According to this site they are sown in the fall.
Mother Nature seems to just let em fall where ever.

Here is a link that might be useful: cooperia drummondii

These flowers drive me INSANE. They'll only come up in the grass in some odd place for me. I've tried seeds in the beds, transplanting bulbs in the beds, babying them, ignoring them.... then they only come back in the grass. So I'm trying to mow but miss the flowers.......

I just don't mow. They are every where when it rains. I plant the seeds when they are fresh. I usually collect them in the first time I see them bloom and get them going fairly well before fall and them plant them. I would plant them now for this nice warm stretch that we are going to have and protect them from freeze and plant them in spring.

I know, I'm no help. I had a hell strip and I would always grab seed and throw them out on it and then one day, 10+ years later it just started to bloom like crazy...Go figure.

But really, when they bloomed I did not mow. And then after they seeded , I would mow. What's wrong with a little long grass for a bit..

Never have had any white copper-rain lilies but my squirrels dig my pink ones up and everyonce in a while I have to transplant them back into my flower bed. I thought I had lost most of mine this year as I did not get many blooms but I watered quite a bit in the area where they were planted and I now see they are doing well but not that many blooms.