How to Kill Moss on Your Lawn
Moss in a lawn competes with grass, looks unsightly and can cause heart ache for your lawn mower as it struggles to cut through the thick growth at the start of the season. Some people like daisies, moss and other wild flowers spread throughout their lawn. However if your are puritanical like me about your lawn and believe only green grass should grow in it, you will likely want to eradicate moss. Fortunately it is easy to treat and measures can be taken to prevent it growing in the first place.
What Causes Moss in Lawns?
Moss thrives in damp and wet, cool conditions.
- Bad drainage. If the underlying soil is compacted and clayey, and doesn't drain well, it can remain excessively wet after rainfall
- Topology of the Lawn. If your lawn isn't flat and at the bottom of a slope, or has slightly hollow shaped sections like a saucer (the whole lawn could be like this), water will tend to collect. This in addition to bad drainage can make the lawn excessively wet
- Lack of Nourishment. If you don't feed the lawn, moss can out compete grass
- Shade. Sun tends to dry out the lawn and reduce the effects of water logging. However in the shade of buildings or under trees, moss can thrive in low light conditions while grass struggles. You may have noticed this in a forest where conditions are damp and moss grows on trees but very little grass is to be seen
- Climate. There's not much you can do about this. If you live in a country which has a mild climate and lots of rain, moss can be a serious problem although growth will stall somewhat during long dry spells in the summer
How Do I Prevent Moss?
- Feed the lawn using a composite lawn feed which contains nitrogen, phosphorus and potassium (NPK). Alternatively if you want to go organic, you can apply a handful of bone meal per square yard in the fall. This is a source of phosphorus. In the spring, apply well rotted, crumbly leaf compost or leave the clippings on the lawn every second time you cut the grass. Either of these methods return nitrogen to the soil.
- Scarify the lawn using a rake or electric scarifier - This pulls out all the dead vegetation and helps to remove moss and thatch (dead grass and weeds)
- Improve Drainage and Aeration - Use a hand fork and drive it down into the ground at 1 foot intervals. Alternatively you can use a hollow tine aerator. This tool has a series of hollow tubes rather than prongs. When you walk on it with your foot and drive it down into the ground, it removes cores of soil when pulled back up. A gas powered version of this is also available. Forking the lawn also improves aeration, as it opens up the ground, allowing more air in around the roots of grass. This helps to promote stronger, more vigorous growth of roots
How to Treat Moss in a Lawn?
You can treat moss using a chemical called iron sulphate (also known as sulphate of iron. In US English, sulphate is spelled sulfate). It can be spread as a dry powder, however personally I find it's much more convenient to spray it or use a watering can, and this gives better coverage than spreading dry powder by hand which can produce patchy results. When you spray iron sulphate, moss turns black within about 20 minutes, so it's easy to see whether you missed any spots.
Iron sulphate will stain clothes, hands and everything else brown, and the dry dusty powder can irritate eyes. When sprayed, It will stain concrete and tarmac (asphalt or blacktop) to some extent ( sort of a beige color) so you might consider alternative chemicals as a treatment on these materials. Vinegar, bleach or salty water will burn moss and can produce good results. Boiling water also scalds moss and weeds. If you spray iron sulphate solution, the resultant stains are not as bad as those which result from scattering the dry powder.
Once you treat your lawn with iron sulphate, the moss will turn black and also the grass to some extent. This is why it's best to spray in the spring. Firstly you will have dealt with all the moss growth which accumulated over the winter, and secondly any staining of the grass will be eradicated once you start mowing your lawn again.
When Do I Treat Moss in a Lawn?
Ideally in late autumn, winter or early spring so that grass doesn't have any competition when it starts to grow again in spring.
What Equipment Do I Need?
All you need is some string, pegs and a sprayer. You can either use a small sprayer with a capacity of about 1 gallon or 5 litres, or alternatively use a knapsack sprayer which you carry on your back. These hold about 16 to 22 litres of mix and although need refilling less frequently, are more cumbersome to use and weighty for those of you of slight build, and not used to carrying heavy loads on your back. Remember 20 litres of water weighs 20 kilos or 44 pounds. You can of course leave a knapsack sprayer on the ground during use and partially fill it, but it will have to be constantly moved around.
How to Get Rid of Moss in Lawns
- Mark out the lawn in strips
- Mix up the iron sulphate solution. Use 3 heaped teaspoons per litre
- Spray along the strips, while walking, moving the sprayhead backwards and forwards like a windscreen wiper
Step 1 - Mark Your Lawn
The idea is to spray the lawn in strips so that you know where you have covered. I use 2 pieces of string wrapped onto scrap metal rods or rebar. You can also use canes or pieces of tree branch. Start off by marking out a strip about a yard wide. When you complete spraying the first strip, you simply move the strings.
Mixing Iron Sulphate and Water - What Concentration Do I Use?
I did some surfing online, but there didn't seem to be any any consistency as regards mixing concentration. So I decided to do some some experiments.
This is the concentration which seems to provide good results:
3 heaped teaspoons per litre
I worked out the weight of a heaped teaspoon and it's about 7 grams
So make up your iron sulphate solution to the following strength
- 21 grams (3 heaped teaspoons) per litre
- 2.8 ounces (11 heaped teaspoons) per US gallon
- 3.4 ounces (14 heaped teaspoons) per UK gallon.
You can double up on the concentration to ensure more thorough results, but I have found this to be unnecessary. It also tends to blacken the grass as well as the moss (not so much an issue when grass is growing vigorously as this discoloration will disappear after a few cuts). It also tends to stain concrete and gravel driveways.
Instead of using teaspoons which can be tedious, you can use an old weighing scales for weighing out the iron sulphate. Alternatively use a disposable cup or other container and add the required number of spoons into this for the capacity of your sprayer, e.g 5 litres. Mark the inside or outside of the container. Then in future you can just measure out by volume rather than by weight or counting spoons.
So to summarize:
Approximate Equivalent Mixing Concentrations
21 grams per litre (3 heaped teaspoons)
Imperial (Sometimes still used in UK)
3.4 ounces per Imperial Gallon (14 heaped teaspoons)
US Customary Units (USA)
2.8 ounces per US Gallon (11 heaped teaspoons)
1 ounce = 28.35 grams
1 US gallon = 3.79 litres
1 UK imperial gallon = 4.54 litres
Step 2 - Mix The Solution
To prevent lumpiness, add the iron sulphate to the sprayer first. Next add the equivalent of a few cups of warm water and swirl the mixture until it dissolves. Finally top up with warm but not boiling water (which could potentially damage the sprayer). Tighten the lid on the sprayer and turn it upside down a few times and give it a shake to ensure thorough mixing. If you are using a knapsack sprayer, add water about a gallon at a time, and swirl the mixture around each time you add water.
Do I need to filter the solution?
I've never had problems with previous brands, but the iron sulphate I'm using at the moment seems to keep clogging up my sprayer. This could be due to clumps of crystals or insoluble impurities. You can filter the mix before you use it by passing it through a thick sock or decant it after mixing it in a bucket, leaving the insoluble matter remaining as suggested in this video.
Step 3 - Pump and Spray
- Its probably a good idea to wear goggles in case the spray gets in your eyes. Try to spray on a windless day
- If you are using a small 5 litre or gallon sprayer, pump it the recommended number of times or until the safety valve operates. For a knapsack sprayer, you just need to operate the handle a few times, and re-pump as spray pressure starts to reduce
- Spray within the strip you marked out, moving the spray head left to right like a windscreen wiper, and walking either backwards or forwards along the strip
- When you reach the end of the strip, move one of the lines to create a new strip, adjacent to the one you just sprayed. Continue this "leap frogging" process, moving each line in turn until you have sprayed the entire lawn surface
- Within about 20 minutes, moss turns black. Spray any spots you missed
- After about a month and when the moss is completely dead, pick a dry day and rake it out.
Killing Moss on Driveway
You can use iron sulphate on a driveway, but it does have a tendency to stain, and this is more noticeable on gravel or concrete. You could spray a patch in a area that isn't prominent first, and see how it turns out. An alternative is to use a solution of bleach and water. Always use eye protection when spraying bleach solution.
Will Moss Grow Back?
It usually will, however treatment with iron sulphate in winter or spring suppresses it for the summer, until the weather gets colder and wetter in the autumn and it begins to grow again. Grass becomes dormant during winter and without this competition, moss will thrive unless treated.
Where Can I buy Iron Sulfate?
It's widely available from home stores, gardening shops and the gardening section in supermarkets. You can also buy it online from Amazon. It's available in various sized bags from 2 to 50 pounds. An alternative is a combined iron sulfate/lawn fertilizer mix such as Scotts MossEx. The pack contains 17% iron sulfate and other nutrients to develop green, healthy grass. This product is sufficient to treat 5000 square feet (555 square yards).
Questions & Answers
Question: Where can I buy iron sulfate?
Answer: It's widely available from home stores, gardening shops and the gardening section in supermarkets. It's available in various sized bags from 2 to 50 pounds.
Question: Does moss ever die in dry weather, or can it stay dormant when dried out for months?
Answer: Some moss species may die or produce spores so that new plants will develop once rain arrives. Generally, though, moss can become dormant and will re-sprout once weather becomes wetter.
Question: We are planning to remove some point trees in our side yard which is full of moss. Will opening up the space to sunshine prevent the growth of moss?
Answer: It may help, because lack of sunshine can reduce the rate at which ground dries out and also moss seems to need less sunshine than grass to thrive. However, it's not guaranteed that you'll have less moss. My lawn is full of moss after a long, cold wet winter, even though it's in full sun. Feeding a lawn so that grass grows strong and competes with the moss will help. Also forking the ground by hand or with a special machine will improve drainage so that the ground isn't so sodden all the time. The application of iron sulfate does help, and although it doesn't kill moss completely, it suppresses it for the summer.
Question: I bought fine powder ferrous sulfate. I assume that it's the same, and that I can use it in a 25-gallon electric pump sprayer like the one on the back of a four-wheeler. Do I spray when it's sunny out with rain in the near forecast, or does it matter?
Answer: I usually spray when it's sunny or overcast, and not windy. It's probably not a good idea to spray when rain is imminent to allow a few hours for the solution to be absorbed by the moss.
© 2015 Eugene Brennan
Louise89 on June 28, 2020:
Great advice! Thank you so much.
Gupi on June 11, 2019:
Thank you for sharing that useful advice. Will definitely come in handy.
Eugene Brennan (author) from Ireland on May 16, 2019:
I've never actually measured it, but I reckon I sprayed about 40 square metres with a 5 litre fill of my sprayer. It would of course depend on the size of the hole in the spray head and pressure of the sprayer. If you intend to spray, you could do a trial run with just water and measure up to see what area you cover.
Todd on May 15, 2019:
Maybe i missed it, but I didn't see how much coverage (sq meter, sq ft) you get per litre or gallon of solution.
Eugene Brennan (author) from Ireland on May 05, 2019:
It's normally recommended that you leave a lawn for 3 days after spraying weedkiller for it to be absorbed through the plant (when weeds are growing actively, and not slowed down by drought). I'm not quite sure how long you such wait. Weeds should start wilting after a week or less and fertilizer should make grass grow quicker so that it competes with weeds. Maybe contact the weed killer manufacturer and see what they recommend.
Sharyn Forty on May 04, 2019:
I have just sprayed my lawn for a common variety of weeds. Can I now spray for moss or should I wait for a week or two to let the weed killer do its thing and if I spray for moss how long before I could spray lawn fertiliser. I found your article really informative and easy to understand - thanks
Dr T on April 26, 2019:
Good easy to understand info
Eugene Brennan (author) from Ireland on May 06, 2018:
I don't think it matters, but possibly when the moss is dry, it'll absorb more solution than when it's wet.
Don't spray when rain is imminent because it'll just get washed off.
[email protected] on May 05, 2018:
We live in a very wet climate and I use a spreader to cover the lawn with ferrous-sulphate, is it best to spray with the lawn wet or dry or does it matter?
Eugene Brennan (author) from Ireland on April 12, 2015:
Thank you all for the kind comments!
Writer Fox from the wadi near the little river on April 12, 2015:
You provide excellent instructions and wonderful graphics to explain how to kill moss in a lawn. I agree with you that it is best to prevent it from growing in the first place. Enjoyed and voted up!
Shane M. Ilagan from Philippines on April 03, 2015:
Very informative article! Thank you for sharing!
Amanda M from Unknown on April 03, 2015:
Very useful! Voted up!