn.

Using A Rucksack Sprayer

Using A Rucksack Sprayer

If you’ve got a rucksack sprayer, the smaller holes in the nozzle will mean less water is required. They do, though, mean that it’s quite easy for the nozzle to get clogged up.

For that reason, it’s a good idea to sieve the iron sulphate into the water to remove any clumps. Mixing in some warmer water will also help it to dissolve.

To work out how much iron sulphate you need, use the same guidelines as for watering with a can. That’s between 1 and 2 grams of powder per square metre of your lawn for fertilising. If you want to kill moss, you’ll need between 4 and 5 grams per square metre.

To determine how much water you need, attach your nozzle and spray your whole lawn with water only. Check how much water you’ve used, then multiply it by three. That’s because you’ll need to spray the whole area three times.

Depending on your nozzle, you’ll probably find you use between 8 and 14 litres per 100 square metres.

If you’ve got a large enough vessel, you can mix together the total amount of water together with all the iron sulphate. Bear in mind that this may be a lot more than you can fit into your rucksack in one go.

Alternatively, you can make up just enough for one rucksack at a time. But if you do that, remember to divide up the total iron sulphate by the number of times you’ll be filling the rucksack. That will require a bit more maths!

Stir up the mixture thoroughly and give it a minute to settle before transferring it to your rucksack. Leave the dregs at the bottom, as they may block the nozzle.  If you’re making up another batch, you can reuse these.

When to apply iron sulphate

For the best results, make sure you apply your iron sulphate mixture in the right conditions.

Avoid frosty days or freezing temperatures. And make sure the soil is moist before you apply the mixture.

If you’re using it at lower concentrations as a fertiliser, you can apply it any time through spring to autumn. Just avoid periods of very strong sunshine.

For higher concentrations, as when you want to kill moss, you’ll want wet, cool weather. Autumn through to spring is best, as long as temperatures aren’t freezing. A morning with a heavy dew is ideal. And if you’re worried there’s not enough moisture in the soil, give it spray with the hose first.

Wear rubber gloves as you work. If you get iron sulphate on your hands, it will stain them black.

Time for a green and moss-free lawn!

We hope you’ve enjoyed our guide to using iron sulphate on lawns and moss.

If you prefer to avoid the maths, buying a branded fertiliser or moss killer is a good option. That will make it easier to apply at the right rate.

But if you don’t mind a bit of extra work, iron sulphate mixed with water can give excellent results. And it’s much cheaper too. Just apply it at the right concentration, and remember to check there’s enough moisture in the soil before you start.

Good luck – and enjoy your healthier, moss-free lawn!

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