These are #Kale flowers. This is one of the Kale plants which I grew from seed last year and which I've kept in the ground over winter.
The seeds I sowed much later in the year are still producing eatable leaves, although something or someone has been eating more than I have. I have some new seeds just starting to germinate for this year's crop.
I'll soon be digging up and composting these older plants. It seems a shame but everything returns to the Earth eventually.