Jacki Cammidge is a Certified Horticulturist specializing in frugal, low-input gardening and propagation, with lifelong hands-on experience and years as a wholesale nursery head propagator.
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Cinnamon might prevent fungal infections in seedlings and cuttings, but that doesn't make it a rooting hormone.
As it's not a hormone, plants won't get triggered to make roots.
However, one of the most beneficial uses, besides its delicious smell, is to eliminate fungus.
Using cinnamon as a fungicide on plants is pretty simple; sprinkle it on, wherever you see a fungus starting, or if you want to keep seedlings safe from damping off.
So you can see how it might work on rooting plants. You would dip the cutting into powdered cinnamon, then stick it in the soil.
Wherever the fungus is exposed to cinnamon in its early stages, it would stop it from growing and infecting the plant material before it roots.
What really happens is it gives the plant more time to root instead of rotting.
I can see where the myth was born that cinnamon is a good rooting hormone, but now it's busted.
Keep some cinnamon on hand if you want to eliminate fungus infections and keep your cuttings and seedling safe, but don't expect it to be a magic bullet to make your cuttings root better or faster.