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Hello there,
perhaps you should reconsider whether really to seed this plant.
Beside it beeing an extremly competitive plant and spreading fast by seeds its stalks, leaves and juice are phototoxic. That means you get serious blisters very much like a severe burn when your skin gets in contact with the stuff + sunshine.
I now that it is often an aim to strive for authenticity when restoring a garden beeing of the profession myself.
My mother sufferd those burns on her arm after volunteering to clear those Heracleum from a meadow when beeing a little careless about her shirtsleeve.
I mean, ist is a real danger for curious exploring kids. Perhaps you find some substitute. There are Leucantemella serrotina, Macleya cordifolia ( a real spreader) and forms of Eupatoria which grow into impressiv specimens. Well good luck with your project,
cheers Linaria

5 years ago I was innocently cutting hogweed away from a path, the burns from the sap still have my arms and shoulders scarred to this day. I don't know where to get seeds, but if your set on growing this plant, you should wear clothing to protect every inch of you. And be carefull not to transfer the sap onto your skin when your changing.

First the sap starts to itch and irritate a little, then the sunlight hits. Excruciating!