A Complete Guide to Killing Moss in Lawns
Lawn Care and Eradicating Moss
Moss in a lawn competes with grass, looks unsightly and can cause heart ache for your lawn mower as it struggles to cut 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 kill, and measures can be taken to prevent it growing in the first place.
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What Causes Moss in a Lawn?
Moss thrives in damp or 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 affects 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 during 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
- 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.
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.
Easy Steps to Spraying You Lawn and Getting Rid of Moss
All you need to do is mark out your lawn into strips using string, mix up your solution and spray
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.
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 O-rings, or even soften the plastic of the sprayer to the extent that it bursts under pressure). Usually I half fill my sprayer with water and top up with hot water from a kettle which has been half boiled. Tighten the lid on the sprayer and turn it upside down a few times 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.
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 3 teaspoons equates to 3 x 7 = 21 grams per litre (liter)
For those of you living in the USA, this is equivalent to 2.8 ounces per US gallon
The Imperial measurements equivalent is 3.4 ounces 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)
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.
So to summarize:
Approximate Equivalent Mixing Concentrations
21 grams per litre (3 teaspoons)
Imperial (Sometimes still used in UK)
3.4 ounces per Imperial Gallon (14 teaspoons)
US Customary Units (USA)
2.8 ounces per US Gallon (11 teaspoons)
1 ounce = 28.35 grams
1 US gallon = 3.79 litres
1 UK imperial gallon = 4.54 litres
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
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