It is fairly common knowledge that a crushed aspirin added to the water of a cut flower display will significantly increase its life and beauty. Plants have immune systems just like animals do and when stressed, diseased or "in pain," they exhibit cellular responses to defend against attack. Aspirin in people will reduce pain and swelling and can lower body temperature in the case of fever or local response to a wound. Similarly, plants have responses to injury and damage as well as extreme conditions that aspirin might be able to alleviate.
Induced Systemic Resistance
External stresses on plants trigger them to make metabolic changes and create defensive compounds to combat the intruding organism or disease. Scientists call this induced systemic resistance and it can be reduced in plants that don't naturally produce salicylic acid by watering with aspirin-laced water. Salicylic acid was first discovered in willow bark, and many plants naturally make it as an immune response. This natural response is also termed systemic acquired resistance (SAR).
As an explanation for why aspirin water is beneficial, first consider that a cut flower is wounded where it has been cut. The plant's response will be to send out compounds to aid in healing the wound. Aspirin has been shown to speed up these responses in humans and now in plants. It has also been shown to speed up germination and aid in larger production of crop plants. Some common diseases such as leaf spot and powdery mildew also respond to topical application of aspirin water.
Salicylic acid is a naturally occurring compound found in plants. As such, it would be best for the environment to use such a substance and eschew poisonous chemicals that are man-made. Natural disease prevention and plant maintenance is good stewardship of our earth and safe guards our food and seed supply.
Misconceptions and Warning
It is not as simple as adding aspirin to your plant's water, however. Just as in humans, there can be adverse effects from too much salicylic acid. Roots can burn, leaves can spot or wilt, it can impede the ability of the plant's vascular system to take up water and nutrients, and some plants just can't move the compound through their cells, resulting in little or no beneficial effects. Most horticultural programs or university extensions can provide accurate information on how to use aspirin.
Syngenta has developed a crop protection product that mimics a plant's natural systemic resistance in much the same way that aspirin does. The company is mass marketing it for use against molds, mildews and leaf spot. Scientists are working on other salicylic doppelgangers and new targeted plant immune system enhancers.