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This sounds a lot like a Goosebumps book I remember reading. Does this look familiar?
http://www.scholastic.com/goosebumps/books_audio/index.htm#/classics/22
That sounds almost exactly like The Plant That Ate Dirty Socks, except those plants ate socks instead of meat (obviously) and the one plant said "Ex" instead of "Skoo," though they explanation was the same.
They did start drinking orange juice at some point.
Edit: If it's not either of those, perhaps it's Mc Mummy, by Betsy Byars? [2]
I'm now pretty sure that it was in fact one of the Plant That Ate Dirty Socks books - the title and general plot definitely ring a bell anyway. I'm not sure where I got the greenhouse and eating meat aspects, unless I'm conflating those elements from other books (maybe even the goosebumps book that was mentioned, which I do remember reading).

A young adult book where the main characters were secretly maintaining a greenhouse full of mutated meat-eating plants, which were described as growing on vines but otherwise looking venus-flytrap-like. It must have been part of a series, because I remember a lot of exposition about the backstory. The main detail I recall is that when one of them fed a plant, it burped, then said "Skoo!" - it was explained that they'd been trying to teach the plants to say "excuse me", and that's the closest they could get.