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Spearwort flowers only have five petals, these have eight on each flower. Masses of flowers and the bees and other flying insects absolutely love them.
I am convinced that it is from the sunflower family - and probably something called thin-leaved sunflowers. It is close to my sunflower hearts feeder for the birds, so probably came from that.
I thought it was that terrible Fireweed until I saw that it had grown into a tree. Fireweed is toxic to livestock - comes from Africa. I am constantly pulling it out of my lawn and verges.
Its a lovely plant whatever it is. I initially thought it was one of the Hypericum family. My Hypericum, rose of Sharon sometimes called, reaches over six foot if not pruned.