If you are refering to Eryngium yuccifolium, it seems to me like the seedheads turn brown when they are ready. Then you can carefully (because they are prickly) snip them off into a paper sack. Then you can store them in a dry place, like your dining room table, for a few weeks to dry further. When they are dry, I break up the heads and manually sort the seeds out. If you are dealing with larger amounts this may be too time consuming. Also, if you are going to plant them yourself, sorting out the seeds may be unnecessary work.
Hope this helps.

I just wait until the seedheads are really dry and I snip off the the very ends of the prickles, and crumble everything up. You will have seeds and chaff, and I've yet to be able to tell them apart. Spread the "stuff" where you want it to grow. You will get seedlings from the mess! April

I've just got into prairie plants lately. I have a rattlesnake master. How hard is it to get the seeds to germinate. How long will it take from seed to blooming stage?

We added rattlesnake master seeds to our meadow mix which we sowed in May 2003. Last year we had plants we could identify, and this year we had full fledged plants with seed heads. Some of the plants are 4' tall. An interesting plant, looks like something from outer space.
I was somewhat dissapointed with the germination rate, but I'm hoping to see more plants next year. Jim

Lantana,

I got good germination from my eryingium seed. I didn't count how many seeds I planted, since I don't separate seeds from chaff in this case. I wintersowed them into a styrofoam cup...and I still got 5 or so plants. Since I don't have a large scale area like some of you are fortunate enough to have, the five plants I got were just the right amount.