I’ve heard that using aspirin water on my plants can help promote plant growth and prevent disease. Is this true?
Harvey Ellis, Atlanta, GA
Aspirin water triggers a plant's natural defenses.
Rebecca Brown, professor of plant sciences at the University of Rhode Island, replies: Although the complete effects of treating plants with an aspirin-water solution are unknown, studies have shown that applying salicylic acid (aspirin is acetyl salicylic acid) to plants can induce resistance to pathogens, environmental stresses, and some insects. This protection should allow the plants to grow more vigorously. Under extremely stressful conditions or those highly favorable to the growth of diseases, however, salicylic acid may not prevent all damage from stress or disease. Salicylic acid is a naturally occurring compound in plants. When a plant is stressed or attacked by a pathogen, this compound stimulates the plant’s internal defense pathways. Treating a plant with additional salicylic acid appears to trigger the plant’s defense pathways in the same way.
In the summer of 2005, I directed a group of master gardeners with experiments on tomatoes. We looked at the effects of two strengths of aspirin water and the commercial product Messenger compared to plants sprayed with just water. Messenger activates the same internal defense system as salicylic acid but at a different point along the pathway, which may provide broader protection. Because of the dry conditions, none of the plants developed any disease. All of the plants grew equally well. We found that the plants treated with a solution of 250 milligrams of aspirin to 1 gallon of water and the plants treated with Messenger yielded more but smaller fruit than the control.
An aspirin-water solution of 250 to 500 milligrams (one or two regular aspirin tablets) of aspirin per gallon of water provides a solution similar to what has been tested; solutions at higher rates have been shown to burn foliage. Any brand of aspirin will work, but plain, uncoated tablets dissolve best. The solution should be applied every two weeks and may be used to water germinating seeds and new transplants.
Another form of salicylic acid that is gaining attention is willow water, made from steeping fresh-cut willow branches in water. Willows are naturally high in salicylic acid. The exact amount of salicylic acid in willows is unknown, however, so the amount to use and the timing of application is less certain. Willow water might be worth investigating as a source that could meet organic certification guidelines (neither aspirin nor Messenger do).
While there’s no harm in experimenting with these solutions, keep in mind that aspirin water and willow water are not registered pesticides. All of these products need to be applied preventively, before the first sign of disease. They may prevent infection but will not kill fungi or bacteria already infecting the plant. It may also take two weeks for the salicylic acid to activate the pathway and protect the plant.