There’s a gardening myth that Aspirin tablets can be used to stimulate rooting in plant cuttings, but is it true? There’s a saying that “a little science is a dangerous thing“, and as we’ll see when we dissect this myth, it’s based on some partial facts mixed with lots of misunderstanding.
In this article, we’ll explain the science in way that’s easy to comprehend, and draw some practical conclusions that can help with our understanding.
Aspirin is the commercial name for acetylsalicylic acid (ASA) a common nonsteroidal anti-inflammatory drug that used for relieving minor aches, pains, and fevers, and it’s also used for inflammation and as a blood thinner. Acetylsalicylic acid was synthesized by Charles Gerhardt in 1853 and in 1897 by Felix Hoffmann in the Bayer Company.
Acetylsalicylic acid (Aspirin), which is classed as a salicylate compound, has its ancestry in the related substances salicylic acid and salicin, that are found in the bark and leaves of the willow and poplar trees. These natural products were used as remedies for pain, fever and inflammation in the ancient times by the Sumerians and Egyptians, as well as Hippocrates, Celsus, Pliny the Elder, Dioscorides and Galen. For those unfamiliar with the history of medicine, Hippocrates and Galen are considered the fathers of modern western medicine.
What Role Does Salicylic Acid Play in Plants?
Salicylic acid (SA) is a chemical compound very similar to Aspirin in molecular structure and is also a plant hormone which is involved in signalling a plant’s defences.
When a plant is attacked by a pest of disease, salicylic acid is involved in the process of systemic acquired resistance (SAR), a chemical alarm that is raised by the plant where an attack on one part of the plant triggers the plant’s internal defences in other parts of the plant. This defensive alarm can also warn nearby plants and induce defence responses in them by converting the salicylic acid into a volatile chemical form that can travel through the air. Plants really do communicate with each other!
The Link Between Salicylic Acid, Plant Rooting Hormone and Cutting Propagation
In this section we’ll explain how a misunderstanding of the role of salicylic acid leads to the false idea that Aspirin can be used a s a substitute for plant rooting hormone.
Indolebutyric acid or indole-3-butyric acid (IBA) is a plant hormone that stimulates root growth. It’s present in high concentrations in the growing tips of willow branches, and the same substance is also found in commercial rooting hormone powders, gels and liquids sold for propagation of cuttings.
As we’ve seen from their use in ancient medicine, willow branches also contain salicylic acid (SA) which also plays a helpful role in cutting propagation.
A home-made rooting hormone can easily be made from young yellow or green willow branches, see the article – Home Made Plant Rooting Hormone – Willow Water
One of the biggest threats to newly propagated cuttings is infection by bacteria and fungi. Salicylic acid triggers a defensive response against these disease-causing agents, helping the plant cuttings to fight off infection, giving cuttings a better chance of survival.
Providing the cuttings with salicylic acid from willow water (or acetyl salicylic acid from Aspirin) can be beneficial to pre-prepare them to defend themselves, because when plants are attacked by infectious pathogens, they usually don’t produce salicylic acid quickly enough for their defence.
So, we can now see how the myth probably came about. The Aspirin, when crushed and added to water that cutting are soaking in, acts much like salicylic acid to create a protective response in cuttings, making them more resistant to bacterial and fungal infections. However, it does not induce the cuttings to root, a rooting hormone such as indole-3-butyric acid (IBA) is required for that purpose.
What Does the Research Show?
Looking at the studies on the effects of salicylic acid (SA) on the rooting of cuttings, the results are unclear and inconsistent. To quote one study from 2020, which sums up the understanding at that time:
“Adventitious root formation is critical for the cutting propagation of horticultural plants. Indole-3-acetic acid (IAA) has been shown to play a central role in regulating this process, while for salicylic acid (SA), its exact effects and regulatory mechanism have not been elucidated.”
It’s important to note that some of the studies in this area are not concerned with rooting cuttings, but with inducing whole seedlings or plants to put out additional adventitious roots from the stem.
Other studies have tested the effects of SA after treatment with indole-3-acetic acid (IAA), which is the most abundant and the basic auxin natively occurring and functioning in plants that is responsible for root growth. The synthetic rooting hormone indole-3-butyric acid (IBA) is a precursor of IAA.
Of the studies concerned with rooting cuttings, some find that low concentrations of SA promoted adventitious roots and altered the structure of the root apical meristem, whereas high concentrations of SA inhibited all growth processes in the root. Others show inhibition of root formation outright in some plants, similar studies show root promotion in other plants.
- Norn S, Permin H, Kruse PR, Kruse E. Fra pilebark til acetylsalicylsyre [From willow bark to acetylsalicylic acid]. Dan Medicinhist Arbog. 2009;37:79-98. Danish. PMID: 20509453.
- Taras Pasternak, Edwin P. Groot, Fedor V. Kazantsev, William Teale, Nadya Omelyanchuk, Vasilina Kovrizhnykh, Klaus Palme, Victoria V. Mironova, Salicylic Acid Affects Root Meristem Patterning via Auxin Distribution in a Concentration-Dependent Manner, Plant Physiology, Volume 180, Issue 3, July 2019, Pages 1725–1739, https://doi.org/10.1104/pp.19.00130
- Dong, CJ., Liu, XY., Xie, LL. et al. Salicylic acid regulates adventitious root formation via competitive inhibition of the auxin conjugation enzyme CsGH3.5 in cucumber hypocotyls. Planta 252, 75 (2020). https://doi.org/10.1007/s00425-020-03467-2
- Koramutla MK, Tuan PA, Ayele BT. Salicylic Acid Enhances Adventitious Root and Aerenchyma Formation in Wheat under Waterlogged Conditions. Int J Mol Sci. 2022 Jan 23;23(3):1243. doi: 10.3390/ijms23031243. PMID: 35163167; PMCID: PMC8835647.