Cryptobiosis: A condition in which an organism (or an egg, seed, etc.) is in a dormant and resistant state without visible signs of life, remaining so until environmental or seasonal conditions are favourable for a return to normal activity.*
The seeds in these seed heads are all dried out, waiting for warmer temperatures and water so they can grow. In this protected state they’re highly resistant to the harsh conditions of winter.
*OED Online. Oxford University Press.