There is only one compound available that kills moss in lawns, and that is iron sulphate; also known as Ferrous Sulphate. Its chemical formula is FeSO4. It is the moss killing component of moss killers and lawn sand and is commonly sold in the green keeping industry as a fertiliser. It may also be combined with other elements and can therefore be used as a general fertiliser.
The most common way of applying a ferrous sulphate based moss killer is to apply it a week or two prior to raking out the moss (see below for more details). The moss is partially desiccated and is thus easier to remove. However, there are two other opportunities to use it that may make moss control easier and more efficient.
Cost Effective Moss Control
Firstly we can use it to inhibit moss growth so that we enter spring with considerably less moss had we not taken action. By applying your iron sulphate moss killer, usually by sprayer or watering can during the moss growth period, anytime from late autumn through to spring you will at least kill the exposed upper layer of moss. Though iron sulphate is brilliant at killing moss it has its limitations when the moss is very deep. It will only penetrate 1 to 2cms into the moss so if you’ve got more depth than this the moss underneath may well remain alive. Therefore, if you know your lawn is prone to heavy moss infestations, starting your treatments in October before the moss gets too deep will be beneficial.
Even though the bottom layer of moss may remain alive, the killing of the top layer will stop light getting to the living bottom layer and the progress of the moss will be halted for perhaps 6 to 8 weeks. This is an easy and cost effective approach to moss control without the need for raking, particularly if your lawn is not smothered. This treatment can be repeated every 4 to 8 weeks over the winter months.
If at the same time you have improved the local environmental factors a little, together with improved lawn care practices, you may well have made conditions more favourable for the grass. This together with a moss killer could bring about the desired result. If not, then the moss will have to be removed.
More Efficient Moss Control
Secondly, you can use it after removal of the moss in the spring. Treating moss prior to removal will not kill all the moss; perhaps 20% to 50% will remain to re-infest your lawn. Therefore, if you treat what remains after raking you may well achieve upwards of 90% moss control.
How to Use Ferrous Sulphate as a Moss Killer
We would advise that initially you test a small area with a 1% Ferrous Sulphate Heptahydrate solution (10gms/1L of water). If this does not give the desired effect a stronger solution can be used up to a maximum of 5% Ferrous Sulphate Heptahydrate (50gms/1L of water).
A typical average strength to use would be a 3% solution. To make the solution add 30g of ferrous sulphate for every litre of water (for example for a 15L sprayer you will need 450g of ferrous sulphate).
Application of the solution should be at a rate of 5 square metres for every litre of chemical mix. Apply evenly over lawn etc avoiding overspray contact with other plants and paths, patios etc.
After 2 to 3 days you will see that the moss has turned black in colour indicating that the moss has been successfully killed. At this point one should use a tined rake to loosen and remove dead moss. Application of a lawnmower set to low cut will help in the removal. Please note: the grass cuttings and dead moss should not be added to compost bins.
Please be aware that ferrous sulphate is only for use on grass areas and is not designed to be used on hard surfaces as it may stain. For hard surfaces like roofs, paths, patios etc Zinc Sulphate can be used or a proprietary chemical like MossKill Premium.
If ferrous sulphate comes in contact with concrete for example it will stain the surface brown which effectively is a rust stain. To remove this stain one will need to use an acid solution like Oxalic acid (used as a 5% solution).
When treating lawns etc with ferrous sulphate keep pets off the surface for at least one week. This protects their well being and prevents transfer of residues onto hard surfaces (i.e. brown staining).
For application on agricultural pasture land / paddocks etc use at the same rates as above. Animals like horses must be kept of grasslands for at least 4 weeks before being allowed back to graze.
An over usage of Ferrous Sulphate can be harmful if the land being applied to has a low pH (very Acidic) as Ferrous Sulphate is naturally very acidic with a pH of 2.5.
NOTE: We must stress that this information can only be used as a guideline and is given in good faith. We cannot be held responsible for any adverse effects that may be experienced as a result of use of Ferrous Sulphate.
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