There was a time when I thought the entire story popped out of J.R.R. Tolkien's imagination. Now that I have read some of the Anglo Saxon texts and histories and even some Celtic ones that he was drawing on for inspiration, I look at the story in a different way as I read it.
After noticing that the tall beans in Tom Bombadil's garden had red flowers, I can name that plant although Tolkien did not. It is a 'heritage' variety of bean called Scarlet Runner Bean, a fitting garden plant for an old relic like Bombadil. It made me wonder if there was a real world equivalent to Kingsfoil in the old Nordic or Saxon tales that Tolkien studied.
I have what I think is a good candidate for athelas and that is Wintergreen, as it was known in the Middle Ages. That name covers a few plants and it was what people once called evergreen plants. The specific variety I have in mind is Chimaphila umbellata. The Oxford English Dictionary gives it an obsolete name of "King's Cure" but that name also applied to pyrola as well. Chimaphila umbellata also goes by the common name of 'Prince's Pine' and was used for treatment of wounds, looking at John Gerard's herbal and not Culpepper this time. It's other advantage is that it has long, slender leaves and it grows in forests.
When Ioreth starts babbling on about it in the Houses of Healing, she mentions that she would come upon it growing in the woods and, when Aragorn went in search of it after the attack on near Weathertop, he had to cross the Road to find it in the thickets to the south. The smell is sweet and pungent and by this he found it in the dark. The Numenoreans brought it to Middle Earth and planted it wherever they camped. The Romans were in some part the inspiration for the Numenoreans so a plant with an association with former Roman camps would be great but not essential. There is no plant in Pliny the Elder's Natural History that would suggest a candidate for Kingsfoil.
It could not possibly be herbs like Basil because that requires full sun and the soil in a forest is too acidic to support the growth of many of our common medicinal herbs. I have learned a thing or two about gardening since I first read LotR as a teenager.
Unfinished Tales and other published notes are not helpful on this point which has lead some bloggers to conclude that this was the only plant invented outright by Tolkien. I will disagree with those people because I think Tolkien always drew upon something in the real world like the Roman mines in Lydney Park inspired Moria and the finding of a gold ring in a well with a curse attached to it inspired the One Ring.
There is a tradition of Prince's Pine having been used as a wound covering by warriors in Anglo Saxon tales but I have yet to come across the tale that names it. Sarassen's Woundwort or Consound also has this reputation; has long, narrow leaves; and grows in forests but Woundwort smells bad. Comfrey or Symphytum Officinale is a contender as is Melilot, called King's Clover by Nicholas Culpepper. Unfortunately King's Clover, although a trefoil and sweet smelling, has the rounded leaves of clover. I believe there is an Athelas and Wintergreen/ Chimaphila umbellata has my vote for now.