We have some large dykes, ditches and plots of apparent wasteland
around here, in amongst lots of arable farmland. What would you think
if I were to plant some sunflower seeds evry now and again on this
land?

I did read somewhere one should never plant corn flower seeds anywhere
apartf rom your own garden as they are quite prolific once they get
started and the farmers hate them. I obviously don't want to get in to
trouble.