Well, I wouldn't know from experience, but I've read around here and other places that it won't kill them but _may_ adversely effect their growth. this is especially dangerous to seedlings, but some people don't even like to leave their roots out in the light for 5min when transplanting.

I'm interested to see if anybody else knows for sure how this effects plants.

But yea, I'd say cover the jugs just to be safe. don't use aluminum foil because it will overheat the jugs (i've done this before )