I know that collards can survive after -18oC, kales - after -14oC, cabbage plants - after -7oC. Is there any Brassica oleracea crop tolerant to -22oC (outdoor culture)?
To my knowledge, apical meristems and stems of oleracea crops die back at about 0-10 F, depending on conditions like soil moisture and humidity. I do not know of any Brassicas that overwinter above ground at those extremes, most are mediterranean/oceanic natives.
My cabbage-expert-colleague (Piotr Kaminski) answered the following:
When asking for survival rate, we need to also ask about the time span for the stress. Obviously, the longer the frost holds, the more damage the microcrystals will have caused. If we're talking short time here, -18C will be doable for kale (which overwinters in the fields in Poland). Collard is, generally speaking, a broad leafy brassicas group, with a very likely broad variation in frost resistance. They might be some wild forms withstanding -22C, but they're unknown to him (think about the group's two-year life cycle!).
Head cabbage (depending on the cultivar) may withstand a short exposure to -8C (as per his personal observations).
Finally, snow will increase the chances of survival (Captain Obvious speaking ;) ).
Marcin, thanks a lot, we have kales that stand well -14-18oC, but I look at cabbage reaction with aim to find the range of survival for several accessions.
Александр Николаевич, специальных исследований морозостойкости белокочанной капусты не проводили, однако при небольшом снежном покрове в полевых условиях отдельные растения сохраняются живыми до -15, следующей весной цветут. Морозостойки в сильной степени отдельные образцы озимой цветной капусты, по наблюдениям Дагестанской станции ВИР, при краткосрочных морозах до -20 в стадии сформированной розетки.