When I had four o clocks, (which I love)
I did nothing and they would re-seed in the same are every year and were beautiful. When I collected the seeds and tried to have them grow in another part of the garden, not so lucky.

I live in zone 5 also and have very cold, snowy, icy, nasty winters.

If you can keep the first-year plant alive through the winter, in a pot in a garage or some such, it'll return in the spring & the tuber will grow bigger with every year.

The bigger the tuber, the bigger the plant will grow during the season.

Some of my old plants have tubers the size of sweet potatoes, & the plant will grow to 4' to 5' tall every year.

& as citytransplant says, they self-seed like maniacs.