Nasty stuff! I'm 100% guilty of this mistake. Have you been told to use dish soap on your plants as a home remedy for pests? Me too! Have you done it? Me too :(
But dish soap is NOT safe for plants (it's not even technically soap at all)
What you want is insecticidal soap. And no, you can't DIY insecticidal soap using dish soap (or castile soap). This is why to keep dish soap well away from your plants, plus the soap that IS safe to use on your plants instead...
Why is dish soap NOT soap? and why does it matter?
To understand why dish soap is so risky for plants, you need a (brief, I promise), lesson on what is and is NOT soap and why it matters if you're a plant.
Dish soap or dishwash liquid is actually NOT soap. It's a detergent. It's made from synthetic chemicals, surfactants and solvents, that strip grease, fats and oils from dishes. Dish soap can have all sorts in the mix, including bleach, dyes, fragrances and rinse aid.
True soap is mostly natural (depending on what companies add to it). Usually made from either potassium and fatty acids or sodium and fatty acids. That's what you'll find in most liquid hand soaps and bars of soap (along with other additives like fragrance and moisturiser).
What does dish soap do to plants?
The reason dishwash liquid does a great job on grease, oils and fats to give you sparkling clean dishes, is also why it can be so harmful to plants.
Dish soap can strip the protective natural oils and waxes from plants. Plants NEED that protective layer! Without it, plants are left open to attack by pests, fungal and bacterial diseases.
Dish soap may kill the pests but can also kill or weaken your plants at the same time, leaving them without their natural defences against diseases and bad bugs. No thank you!
What about castile soap as a pesticide?
You might have already heard about the problem with dish soap and plants, and been told to use castile soap instead, like the Dr. Bronner's brand.
Castile soap is made from potassium or sodium, along with vegetable oils. It can also have glycerin, essential oils and fragrance added depending on the brand and formula. Castile soap IS soap, but it's not a pesticide.
Is castile soap safe to use on plants?
Although castile soap is NOT insecticidal soap, it is a safer option than dish soap, provided it's the type made with potassium not sodium. I still wouldn't use it on plants myself (but to be fair, I have used it before I knew better). It's not worth the risk if you want to kill the pests without killing your plants!
Castile soap is NOT a pesticide, it's not recommended or tested for use on plants. How much to use, and what other additives might do to your plants, are unknowns. Plus what might be tolerated by one plant could harm another. I wouldn't risk it. There is a much gentler, safer option...
What is insectical soap?
Insecticidal soap is soap specially formulated for use as a pesticide for plants. It's made using potassium, not sodium, along with special types of fatty acids. Not good for dishes, bad news for pests, and much gentler and safer to use on plants.
What plant pests does insecticidal soap kill?
Always check the label for the product you have, but in general, insecticidal soap tends to be most effective against soft-bodied pests.
That usually includes:
- spider mites,
- aphids,
- whitefly,
- young scale,
- thrips,
- psyllids.
Insecticidal soap doesn't usually work against mealybugs and adult scale, caterpillars, beetles, wasps or insect eggs (but again, always check the label).
Bonide Insecticidal Soap for example (that brand's available on Amazon for my international plant friends), lists ants, aphids, borer, caterpillars, spider mites, crickets, weevils, flies and thrips, so it does vary.
Can insecticidal soap be used on all plants?
Some insecticidal soaps are certified organic and tested safe for use on crops like fruit and veges, but ALWAYS check the label before use. Unless it says otherwise, stick to ornamental plants (that category includes houseplants, shrubs, plants grown for their flowers like roses, that sort of thing, so not anything you might later eat or turn into a drink).
The one I use (now that I know to ditch the dish soap and castile soap!), is Yates Natrasoap. That one comes in either a 200ml concentrate or a 750ml ready-to-use spray. Both are available locally if you're in New Zealand, but if you're overseas Amazon has you covered. Get yourself Bonide Insecticidal Soap instead.
If you go for a concentrate, once mixed you'll need to safely dispose of leftovers. You can't save the rest for later. Or you can just be clever and get the Yates Natrasoap ready-to-use so leftovers stay stable and can be stored for next time.
The Bonide option (for my international plant friends), is also a ready-to-use. I always prefer a ready-to-use for my indoor plants in particular for pesticides, otherwise too much goes to waste.
The Yates insecticidal soap is certified organic, biodegradable, has no residual effect and little to no effect on beneficial hard-bodied insects such as ladybirds. It can be used on all plants including fruits, vegetables and ornamentals, but I'd still do a test-spray first on a smaller area if a plant is known to be sensitive (like ferns).
How does insecticidal soap work?
Insecticidal soap is a contact insecticide. It only kills what it touches when it's wet. Repeated applications are usually needed. You want to spray it both on visible adults and also where you can see signs of pest damage, especially good coverage underneath leaves, a common hiding place for pests. It works by damaging the pest's protective outer layer (the cuticle), causing them to dehydrate and die.
What's the mixture for making insecticidal soap?
It'll depend on the brand you buy of course, but for Yates Nutrasoap, the concentrate is 20mls per 1 litre of water and once mixed with water, should be used within 24 hours. You'll also need to buy a fine spray mister to apply it, like the Egmont 360 or Mister 360.
TIP: The '360' in the name tells you those misters also spray upside down, handy for getting full coverage even under leaves without tipping plants upside-down!
Or if you can find a ready-to-use option, they normally have a spray bottle included, with no prep required, and leftovers can be stored for next time so nothing goes to waste. Yates Natrasoap makes a ready-to-use option, and for those overseas, Bonide has a ready-to-use option also.
Safer but don't be silly!
Just because insectical soap is natural and organic, doesn't mean you can ignore the usual pesticide precautions.
Avoid spraying on hot days. Pick a cool day and apply early morning or early evening when temperatures are cooler. Keep plants out of direct sun while still wet. Don't spray if soil is dry or the plant's already under moisture-stress (water first and let them recover before spraying with pest treatment).
Baby plants such as seedlings and cuttings are also a no for using pest treatment on, but if you need to for babies or any senstive plant, do a test spray first and wait 3 days to check for a reaction before you spray the whole plant.
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident writer for NZ Gardener magazine on the topic of indoor plants. You'll see her houseplant care articles in magazines in-stores NZ-wide, and online on Stuff, The Post, NineHoney and more.