In November or so, I put a mini greenhouse on top of half my kohlrabis. I eventually added some milk jugs full of water – I think about twenty of them, all that would fit.
Today, I decided to take the mini greenhouse off. The kohlrabis have leafed out entirely, and now have nice big flower buds. Woo hoo!
Meanwhile, the kohlrabis that I didn’t cover, which are right next to them, not only don’t have flower buds, they haven’t even started leafing out yet. (I don’t think they’re dead, because they’re still green and fat, but it’s possible they are.)
And the brussels sprouts on the other side of where I put the mini greenhouse, which I didn’t cover, started leafing out a month later than the kohlrabis under the greenhouse, and don’t have flower buds yet.
On top of this, I took the plastic sheet off my potatoes on the other side of the garden. The grass that was under the plastic is not only three times taller than the grass next to it, it has already completely gone to seed and looks like it has fully viable seeds. No other grass in my yard has done that yet, including the grass right next to those patches!
So! I think putting plastic on overwintering crops makes them flower much faster than normal!
This may be very useful information if you have two crops that tend to flower at different times, and you want to get them to sync up so you can cross them. 