First, a short history of aspirin and how it came about. Aspirin is acetylsalicylic acid, a synthetic compound and a first cousin to salicin and salicylates. These compounds are found in almost all plants, but historically found in large amounts in willow bark. White willow bark has the highest concentration of the salicylates.
Using willow bark as an analgesic and anti-inflammatory goes back to the beginning of mankind's history. Almost all cultures were aware of these amazing compounds and used them to treat headaches, joint and dental pain, fever, and other maladies.
A lot of research has been done on the chemical paths that aspirin takes in the human body, but it has only been relatively recent that studies have focused on aspirin's uses and effects on plant growth and health.
There are many of these studies on the web; I'm not going to inundate you or bore you to death with them. Type in 'aspirin' and 'salicylic acid' and you'll be entertained for hours! I'm going to condense the information and tell you how I've used aspirin, and my short theory of how and why it works.
Salicylates are hormones found in almost all plants. SAR, or Systemic Acquired Resistance, is a plant's reaction to pathogens, diseases and insect attack. It helps build up the plant “immunity system.” When the plant becomes stressed (including lack of water), several compounds are released to assist the plant in fighting off the offenders.
You'll recall my two blogs on hydrogen peroxide and H2O2's uses for growing hydroponic plants? Hydrogen peroxide is one of the major compounds and salicylates are another major compound! Both are found naturally in almost all plants, and they work together in the complex SAR cycle to fend off pathogens, and even some insects. A lot of research is being conducted toward using salicylates as natural insect repellents.
Like foliar feeding, the pure science of how it works might be complicated, but the application is very simple and inexpensive! I'm going to share a couple of stories of how I used aspirin with my struggling plants. You will find numerous references to aspirin use in my twenty-something videos on my Plain 2 Grow Jim You Tube channel.
Story # 1: My tomatoes were “gasping” for survival in my small greenhouse (find how to build it here) during a fierce summer. I used a combination of foliar feeding (the formula had a small amount of aspirin in it; 40 mg crushed and unbuffered), hydrogen peroxide (also in the foliar feed), and plain water flushes of the water reservoir (also with aspirin, 40mg crushed and unbuffered and 1.5 ounces of 3% hydrogen peroxide) alternating with nutrient solution changes. Watch my videos and see how well they survived and grew!
Story#2: Tomatoes take center stage here because they are easily stressed: I have also used aspirin with many of my other plants, including a hydroponic watermelon! But back to the tomatoes: I was growing cherry tomatoes in a cold, over-winter greenhouse experiment, with temperatures dropping down into the low 'teens. I couldn't get the greenhouse heated over sixty degrees F and so the plants grew but did not produce very well. In early spring, I decided to do another experiment and put the tomatoes out into the big bad real world without any greenhouse protection from deer, the elements, or insects. I cut the tomatoes way back, almost sticks of stems, placed them on a bench, and filled the reservoir with plain water, 65mg of crushed unbuffered aspirin, one teaspoon of magnesium sulfate, 1.5 ounces of hydrogen peroxide, and two eyedropper DROPS of sodium hypochlorite (household bleach). The tomatoes roared back to life under the spring sun and were never attacked by deer or insects! You have to consider that these were completely passive wick plants without bubbling or sprays!
Aspirin in large amounts is toxic to humans. I haven't found any data about aspirin toxicity while growing plants. One agricultural study stated that they used up to 250 mg to a gallon of water. The most I've used was 65mg to a half - to three-quarter gallon of water with no discernible negative effects. Again, my stuff is experimental and you might get a completely different result. If you've got a sick plant and you don't think it will make it then give aspirin a try. I would use 40 to 65 mg per gallon to start and work up from there.