Can You Spray Rubbing Alcohol on Plants?

Rubbing alcohol can be an effective insecticide and herbicide, but it's indiscriminate in that it kills both weeds and any plants you want to keep alive.

There are ample alternative natural insecticides to spray directly on plants, most of which are safer and, in practice, more effective. There are also more sustainable natural herbicides than rubbing alcohol. But if you use rubbing alcohol on plants, it's important to know how to apply it—which kinds, what amounts, and which methods—and, more importantly, what not to do.

How Not to Use Alcohol on Plants

The most common types of alcohol are ethanol, methanol, and isopropyl (or rubbing) alcohol, and each comes with its own do's and don'ts.

Methanol

Methanol is the simplest form of alcohol, commercially available as racing fuel and for other applications. Some studies have found methanol stimulates plant growth, especially C3 grasses like wheat, fescue, rye, bluegrass, and many others, yet a general review of the literature found it is ineffective or inhibits plant growth.

Methanol can be an effective herbicide at high concentrations but it's a pricey item to kill plants. It's also an indiscriminate killer, so if you want to kill all the plants in a single area, solarizing your soil is a far less expensive and far less-flammable method.

Ethanol

Like methanol, ethanol has been found to stunt plant growth. One practical use is to add a diluted ethanol solution to paperwhites or other daffodils to slow their growth so they last longer as forced bulbs. Research conducted by Cornell University found solutions of between 4% and 6% ethanol prevented paperwhites from growing too tall or floppy. But be warned: the same study found solutions with ethanol content greater than 10% killed the plants.

Rubbing Alcohol

Rubbing alcohol (isopropyl alcohol) is the most commonly recommended, most practical (or, rather, least impractical), and least expensive alcohol to use on plants. Rubbing alcohol is usually 70% alcohol, so it needs to be heavily diluted to be used correctly. As with ethanol, too strong of a solution will do more harm than good.

It's generally agreed that a solution of at least 20 parts water to one part rubbing alcohol, which yields a solution of 3.33% rubbing alcohol, can be an effective insecticide. Unfortunately, it's also an effective but indiscriminate herbicide, so use carefully, if at all.

Using Rubbing Alcohol on Plants

Isopropyl alcohol can kill mealybugs, aphids, spider mites, thrips, slugs, snails, and whiteflies by melting their protective wax coatings and drying out their soft bodies. Eggs and pupae are likely to not be affected, so you will need to reapply your solution once new predators emerge.

Once you have created a less-than-4% alcohol solution, test it on your plants first. Apply a small solution of rubbing alcohol on a plant's leaves, then wait at least a day. Alcohol acts as a desiccant; you may find burn marks, curled or withered leaves, or other negative reactions.

If your plants seem unharmed by spraying alcohol for pest control, use it in moderation. Over-spraying an alcohol solution so that it runs off leaves and into the soil can turn your insecticide into an herbicide. Plants absorb alcohol via their roots, which can cause severe dehydration and kill the plants.

To be safe, use a cotton swab to apply the solution directly onto the mealybugs or their eggs rather than using a spray, which will coat the leaves and potentially damage them. Of course, using a cotton swab to apply rubbing alcohol to each pest is probably a waste of your time when easier, safer, and more effective alternatives are available.