Home remedies against pests: what helps and what harms

You turn over a leaf of your nasturtium and the underside is black. Not a little bit black, completely black. The next morning you type “home remedies for aphids” into your phone and get seventeen recipes, five of which contradict each other and two of which are poisonous.

That is exactly where the trouble starts. Home remedies have a good reputation because they sound like the kitchen rather than the chemistry shelf. But “from the kitchen” does not mean “harmless”, and “my grandmother always did it this way” does not mean “it works”. Some of the old recipes are actually on the EU list of basic substances. Others are superstition that costs you a lettuce. And a few are genuinely dangerous, usually the very ones passed around garden forums with the most confidence.

So let us go through them: which recipes survive the practical test, which ones you can save yourself, and where German plant protection law draws a line that hardly anyone knows about.

Soft soap against aphids: the recipe that holds up

The soft soap solution has the best ratio of effort to effect. Potassium salts of fatty acids dissolve the waxy cuticle of aphids, spider mites and whiteflies, the insects lose their protection against evaporation and dry out within a few hours. The soap itself is what matters: real unscented potassium soft soap, never washing up liquid.

One thing up front, so there is no surprise later. Strictly speaking, this treatment is only lawful with an approved potassium soap product that carries a clearance for home and allotment gardens. A solution you mix yourself counts as an unauthorised plant protection product. Why that is so is explained further down in the section on the legal framework. The figures below apply either way, and they also explain why the label on the ready made product says what it says.

1. Mixing it

   10 to 20 ml of liquid potassium soft soap per 1 litre of lukewarm water, so 1 to 2 per cent of product, not 1 to 2 per cent of pure active ingredient. Going stronger does not help, it scorches leaves. What decides the outcome is not the concentration but whether you hit every single insect.

2. Test it first

   Spray a handful of leaves, wait 24 hours, then look again. Young shoots, strawberries and hairy foliage are the sensitive ones.

3. Spraying properly

   Early in the morning or in the evening, never in full sun. The underside of the leaf is the target. On roses and beans, a strong jet of water beforehand knocks off most of the colony.

4. Repeating

   Again after 7 days, and a third time if the infestation is heavy. Otherwise the insects you missed will rebuild the colony within a fortnight.

5. Rinsing off

   Rinse with clear water a day later, because sticky honeydew residue attracts sooty mould. For identification, see Spotting and controlling aphids.

Above 1 per cent, bicarbonate stops being well tolerated. Repeat the spray every 10 days, at most eight times a year, and no more than 2 to 5 grams of bicarbonate per 10 square metres per year in total. A dash of rapeseed oil helps it stick. You still have to be honest about it: bicarbonate works preventively, and a courgette leaf that has already turned white across its whole surface will not go green again. One exception has been added since 2025: on wine and table grapes, bicarbonate may no longer be used as a basic substance in Germany, because there is now an authorised product with the same active ingredient.

Cow's milk is likewise approved as a basic substance against powdery mildew, on cucumbers and other cucurbits under glass, at one part milk to 10 or 20 parts water depending on the crop. The catch is the time window. The approval only covers the period up to flowering, and stops once fruit has set. In other words, just as your first courgette starts swelling, you are out. What is approved is raw milk, not the long life milk from the chilled aisle, and on lettuce milk is not approved at all.

Whey is the next of kin, and this is where people usually miscalculate. The 60 to 80 grams per litre is the stock solution made from whey powder, not the spray itself. It gets diluted a second time: 6 to 30 millilitres of stock solution in 1 to 1.5 litres of water, three to five treatments seven days apart, on cucumber, courgette and squash under glass, and only until the first flower buds appear. This is where you find the condition that many people wrongly attach to milk: treated leaves are not fit to eat. On a courgette leaf that hardly matters. On leafy vegetables it would matter a great deal, but whey is not approved there anyway.

Horsetail belongs in this group too. Equisetum arvense is approved as a fungicide against scab and mildew on apple, at 2 grams of dried herb per litre, two to six treatments seven days apart. But only in spring, from bud break to the end of flowering. By late July that window has long since closed. In any case your strongest lever needs no product at all: give plants space and water them from below in the morning. See Spotting and treating powdery mildew.

On top of that, plenty of them never drown. Some feed at the rim and are back the following evening. And the trap is not fussy about its victims: ground beetles end up in it, and so does the leopard slug Limax maximus. That it eats the eggs of the Spanish slug Arion vulgaris has been observed; that it measurably reduces their numbers has not been demonstrated. A Swedish experiment in 2022 found no stronger effect than simply adding more slugs of the same species. That is still no reason to drown one. So do not sink the rim of the trap flush with the soil, leave it standing one to two centimetres proud, and put it at the edge of the bed rather than in the middle of it.

Coffee grounds and eggshells: the most stubborn myths

The coffee grounds tip goes back to a real observation. Caffeine kills slugs at concentrations of roughly 1 to 2 per cent, and it deters them from feeding at as little as 0.01 per cent. Freshly brewed coffee sits at 0.06 to 0.1 per cent, which would put it above the deterrence threshold. The trouble is that what ends up on the bed is not fresh coffee but spent grounds, and there is barely any caffeine left in those. Whatever remains is washed away by the first rain, which is exactly when the animals are out. And there is a point no gardening guide mentions: caffeine appears on no basic substance list at all. Scatter coffee grounds deliberately against slugs and you are formally applying an unauthorised plant protection product.

With eggshells it is the same problem in another form. Sharp edges do slow a slug down, as long as everything is bone dry. But slugs travel when it is damp, and then they carry enough mucus to glide straight over shells, sawdust and sand alike.

What actually helps against slugs

The metal slug collar is inelegant and it works. You bury it 3 to 5 centimetres deep, the folded over rim is the barrier, and it keeps working at two in the morning. For planted out seedlings of lettuce, cabbage and courgette it is the best investment you can make. For sown rows it is useless, and there a slug fence around the bed or a crop protection net is the answer.

Hand picking remains the second pillar, at dusk or early after rain, and a damp board as a daytime hiding place makes it predictable. Slug pellets based on iron phosphate are authorised. See Spotting and controlling slugs.

Herbal brews: one is permitted, three are not

Nettle extract is the winner. It is approved as a basic substance, explicitly against aphids on fruit trees, beans, potatoes, cabbage and roses. The official recipe is surprisingly precise: 75 grams of fresh or 15 grams of dried nettle, young leaves, clean and washed, per 1 litre of drinking water, soaked for at least 24 hours at around 20 degrees. The preparation instructions actually specify 3 to 4 days with daily stirring. Then filter it, dilute with five times the volume of water, and observe a seven day waiting period.

One detail here is not a detail at all: the container has to be clean and sterilised. The extract ends up on leaves that people eat, and the approval explicitly requires clean vessels and testing for E. coli and salmonella. On lettuce and cabbage it also only applies up to around nine true leaves. And here is a sentence you rarely read: as a preventive treatment the extract is useless. You spray when the aphids are there, and you have to hit them. See Making your own plant brew.

Tansy brew is the loser. Tanacetum vulgare is named on the list of substances that were assessed and explicitly not approved, alongside wormwood, mugwort, yarrow and comfrey. The reason is its thujone content. Tansy is welcome to flower on any verge it likes, but it does not belong in your sprayer.

Garlic infusion sits somewhere in between. There are indications of a deterrent effect, but no solid figures, and garlic is not approved. How closely these things are examined is clear from its neighbour on the list: onion oil is approved, but only as a scent in a dispenser against carrot fly, and only for professionals.

The legal framework hardly anyone knows about

German plant protection law is uncomfortably clear on this. You may only apply products that are authorised, and only in the way the authorisation describes. For home and allotment gardens you also need a clearance for that specific setting. A professional preparation from your neighbour is off limits, even if the active ingredient is identical.

And now the part that surprises people. The moment you apply something against a pest, it legally becomes a plant protection product. The soap solution from the start of this article is, strictly speaking, not permitted, even though ready made potassium soap products with the same active ingredient are authorised. If you want to stay on the right side of it, buy one of those with a home and allotment clearance and follow its label.

The exception is called a basic substance: a substance not intended for plant protection that turns out to be useful there anyway. The list arrived with EU Regulation 1107/2009 and it is the most honest compass we have for home remedies.

What is actually due in late July

We are in CW 31, and the season is shifting right now. The big aphid wave is behind us; it usually runs from mid May to late June · CW 20 to 26.

Powdery mildew is having its best moment: warm days, cool nights, dew in the morning. On courgette, cucumber and gooseberry the bicarbonate spray is worth it, at the approved interval of 10 days. The second slug wave arrives with the first steady August rain · CW 32 to 35, and then it is lamb's lettuce and endive that are at risk. Put endives in as young plants with a collar straight away, and cover the lamb's lettuce with fleece after sowing.

The honest verdict

Home remedies are not a substitute for a healthy garden, they are the repair job. Real plant protection happens beforehand: the right variety, enough spacing, crop rotation, companion planting and enough structure to keep ladybirds and ground beetles around. “Natural” is not a safety rating.

What helps most is your own memory, and you can outsource that to a garden journal. Once it tells you that the aphids on the rose by the south gable arrive in CW 21 every single year, by the third year you are there a week ahead of them. That is how seventeen contradictory internet recipes turn into a method that works in your garden.

Common questions about home remedies for pests

Can I use washing up liquid instead of soft soap?

Better not. Washing up liquid contains surfactants, fragrances and degreasers that attack the waxy layer on the leaves and can cause scorching, especially on young shoots and in sunshine. Use pure potassium soft soap, 10 to 20 millilitres per litre of lukewarm water.

Do coffee grounds really work against slugs?

No, at least not in the state they reach the garden in. Caffeine only kills at around 1 to 2 per cent, though it deters feeding from as little as 0.01 per cent. Freshly brewed coffee at 0.06 to 0.1 per cent would be above that threshold, but what lands on the bed is spent grounds with hardly any caffeine left. On top of that the barrier washes away with the first rain, which is exactly when the animals are out. And formally it is not clean either: caffeine is not an approved basic substance.

Is milk against mildew allowed?

Yes, but within narrower limits than most people assume. Cow's milk is approved in the EU as a basic substance against powdery mildew, including on cucumbers and other cucurbits under glass. Depending on the crop that means one part milk to 10 or 20 parts water, sprayed every 7 days. What is approved is raw milk, and only up to flowering: once fruit has set, you stop. On lettuce and other leafy vegetables milk is not approved at all.

Why am I not allowed to spray tansy brew?

Tansy was assessed as a basic substance in the EU and explicitly not approved. The reason is its thujone content, which is considered a concern for both the user and non target organisms. Wormwood, mugwort and comfrey are on the same list.

When do I actually need to intervene against aphids?

Only once the colony is weakening the plant: distorted shoot tips, curled leaves, everything sticky with honeydew. Small colonies are the food that brings ladybird larvae and lacewings into your garden in the first place. Spray at the first five aphids and you prevent the very beneficials that would otherwise solve the problem for you.