comfrey tea

Dont know myself but a few people on my allotment site store some and use next year. They have done for a long time and appear to get good results.
I have just used my last on the overwintering veggies and compost heap.(works well for me) Will start a fresh brew in the Spring

Might be worth getting it out of the cold for the winter?  Dunno if it would effect he chemical composition at al, it sure does for some organic chemicals, but even if not the bottles might break if they freeze!

Don't use the stuff myself, but found this information for you, although you might wish I hadn't! 

I  never finish the tea ( can't drink it in one session)by the end of the growing season so I always have some left. I use it in strict rotation using the oldest first!!