The blooming period usually lasts 1 to 2 weeks, so if a population was blooming in early April, you could find ripe or nearly ripe seeds at the end of April. Twice in the last 10 years, we've harvested seed in the last week of April. We've also gotten seed as late as June 22. So the typical blooming period is May - July; and, because the blooming period is brief, the typical time to look for seed is also May - July. However, some years "start" a little early, and you could begin looking for seed in April if we are having an early spring.
If you have these plants, or know where they are growing, visit them frequently in the spring to see what they're up to. You may see some plants blooming while others have already formed seeds. You'll be able to get some seeds a few weeks after you see them blooming. The seeds will be tiny and black when ripe. Sow them right away for the best results.
We should probably add April to the bloom time. Thanks for noticing.