Linda, the 250 hrs vernalization is what I remember also, but it doesn't have to just be at night. And that info came from Johnny's Seeds. I just checked their site and they have now changed it to:
"Time transplanting so plants get 8-10 days of temperatures around 50°F (10°C) to induce earlier budding. Protect from frost."
Go figger, eh?
Last year when I grew them I was fortunate and had the required cold temps and actually ate artichokes! Yay! Unfortunately I found out the deer also like them and ate their share, too!
I'm growing them again and they have met the original 250 hrs vernalization already so maybe I'll have good luck, or good fortune again this year.
Hope this helps.
Shoe
