You buy little packets of the seeds; they’re available everywhere around here, even in grocery store seed displays. I’m actually very surprised that some of you folk seem to have had so little experience of forgetmenots as a decorative garden flower; they’re quite commonplace both here and in England as well. And Ken, in West Grove (There’s a West Grove in PA, in Alberta, and in Cardiff, so (?) ) clearly has 'em.
I never bought my original seeds, though. All my forgetmenots started about ten years ago, before I got into Backyard Railroading, with five seeds from one very sickly, neglected little flower growing in hard, dry clay in a dark corner. The next year I nurtured and protected my first five seedlings - they were my special little treasures that year. Everything else started there. At that time I had no idea that my love of forgetmenots would develop into this weed-defense strategy.
Answers to questions: Depth of root, I dunno, never a problem - Root is fibrous, 2-3 inches, maybe 4. But they pull out easy and complete and do not sprout again like taproots such as dandelions. That’s the whole point. You now have a forgetmenot instead of a dandelion. It’s a choice I’d make anytime.
As for Wisconsin’s attitude, that’s their business. I know I can eliminate forgetmenots from any area in my yard that I want clear of them, for example, a part of the Railroad now fully scenicked or planted with groundcovers and bonsai.
I can just pull them up and that’s it.
Their seeds don’t blow around in the breeze like thistledown or dandelion fluff; they just drop.
When the seeds are dry in late June, my wife pulls up the plant skeletons in great bunches, and I treck around shaking the seeds off wherever I want to encourage forgetmenots. Meanwhile, baby forgetmenots do their thing, growing their greenery low to the ground through the rest of the summer, and occupying any vacant ground that ordinarily would be a magnet for weeds. To Wisconsin, “Better forgetmenots than dandelions” is all I have to say.
Last year I almost entirely neglected both my garden and RR because I had to deal with two very aged and sick parents, driving them to endless appointments, chasing ambulances, finally moving them into a home, and then renovating and selling the old homestead, so I had no time for anything else. Despite this almost total neglect of my garden, this Spring I only had ten young dandelions and two little clusters of goldenrod to deal with. That’s a pretty darn good record. This year I assume I’ll be able to keep up with things in the garden a little better, and get back to a pretty well weedless situation.
Somebody asked about light conditions. They like full sun for sure, but they do well almost anywhere. Remember they put on their big act before the trees have leafed out fully in any case. That patio picture of mine, above - the patio is in deep shade all summer from a huge overhanging maple tree growing in the southwest corner. That didn’t prevent last year’s babies from growing up just fine, or coming into full flower over the last week or two.
If you can’t get seeds, send me a buck for postage and a SAE & I’ll fire down an envelope full. If yr in Canada shoot me a SSAE, not a looney. 
When I do the seed thing I’ll just shake a bunch onto a tarp & save them for anyone who requests 'em, no sweat.
The last couple of years we have thrown thousands and thousands of seeds into the composter or garden trash.
I might just add here that two of my neighbours have copied my forgetmenot strategy; their yards look great! You can guess where they got their first handfuls of seed…
Steve: that’s viper’s bugloss, a very invasive alien from Europe, capable of growing in either dry soil; or marshy conditions and a serious threat to wetlands and to native plants such as bullrushes. It has a horrendous root network allowing a single plant to take over acres of ground, and is very difficult to eradicate. Be careful what you wish for, and even more careful about what you plant. Good luck with that. I wouldn’t be surprised if in the end you’re forced to use Roundup on it. Whatever you do, be sure to get it ALL.
I still intend to tell you about my other anti-weed techniques. There are several, and used all together they constitute a fail safe method of winning the war. Stay tuned! 
