I've always thought the erect and smart looking Melancholy Thistle deserved a more cheerful name. But it pays to look at my old edition of Culpepper from time to time. It has the explanation for the name.
It apparently works wonders for a gloomy disposition.
Culpeper tells us, under "Government and virtues":
"it is under both Saturn and Mars. One rids melancholy by sympathy, the other by antipathy; their virtues are but few but those not to be despised; for the decoction of the thistle in wine being drank expels superfluous melancholy out of the body and leaves a man as merry as a cricket"
More sad to look at are the thistles on sheep pastures which are ignored by the white plague when they're scoffing all the wild flowers. These are Creeping Thistles usually. .
It's even sadder when the local management appears and cuts them down for lack of anything more useful to do