The greenhouse radical and the greenhouse kanji are exactly the same!
Note that this isn't an official meaning of the kanji though (you won't find it in kanji dictionaries). If you'd prefer to learn an official meaning, one of them is endless. Imagine an endless greenhouse, that goes on as far as the eye can see.
Your flowers have grown so big that now the only way you can fit into your endless greenhouse is by lying on your back (ばく).