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I don't know if trimming off the flower stalks would encourage them to go back to normal growth, or if I'm just SOL with this batch of seedlings.
[Image description: a close up of a mulched garden bed with a bok choy seedling putting out a thin flower stalk.]
Once they start, they really don't wanna stop. Which hemisphere are you in?
Northern hemisphere, I'm in inland southern California. From experience in past years, my best timing is to plant seeds the beginning of August for a harvest in December through mid-February. Later plants don't mature before the aphids start to kick off in spring. This year I was a little late, I only started them on August 14th, but now we've had temps in the upper 90s and even triple digits for a week and one by one they're all bolting.
I've already started another batch of seedlings, maybe with netting I can keep the aphids off enough to get a harvest.
Too late to do anything once they've begun bolting. If you're expecting warm temps you can cover with ag fabric and spray it down, morning and afternoon, to keep temps underneath a bit lower, but that has to be done at the time/just before the heat sets in.