EM Previsan S: spraying effective microorganisms on foliage, how to use it and its limits

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Spraying microorganisms on the leaves rather than on the soil: it is an idea that appeals to many gardeners. Leaves naturally host a multitude of bacteria and yeasts, and some EM products are designed to be applied directly to foliage. This is the case with EM Previsan S, a ready-to-dilute preparation that we offer.

This guide explains how to use it properly, when, at what dose and, above all, what you can honestly expect from it. A foliar EM spray is not a treatment: it replaces neither good growing practices nor an authorised product once a disease has taken hold.

Why spray the foliage?

The surface of a leaf is a living environment. Billions of microorganisms settle there, feed on the sugars and dust deposited on it, then disappear with the sun, wind and rain. The idea behind an EM-based foliar spray is to bring lactic acid bacteria and yeasts from fermentation to this very spot.

It is worth knowing that a leaf is a tough place for a microorganism. The sun's ultraviolet rays are the main factor limiting their survival, followed by heat and drying out. That is why the timing of the spray matters as much as the dose: a living product sprayed in the full midday sun loses much of its value within a few hours.

Did you know?

On a leaf, it is not the rain but the sun that does the most damage to microorganisms: ultraviolet light damages their genetic material. Spraying early in the morning or in overcast weather therefore gives them a much better chance.

EM Previsan S: what it is, and what it is not

EM Previsan S is a liquid preparation based on effective microorganisms, combined with a plant-based binder. This binder helps the solution stay on the leaves, including waxy or glossy foliage such as box, bay or rhododendron, on which water tends to bead and run off.

EM Previsan S is a preparation of microorganisms with a plant-based binder that forms a natural film on the leaves. It is not a plant protection product. In the UK and Ireland, as throughout the European Union, a product intended to protect plants against diseases, fungi or pests must be authorised before it can be sold for that use. We therefore do not present EM Previsan S as a treatment, and you should not rely on it to cure a diseased plant. The legal framework is explained in our guide EM and regulations.

The research points the same way. Independent trials of EM give highly variable results depending on the plants and conditions, and several find no measurable effect. Work published in 2015, combining a review of studies with greenhouse trials, even noted something unexpected: maize plants treated with EM were around half as appealing again to a caterpillar as untreated plants, and responded less to its attacks. This does not prove that EM weaken plants, but it is a reminder not to credit them with unproven benefits. Our guide What science says about EM gives the full picture.

Key points

EM Previsan S is used as regular foliage care, alongside a well-managed garden. It is neither a remedy nor an insurance policy against disease.

Dilution and frequency

Always refer to the label on your container: it is the reference. As a guide, the manufacturer recommends a dilution of 1 litre of EM Previsan S per 100 litres of water, i.e. 1%. Its professional guidelines, around 1 litre of product per hectare in 100 to 150 litres of water, work out at the same concentration. For a garden, this gives:

• 10 ml per litre of water in a small hand sprayer;
• 50 ml per 5 litres or 100 ml per 10 litres in a pressure sprayer or knapsack sprayer.

The programme suggested by the manufacturer for professionals runs from April to September, with 8 to 10 applications per season, roughly one every two to three weeks, and up to one a week in very wet periods. In the garden, one application every two to three weeks during the growing season is considered a reasonable rhythm to start with.

One rule comes up with every EM manufacturer: it is better to increase the number of applications than the dose. A more concentrated solution is not more effective, and an overly strong acidic preparation can mark young leaves.

Preparing the solution

• Use rainwater, or tap water left to stand in the open air for 24 hours: chlorine harms microorganisms. The other basic precautions are gathered in our guide Getting started with EM: the golden rules.
• Only prepare what you will use that day. Once diluted, the microorganisms do not keep.
• Do not mix EM Previsan S with a plant protection product, fungicide, insecticide or foliar feed: one can destroy the other.

Emy's tip

Keep one sprayer just for microorganisms and label it with a marker pen. A sprayer that has held a weedkiller or fungicide retains residues, even after rinsing, which can damage your plants or destroy the microorganisms in the solution.

When and how to spray

The right time

• Early in the morning, while the dew is still present, or late in the afternoon once the heat has eased. The manufacturer recommends dry weather with dew where possible.
• In overcast, still weather: with no wind, the solution lands on the leaves and not in your neighbour's garden.
• Never in full sun or during the hottest hours: the droplets dry too quickly and the microorganisms suffer from ultraviolet light.
• Avoid spraying just before a heavy downpour, which would rinse the foliage.

The right technique

• Set the nozzle to a fine mist.
• Aim at both the upper and lower sides of the leaves. The underside, more sheltered from the sun, is often overlooked.
• Stop when the foliage glistens with a damp film, before it starts dripping. Whatever runs off falls to the ground and is lost to the leaf.
• Let the foliage dry within a few hours. A plant that stays wet all night is more vulnerable to disease: one more reason to prefer the morning.

Plant health starts in the garden

No product of any kind can make up for a plant that is badly placed or weakened. The proven measures are simple, free and far more effective than any spray:

• The right plant in the right place: respect its needs for sun, water and soil type.
• Resistant or tolerant varieties, increasingly available for tomatoes, roses and fruit trees.
• Crop rotation in the vegetable garden, so as not to grow the same family in the same spot every year.
• Space between plants: well-aired foliage dries faster after rain or dew.
• Watering at the base, in the morning, rather than on the foliage in the evening.
• A living soil without excess nitrogen: a plant overfed with fertiliser produces soft, more vulnerable tissue. Adding compost and fermented organic matter is covered in our guide Using EM in the vegetable garden.
• Beneficial wildlife: hedges, flowers and nest boxes attract ladybirds, birds and hedgehogs.

What if a disease has already taken hold?

If you notice spots, a powdery coating, yellowing leaves or a colony of pests, EM Previsan S is not the answer. Work through these steps in order:

1. Identify the problem. A sharp photo of both sides of the leaf, shown at a garden centre, avoids many mistakes: many symptoms come from lack of water, a cold snap or a deficiency.
2. Remove affected parts at the first signs, using clean secateurs.
3. Correct the cause: watering, exposure, planting density.
4. If treatment is needed, choose a product genuinely authorised for that use. Rules on which plant protection products amateur gardeners may buy and use vary from country to country: check the label to make sure the product is approved for home garden use, and follow your country's regulations.

Once the plant has recovered, resume your EM sprays using thoroughly rinsed equipment.

In summary

EM Previsan S is a foliar spray based on effective microorganisms, with a binder that helps it stay on the leaves. Dilute it to around 1% in non-chlorinated water, following the label, and spray both sides of the foliage with a fine mist, early in the morning or in overcast weather, increasing the number of applications rather than the dose. It is used as a form of care, alongside proven practices: good varieties, rotation, airflow, watering at the base and a living soil. It is not a plant protection product: if a disease breaks out, identify it and turn to a product authorised for garden use.

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