Weed?

Weed: "A herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation... Applied to a shrub or tree, especially to a large tree, on account of its abundance in a district... An unprofitable, troublesome, or noxious growth."

-- The New shorter Oxford English dictionary on historical principles[3]

Three of these plants have appeared (among other weeds) in the vegetable patch this year, growing up to 30inches tall. I thought they might be teasel until these flowers opened - they're new to me and none of my books seem to know what they are. Curiously all the flower stems are in threes.

The insects find the flowers attractive so they'll stay until the potatoes are cropped, but do I save the seed or burn them?