Growing Biannuals Year Round without going to seed

I live near the coast where they lowest temperature on record is -4C. (In New Zealand, so we are just starting winter).  Most biannual vegetables will survive outside in winter, and definitely inside a basic greenhouse

What is it that triggers them to go to seed in spring - the increase of light or temperature?  What is the earliest that these seeds can be planted and not go to seed?  If you want a year-round supply, how is this maintained without having a spring gap? Esp my celery and parsley and brassicas. This is one year of biannual vegetables I found:

Beetroots
Brussels sprouts
Cabbages
Caraways
Carrots
Onions
Celery
Parsley
Parsnips
Silver-beet
Turnips

Have you thought about succession planting?
You’re not going to change the nature of biennials.
First year is for eating, second year is for saving seeds. Most aren’t too palatable in the second year anyway.
So plant succession plantings as often as your climate allows, and in quantities that you want for the table.
And each year, just leave a few to make you some more seeds.