We've had an overnight low of 21°F already, despite the unseasonable daily high temps. One of the things I prefer to do after our first few frosts is harvesting our Jerusalem artichokes (Helianthus tuberosus) because of the chemical changes they undergo - the starches change and they become a little sweeter.
If you haven't yet (and you haven't), please suggest a plant nursery doing good things in your region (or a region other than your own, that's fine too) so others visiting can put their plant dollars towards nurseries engaged in good works and social equity.
i have photos somewhere on my phone, but we began clearing the back section of our yard for the eventual meadow installation in a year or so! we raked up all the leaves/sticks/junk and got about 1/3 of it covered in cardboard to kill everything off. i need to read back over my materials, but i think the idea is to uncover for a month every few months to let the weed seeds germinate, then cover it back up to kill them off.
actual seeding will happen in early october of next year, likely with a mix from prairie moon.
The seeds should germinate just fine under your cardboard, unless you've got a ton of the species that need light to germinate (most don't, in my experience). That's awesome though, I can't tell you how excited I am for your project!