Vernalization

Many plants grown in temperate climates require vernalization and must experience a period of low winter temperature to initiate or accelerate the flowering process. This ensures that reproductive development and seed production occurs in spring and summer, rather than in autumn. http://en.wikipedia.org/wiki/Vernalization).

In my understanding vernalization  turns on reproductive genetic clock. It works mainly on plant with metabolic activity or imbibed seeds (germinating seeds, seedling etc). It affects not only plants grown from roots but plants on any stage of development including sprouting  seeds. Even vernalization documented on seed stage prior to seed desiccation. If it case for black radish it will explain difference in bolting related to seed source history. Vernalized seed at stage of maturity will be prone to bolting even  under optimal seeding time.

Would vernalization  be a reason for bolting in case of early seeding? Certainly yes, if you get long cool spring.  It is interesting to note that many publication  report  good resistance to low temperatures as low as minus 5 for black radish seedlings, but it is also go with recommendation for seeding when warm weather established. Thus, shifting to late seeding protect plant from vernalization.. A week of cold weather probably will not end up with vernalization. The most often referred parameters  required for black radish vernalization is 10-20 days with temperature   plus 5-8 C. It is also interesting to know that there is term “devernalization” when hot weather turn off vernalization. If you interested to know more about this subject I recommend you to read the article “Memories of winter:Vernalization and the competence to flower” (http://www.biochem.wisc.edu/faculty/amasino/pdfs/MichaelsPlCellEnv23_1145-00.pdf)