It depends. I havent checked, but right now you are likely getting more than 12 hours of darkness, meaning they would start flower right now. You have to wait until you are getting over 12 hours of light a day.
For instance in new york city right now gets
Sunrise: 6:39am
Sunset: 5:39pm

Means they are not even close to getting 12 hours of light yet


But on march 18

Sun Rise: 6:03am
Sun Set: 6:05pm


Thats when you get over 12 hours of light, i would wait a week or 2 longer at least to avoid early flowering.
The date is based on your latitude too, so if you live in the scottish highlands its later than if you live Brighton. Frost probably isnt an issue for you guys. If you dont want them too big, then wait until june to plant them, if you plant them in april the will be huge come next oct.