I spent some time last year using aspirin to preventatively manage tomato diseases. I was not vigilant about the routine, but can say my aspirin treated tomatoes did better than the untreated tomatoes. Early blight is a disease that gets tomatoes in my area. Last year the blight came, but it appeared to do a lot less damage to the treated tomatoes. The treated tomatoes also seemed to recover more quickly. This year I intend to follow a strict routine of aspirin watering. All I can say at this point is there seemed to be some benefit to using aspirin. Enough of a benefit, that I will revisit its use as a preventative disease measure this year.
I read that the salicylic acid found in aspirin binds to a receptor site in the cells of the tomato plant. Essentially, the salicylic acid mimics the chemical trigger(s) that sets off the natural defense response in tomatoes and other garden plants. It is technically called the salicylic acid-binding protein 2 (SABP2) gene. It was discovered by scientists at the Boyce Thompson Institute for Plant Research out of Cornell University.
The use of aspirin (for the salicylic acid) to grow healthier tomatoes has scientific bases. Spraying or watering plants with an aspirin solution triggers their natural defense systems that fight bacteria, fungi, and viruses. What you are doing by using aspirin in your garden is triggering the tomatoes to fight off diseases and threats early. You are turning on all the natural defense systems of the tomato plant before a threat actually arrives.
Last year I planted my tomatoes with aspirin in the soil and also directly watered the leaves with the aspirin solution. The watering, over the season, was more random then scientific. I am not going to plant with aspirin this year but I am going to stick with a regular aspirin watering routine. I may just water the foliage of the plant with a watering container or I may use a sprayer. I don’t think it matters how you deliver the water and aspirin to the leaves. I plan to plant the tomatoes as I normally do and let them grow for 4 weeks. At four weeks, they will get their first dosing of aspirin water. I will follow up with dosing each plant every 2 weeks. Each plant will get drenched. I will stop dosing the plants come September.
The mix that I am going to use is one regular strength aspirin per gallon of water. The strength of a regular aspirin is typically 325 mg. The aspirin should not be safety coated. I will add just under1 teaspoon of dish soap to the gallon of water. I do this, in theory, to help stick the water to the plant. I don’t believe it is needed. The plant will be able to absorb the salicylic acid through its leaves and through its roots. From what I understand the salicylic acid will easily find its way to the plants receptor sites.
If you want to try this out on your tomatoes, I suggest leaving some untreated. In theory you should be able to see a difference. The tomatoes will have to be planted in different locations in your garden. The treated tomatoes will have aspirin water running all over the immediate garden area. All plants that have roots in the treated area will be getting the aspirin treatment.
*Some studies say it fights off insect pests too. I can't confirm that.