Trifid bur-marigold: the day of the trifid



I remember quite well the 'scary' sci-fi film of the 1960's based on the book "The day of the trifids". It was with some trepidation, then, that I had my first encounter with the trifid bur-marigold (Bidens tripartita) but it proved to be quite harmless and I emerged unscathed!
Trifid bur-marigold is a member of the daisy family; it is one of those daisies that has the yellow centre to the flower but not a ring of white petals around it. Instead the central flower is backed by a ring of seven pointed green 'bracts'. These give the flower head a rather unique appearance. The central stem is reddish in colour and forms into branches, each branch having a flower on it. It is a fairly typical bushy daisy, about two feet tall, that flowers from July right through until October and is usually found around the dry margins of ponds, lakes and reservoirs. When the seed heads form they have burs on them that attach to animal fur or human clothing as a means of seed dispersal.
Being a daisy it is easy to see why it is called 'marigold' and as it produces burs then bur-marigold is a pretty obvious development but where does the trifid come from? Trifid means split or divided in to three lobes or parts but I cannot see how this relates to this plant, maybe someone can tell me?
Trifid bur-marigold: the day of the trifid

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