We had a really hard frost, -9 C. I had picked a lot of the swiss chard the day before and thought the rest was dead. When I went out to pull up the rest, there were lots of nice leaves that had survived just fine. My Mom would grow Swiss Chard on Saturna Island and it would survive all winter. It's a bit milder there than the Lower Mainland, but I wouldn't be in too much of a rush to pull it out. It will probably survive under your plastic greenhouse, and you will probably get to harvest leaves for quite awhile yet. If you leave it there, you may even get an early crop in the spring.