Aspirin rooting hormone is recommended as one of the best rooting hormones for plant cuttings. Dissolve an aspirin tablet in water and soak cuttings in it for an hour. It increases the chance of rooting – or so it is claimed on many DIY sites – but does it work?
Aspirin is Acetylsalicylic acid (ASA)
Aspirin is a common drug used to relieve pain and its main ingredient is ASA (acetylsalicylic acid).
It was known for hundreds of years that chewing willow bark could ease pain. Eventually the active chemical was identified and called Salicylic acid, after its origin Salix, the genus name for willow.
Salicylic acid (SA) is a natural phytohormone found in plants where it plays many roles in plant growth, photosynthesis, transpiration, ion uptake, and defense against pathogens.
Gardeners have taken this information and made the following connection. SA exists in plants and is important for plant growth. ASA reduces pain in humans just like SA, and its name sounds similar, so it must also be good for plant growth.
It is important to understand that although SA and ASA are similar chemically, you can’t conclude they will have the same effect on plants.
What is a Rooting Hormone?
- Indole-3-acetic acid (IAA)
- Indole-3-butyric acid (IBA)
IAA is the most abundant auxin in plants, but it is not used very much for propagation since it breaks down quickly in plants and when exposed to light. IBA is found in very small amounts in plants because plants convert it to IAA.
Man has also synthesized compounds that act like auxins in plants and these include:
- alpha-Naphthalene acetic acid (NAA)
- 2,4-diclorophenoxyacetic acid (2,4-D)
These rooting hormones cause cuttings to initiate root growth. Neither SA nor ASA are rooting hormones, although SA is a plant hormone.
Cuttings Root On Their Own
Many cuttings root on their own without any added rooting hormone because cuttings naturally contain it. Most softwood cuttings from herbaceous plants and house plants root very easily. Most shrubs are easily propagated from softwood or semi-hardwood cuttings.
This fact has led to a lot of confusion about home-made rooting hormones. Place some aspirin in water, treat the cutting and it roots. Then people conclude that the aspirin caused the rooting. It didn’t. The water, along with the natural rooting hormones in the plant caused the cutting to root.
To test a rooting hormone, you need to use a plant species that does not root easily and you need to run controls by trying some cutting with and without the test material. Without a control you can’t claim success with any home-made rooting hormone.
What Does Science Say?
Although aspirin is not a rooting hormone, there is some evidence that adding SA to a rooting hormone like IBA can increase root development and some testing of this has been done.
Olive trees are very difficult to root from cuttings. When tested, SA alone or in combination with IBA had no effect on rooting. In fact, pre-treating cuttings with SA, before using IBA, inhibited root growth.
When mung bean seedling cuttings were exposed to SA, it did increase rooting, however this type of plant sample is quite different than the cuttings used by most gardeners.
When germinated sunflower seeds were exposed to aspirin (ASA), low levels improved root growth, but higher levels stopped root growth, showing it can be toxic to roots.
I found almost no testing of aspirin (ASA) as a rooting hormone, which is not surprising since it is not a known rooting hormone. There is some testing of SA which has mixed results. The effect of any rooting hormone will depend very much on the species being tested, and even on the cultivar. In one case one cultivar showed some rooting and the other did not.
Based on the science it is incorrect to say that aspirin helps all cuttings form roots, and at higher doses it actually prevents roots from growing.
Is Aspirin a Rooting Hormone?
Aspirin is not a rooting hormone and it probably has limited if any positive effect on rooting.
The reality is that most cuttings taken by gardeners root very easily without any rooting hormone. If you feel you need to use a rooting hormone, use a commercial product.