I visited some friends this June in the UK and couldn't believe how extensively they just grow wild in virtually all the fields. This goes for both fields left fallow and even those that had been cultivated and planted. One of my friends in the West Midlands has sent me packages of various colours and flower confirmations, but I have been reluctant to waste them in my yard yet. However, I did buy a potted/flowering Icelandic Poppy at one of the local nurseries nearby and it did well until things started heating up by the end of May. It had several seed pods that I am told will allow them to self seed from year to year. I hope that one will, but it was so beautiful that I figure I will just go ahead and scatter the seeds that I have this Fall when things cool down.
My friend tells me that he thinks that they actually do better if you just scatter them in and amongst the grass along a fence line, or just about anywhere around other plants and shrubs. He says they should overwinter just fine, germinating in the Spring just fine.