Aspirin Spray for Tomatoes and the Vegetable Garden

Do you want to grow great tomatoes? Most gardeners do and they are on a constant hunt for magical concoctions that will create the best yield and most flavorable tomato. The latest craze involves the use of aspirin. Just mix a few aspirin in water and spray your plants. Spraying tomatoes or any other kind of plant with aspirin will make them grow better, have less diseases and ward off insects.

My regular readers will know that I love to find the origin of a myth, and I think I know how this one got started. Lets have a closer look at how some science can result in an online sensational cure-all.

Aspirin Reduces Insect Pests and Diseases in the Vegetable Garden

Do a quick google and you will find headlines such as these.

• Aspirin Increases Growth and productivity
• Aspirin Prevents Blight in Tomatoes
• Aspirin Wards Off Insects

Reading these posts you quickly realize that almost none of them reference any kind of scientific study to support their claim. Occasionally you will see mention of work done by Martha McBurney, a Master Gardener at Rhode island University, but no real data is presented. I have followed up on many of these links and I think I’ve tracked down the series of events that has led to this popular internet solution of using aspirin.

Science Looks at Aspirin in Plants

The science did not really look at Aspirin; they studied the role of salicylic acid (SA). Early work on tobacco plants showed that higher levels of salicylic acid helped plants fend off diseases, including mosaic virus. Similar results were found in cucumber plants. It should be noted here that this worked looked at natural salicylic acid levels in plants – plants were not sprayed.

Work by the U.S. Department of Agriculture found that spraying tomato seedlings with salicylic acid, before they were exposed to a phytoplasma, reduced the incidence of disease from 94% to  47%. They suggested that “the treatment triggering systemic acquired resistance (SAR), a kind of general readiness state that primes plant defenses against pending microbial or insect attack”. This work was all lab work.

Another study found that both acetylsalicylic acid (Aspirin) and salicylic acid induced stress tolerance to heat, chilling and drought in bean and tomato plants, by either soaking seeds or as a soil drench.

Foliar application of SA in greenhouse experiments showed better growth and fruit development.

Fast forward a few years and SAR (systemic acquired resistance) is better understood. When a pathogen infects a plant, SA is produced by the plant at the infection site. Some of this SA is transported to uninfected parts of the plant, where it turns on the plant’s defenses. SA does not fight infection directly, but it triggers plants to fight the infection. “The activation of SAR provides a broad-spectrum resistance against a wide range of related or unrelated pathogens.”

A summery of the science on this subject, dated 2019, had the following to say, ” Exogenous application (ie spraying) of SA are reported to activate the defense responses including SAR. However, rapid glycosylation of SA and its phytotoxicity has prevented the efficiency of SA as a plant protection chemical”.

To paraphrase this, SA has a short life in plants and it can be toxic, which makes it impracticable to use.

Aspirin vs Salicylic Acid (SA)

A lot of people assume these are the same, but they’re not.

The active ingredient in aspirin is acetylsalicylic acid. When it is dissolves in water it produces salicylic acid and acetic acid. It is quite possible that the SA produced from aspirin will have a similar effect on plants as pure SA, but none of the scientific work tested aspirin.

Citizen Science on Aspirin and Plants

I mentioned Martha McBurney, a Master Gardener at Rhode island University. Her work is routinely used in social media to validate the use of aspirin on plants. In the summer of 2004, after seeing some of the above science, she applied aspirin to vegetable plants; tomatoes, cucumbers, beans and basil. It is reported that Martha said, “by the end of the season, the plants in the raised beds with the aspirin water looked like they were on steroids! The plants were huge, and green and with no insects. We even saw some disease problems that reversed themselves. We think we got a virus on the cucumbers, and the aspirin water seemed to reverse it. The cucumbers ended up being very healthy. Seed sprayed with aspirin water gave 100 percent seed germination, compared to spotty germination in the other trial beds.”

Note the lack of any real detail of either the experimental setup or the results. I have not been able to find any documentation of this work.

The following year, Dr. Rebecca Brown, Plant Sciences at the University of Rhode Island worked with Martha to repeat the experiment in a more controlled fashion, including replicates. They tested Early Girl tomatoes with two concentrations of aspirin and a commercial product called Messenger.

The conclusion from this work is, “Aspirin and Messenger will not decrease fruit yield under low-disease conditions if used to trigger systemic resistance in tomatoes. Messenger and Aspirin applied at the rate of 250 mg per gallon of water may actually increase the number of fruit. We cannot make any conclusions as to the efficacy of these compounds in reducing disease, because of the lack of disease in the study. Variation among plots was quite high, perhaps because of edge effects resulting from the lack of border rows. It would be beneficial to repeat the study under conditions more favorable to disease development, and to use larger plots with border rows.”

Science vs Anecdotal Information

We can learn a lot from the above experiments.

Notice the very different conclusions reached in 2004 and 2005. In 2004, the results were not properly measured (as far as we know) and there were not enough replicates (confirmed by Dr. Brown). The results are anecdotal and the opinion of the person doing the work.

When that same person repeated the work under stricter conditions, the conclusions were vastly different. The two years might have been very different disease wise – that is one reason these types of experiments are usually carried on for several years. Results in a single year can be an anomaly.

The use of replicates allows one to apply statistics on the data providing a standardized way to measure results. This is always part of any published study, and never part of anecdotal information.

Martha reported her 2004 results to media and they have been published in a few places. When the 2005 results became available, showing no disease control, they did not become published in the media, and the previous reports of success have still not been corrected. That is very common. The media and much of the public only care about sensational stories, not the facts.

Consequently, the general public is only exposed to the initial anecdotal report showing positive results. Anecdotal reports are of limited value.

Aspirin and Insects

What about keeping pests off plants? There is no real evidence of this. One thing that happens a lot is that when a product does one thing in the garden i.e. prevent diseases, it is automatically assumed it will do even more. Claims get exaggerated as they get repeated.

Others feel that since aspirin makes plants healthy, they will naturally develop defenses against pests. Unhealthy plants do tend to have more pest problems, but that does not mean spraying healthy plants with aspirin will keep pests off the plant.

Does Aspirin Work?

Based on the science, there is a logical explanation as to how aspirin could help control diseases. The problem is that the science is all lab work, and none that I found proved anything in the field. SA is effective if applied before a disease attacks, but will not do much once the disease is there because it triggers defense mechanisms in plants, it does not fight disease.

All of the research, except the report from Rhode Island University is with SA, not aspirin and this one report did not show disease control.

It is also not clear which diseases are affected. The chemical mechanism involved does seem to be generic, but it still needs further study to fully understand it. So claims that it will prevent blights in tomato might be true, but I have not found evidence that this works.

Aspirin is also toxic to plants and the concentration of any spray should be tested before use. There is limited information about toxic levels on different kinds of plants.

Will aspirin work as an insecticide? There is no evidence for this.

Will aspirin make plants grow better? If a plant is stressed, aspirin may help the plant deal with the stress. In such a situation it might grow better, but I found little evidence that spraying non-stressed plants makes them grow better. Some limited research indicates tomato yields may be higher.

Aspirin might work to prevent diseases but it needs to be used before the disease attacks.

Most of the scientific studies looked at spraying the plants directly, but a few looked at soil drenches, which also seemed to work. However, this was lab work in potted plants. Trying to treat plants in the field using soil drenches is much more complicated. If you decide to try this method, foliar spray would be a better option.

Aspirin might help plants grow better, but the evidence is limited and until we know more, it is probably best for gardeners not to use it. Instead of using untested chemicals, use the correct level of fertilizer.