Can alcohol be sprayed on plants?
Alcohol can be an effective insecticide. It can also be an effective herbicide, but it’s indiscriminate in that it kills both weeds and any plants you want to keep alive.
There are many alternative natural insecticides that you can spray plants directly, most of which are safer and, in practice, more effective. There are also other natural herbicides that are more durable than alcohol. But if you’re going to use alcohol, it’s important to know how to apply it to plants – what kinds, how much, and what 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 has its own do’s and don’ts.
Methanol is the simplest form of alcohol. It is commercially available as a racing fuel and for other applications. Some studies have shown that methanol stimulates plant growth, especially C3 grasses like wheat, fescue, rye, bluegrass and many others, but a general review of the literature reveals either that it is ineffective , or that it actually inhibits plant growth.
At high concentrations, methanol can be an effective herbicide, but it is expensive to use only to kill plants. It’s also an indiscriminate killer, so if you want to kill all the plants in one area, solarizing your soil is a much cheaper and much less flammable method.
Like methanol, ethanol retards plant growth. One practical use is to add a dilute solution of ethanol to paperwhites or other daffodils to slow their growth so they last longer as forced bulbs. Research by Cornell University found that solutions containing between 4% and 6% ethanol prevented paper blanks from becoming too large or soft. But beware: the same study revealed that solutions containing more than 10% ethanol killed plants.
Rubbing alcohol (isopropyl alcohol) is the most commonly recommended, most convenient (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 properly. As with ethanol, too strong a solution will do more harm than good.
It is generally accepted that a solution of at least 20 parts water to one part rubbing alcohol, which yields a 3.33% solution of rubbing alcohol, can be an effective insecticide. Unfortunately, it is also an effective but indiscriminate herbicide, so use it with caution, if at all.
Using Rubbing Alcohol on Plants
Isopropyl alcohol can kill scale insects, aphids, spider mites, thrips, slugs, snails and whiteflies by melting their protective wax coatings, drying out their soft bodies. Eggs and pupae are unlikely to be affected, so you will need to reapply your solution once new predators appear.
Once you have created a solution of less than 4% alcohol, test it on your plants first. Apply a small solution of rubbing alcohol to the leaves of a plant, then wait at least a day. Alcohol acts as a desiccant, so you may find burn marks, curled or wilted leaves, or other negative reactions.
If your plants seem unharmed by spraying alcohol for pest control, use them sparingly. Overspray of an alcohol solution so that it runs off the leaves and into the soil can turn your insecticide into a herbicide. Plants absorb alcohol through their roots, which can cause severe dehydration and kill plants.
To be on the safe side, use a cotton swab to apply the solution directly to mealybugs or their eggs, rather than using a spray, which will also coat the leaves and could damage them. Of course, using a cotton swab to apply alcohol to each pest is probably a waste of time when simpler, safer, and more effective alternatives are available.
Frequently Asked Questions
Can I use alcoholic beverages in the garden?
Alcoholic beverages contain ethanol, but they can also contain sugars and other organic matter that can promote the growth of bacteria and fungi that can kill plants. Thus, it is not recommended to use alcoholic beverages on plants.
Are there other uses of alcohol in the garden?
You can use rubbing alcohol to disinfect your gardening tools. Dip the ends of your trowels, shovels or other gardening tools in a 2-3% rubbing alcohol solution. Rinse thoroughly with clear water before reusing in the garden.