The English word fern comes from the Anglo-Saxon fearn, which is just the regular word for "feather." This is true across Germanic and some other Indo-European languages, including Sanskrit; the identity of ferns and feathers seems to go back to proto-Indo European on the Bronze Age steppes. The name is the first step in our journey, connecting ferns to a whole world of things that ferns are not: lightning, demons, lightning demons, firebirds, ghosts, witches, the devil himself.
The mad folklorist at Old European Culture has this:
Ferns are non flowering plants. But, according to the lore found in Slavic, Baltic and Germanic folklore from Russia to England and from Estonia to Serbia, people once believe that ferns did flower. But only once a year, on the eve of the Summer Solstice (St John's Eve). The flower appeared and disappeared almost instantly, so the gatherer had to be lucky in order to be at the right place at the right time, and quick in order to pick the flower before it vanished.
The fern flower was believed to bring fortune to the person who finds it. In various versions of the tale, the fern flower brought luck, wealth, ability to defeat demons, fulfill wishes, unlock secrets, and understand the language of animals and trees.
But getting this flower was not easy. As a matter of fact it was an almost impossible task. And very very dangerous...Because the flower was closely guarded by evil spirits, or devil himself. They did all they could to prevent the gatherer from obtaining these "fiery blossoms."
The demons' most common ruse was to throw mist over the searchers' eyes, so they could not see their prize; or, in another common variant, they put the searchers to sleep so they would wake in the morning, too late.
Ferns have an ancient association with lightning, which comes in various forms. Some traditions have held that ferns grow where lightning has struck, while others that it deters lightning, so that lightning will not strike a house around which ferns grow. In Serbia they were sometimes called Perun's Flower, after Perun the old thunder god. In northern England fern was sometimes known as lightning plant. This is from Charles Hardwick's Traditions, Superstitions, and Folklore (1872):
Besides the powers already mentioned, fern has others which distinctly mark its affinity with thunder and lightnings. "In places where it grows the devil rarely practises his glamour. He shuns and abhors the house and the place where it is, and thunder, lightning and hail rarely fall there." This is in apparent contradiction with the Polish superstition, according to which the plucking of fern produces a violent thunderstorm; but it is a natural superstition, that the hitherto rooted and transformed-thunderbolt resumes its pristine nature, when the plant that contained it is taken from the ground. In the Thuringian forest fern is called irrkracety or bewildering weed (from irren, to err, go astray), because whoever treads on it unawares loses his wits, and knows not where he is. In fact, he is in that condition of mind which we English call 'thunderstruck,' and which Germans, Romans, and Greeks have agreed in denoting by exactly corresponding terms. He has been crazed by a shock from the lightning with which the fern is charged like a Leyden jar.
The summer solstice is a favourite season for gathering plants of the lightning tribe, and particularly the springwort and fern. It is believed in the Oberpfalz that the springwort, or St. John's wort (johanniswurzel) as some call it, can only be found among the fern on St. John's night. It is said to be of a yellow colour, and to shine in the night like a candle; which is just what is said of the mandrake in an Anglo-Saxon manuscript of the tenth or eleventh century. Moreover, it never stands still, but hops about continually, to avoid the grasp of men. Here, then, in the luminosity and power of nimble movement attributed to the springwort, we have another remarkable tradition signifying the transformation of the lightning into the plant.
And some traditions make this one-night-blooming St. John's Wort to be the fabled fern flower.
Fern seed was also fabulously difficult to obtain, and also possessed great powers. Two in particular show up in many stories: invisibility and love magic. The Grimm Brothers printed the tale of a man who accidentally got some fern seed in his shoe, and when he got home his family shrieked with terror at the sound of his voice; it took him some time to figure out that he was invisible and then yet more to realize that the annoying thing in his shoe must be the cause.
Hardwick passes on the story of a certain Bangle of Lancashire, who fell in love with a neighbor girl but was spurned by her. He fell into despair.
Doctors had been applied to, but he was no better; philters and charms had been tried to bring down the cold hearted maid, but all in vain. ... At length sorcerers and fortunetellers were thought of, and 'Limping Billy,' a noted seer, residing at Radcliffe Bridge, having been consulted, said the lad had no chance of gaining power over the damsel, unless he could take St. John's fern seed; and if he could but secure three grains of that, he might bring to him whatever he wished, that walked, flew, or swam.
(The motif of powerful wizards having a limp or other physical deformity is so old as to be found worldwide, and here we have it again in 19th-century Lancashire.)
Two friends, Plant, a country botanist, and Chirrup, a bird catcher, agreed to accompany Bangle in his expedition in search of the potent fern seed. Plant said he knew where the finest specimens of the herb grew, and led the way to the Boggart Ho' Clough, referred to in the preceding chapter. The three worthies assembled on the Eve of St. John, at midnight, in this then thickly wooded glen.
As a part of the necessary cabalistic implements. Plant brought an earthen dish, "brown and roof " [rough], Chirrup a pewter platter, which he regarded as "breet enough" for their purpose; Bangle's contribution, which he described as "teed wi' web an' woof," and "deep enough," was "a musty dun skull, with the cap sawn off above the eyes, and left flapping like a lid by a piece of tanned scalp, which still adhered. The interior cavities had also been stuffed with moss and lined with clay, kneaded with blood from human veins, and the youth had secured the skull to his shoulders by a twine of three strands made of unbleached flax, of undyed wool, and of woman's hair, from which also depended a raven black tress, which a wily crone had procured from the maid he sought to obtain." A silence, like that of death, was around them, as they entered the open platting. Nothing moved either in tree or brake. Through a space in the foliage the stars were seen pale in heaven, and a crooked moon hung in a bit of blue, amid motionless clouds. All was still and breathless, as if earth, heaven, and the elements were aghast. . . . Gasping, and with cold sweat oozing on his brow. Plant recollected that they were to shake the fern with a forked rod of witch hazel, and by no means must touch it with their hands. . . . Plant drew his knife, and stepping into a moonlighted bush, soon returned with what was wanted, and then went forward. The green knowe [knoll], the old oaks, the encircled space, and the fern were now approached; the latter stiff and erect in a gleamy light. . . . Plant knelt on one knee and held his dish under the fern. Chirrup held his broad plate next below, and Bangle knelt and rested the skull directly under both on the green sod, the lid being open. Plant said,
Good St. John, this seed we crave;
We have dared; shall we have?
A voice responded,
Now the moon is downward starting,
Moon and stars are now departing;
Quick, quick; shake, shake;
He whose heart shall soonest break, let him take.
They looked, and perceived at a glance that a venerable form, in a loose robe, was near them.
Darkness came down like a swoop. The fern was shaken; the upper dish flew into pieces; the pewter one melted; the skull emitted a cry, and eyes glared in its sockets; lights broke; beautiful children were seen walking in their holiday clothes, and graceful female forms sung mournful and enchanting airs. The men stood terrified and fascinated; and Bangle, gazing, bade 'God bless 'em.' A crash followed as if all the timber in the wood was being splintered and torn up; strange and horrid forms appeared from the thickets; the men ran as if sped on the wind. They separated and lost each other.
Plant lay unconscious at home for three days and Chirrup was found at White Moss, raving mad and chasing the wild birds. As for poor Bangle, he found his way home over hedge and ditch, running with supernatural and fearful speed, the skull's eyes glaring at his back, and the lower jaw grinning and jabbering frightful and unintelligible sounds. He had preserved the seed, however, and having taken it from the skull, he buried the latter at the crossroads whence he had taken it. He carried the spell out, and his proud love stood one night beside his bedside in tears. But he had done too much for human nature, and three months later she walked in his funeral procession.