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 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. That's the case with EM Previsan S, a ready-to-dilute preparation that we carry.

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 authorized 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's 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's 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's 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 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 boxwood, bay laurel, or rhododendron, where water tends to bead up 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 pesticide or plant protection product. In France, as throughout the European Union, a product intended to protect plants against diseases, fungi, or pests must be granted marketing authorization before it can be sold for that use. So we 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: corn plants treated with EM were about 50% more appealing to a caterpillar than untreated plants, and responded less to its attacks. This doesn't prove that EM weaken plants, but it's a reminder not to credit them with unproven benefits. Our guide What science says about EM gives the full picture.

Key takeaways

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 has the final word. As a guide, the manufacturer recommends a dilution of 1.1 quarts (1 liter) of EM Previsan S per 26.4 gallons (100 liters) of water, i.e., 1%. Its professional guidelines, around 1.1 quarts (1 liter) of product per 2.5 acres (1 hectare) in 26–40 gallons (100–150 liters) of water, work out to the same concentration. For a home garden, that means:

• 1.3 fl oz per gallon of water (10 ml per liter) in a small hand sprayer;
• 1.7 fl oz (50 ml) per 1.3 gallons (5 liters) or 3.4 fl oz (100 ml) per 2.6 gallons (10 liters) in a pressure sprayer or backpack sprayer.

The program the manufacturer suggests 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 during very wet spells. In a home garden, one application every two to three weeks during the growing season is considered a reasonable pace to start with.

One rule comes up with every EM manufacturer: it's 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 uncovered for 24 hours: chlorine harms microorganisms. The other basic precautions are gathered in our guide Getting started with EM: the golden rules.
• Only mix what you will use that day. Once diluted, the microorganisms don't keep.
• Do not mix EM Previsan S with a pesticide, fungicide, insecticide, or foliar fertilizer: one can destroy the other.

Emy's tip

Keep one sprayer just for microorganisms and label it with a marker. A sprayer that has held a weed killer 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 whenever possible.
• In overcast, calm weather: with no wind, the solution lands on the leaves and not in your neighbor's yard.
• Never in full sun or during the hottest part of the day: the droplets dry too quickly and the microorganisms suffer from ultraviolet light.
• Avoid spraying just before a heavy downpour, which would rinse off the foliage.

The right technique

• Set the nozzle to a fine mist.
• Aim at both the tops and undersides 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 spray in the morning.

Plant health starts in the garden

No product of any kind can make up for a plant that is poorly 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, more and more widely available for tomatoes, roses, and fruit trees.
• Crop rotation in the vegetable garden, so you don't grow the same plant family in the same spot every year.
• Space between plants: well-ventilated 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 fertilizer 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 ladybugs, 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 center, prevents a lot of mistakes: many symptoms come from lack of water, a cold snap, or a nutrient deficiency.
2. Remove affected parts at the first signs, using clean pruning shears.
3. Correct the cause: watering, sun exposure, planting density.
4. If treatment is needed, choose a product that is genuinely authorized for that use. Rules on which pesticides home 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 local 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. Use it as a form of care, alongside proven practices: good varieties, crop rotation, airflow, watering at the base, and a living soil. It is not a pesticide: if a disease breaks out, identify it and turn to a product authorized for home garden use.

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