Miss Havisham doesn't exactly have a green thumb, because her garden is, well, a hot mess. It's "rank" and "overgrown with tangled weeds" (8.98); it's "miserable" (11.4) and "quite a wilderness" (11.105). But to Pip, it's paradise. When he walks around the "ruined garden" with Estella, it's "all in bloom" (29.77).
On the one hand, the ruined garden symbolizes the ruin of Miss Havisham's life. Instead of being, you know, fertile and blooming, she's let herself dry up in a tattered wedding gown. (Gross. Sorry.) It destroys everything it touches, almost ruining Pip's life and definitely ruining Estella's, depending on what ending you read. So, you could definitely read the garden as symbolizing the loss of innocence—or a ruined innocence that never really existed.
At the same time, it symbolizes the wealth and privilege of high society—which is decaying and rotten. It is no coincidence that the land is sold off and divided up at the end for, presumably, what's going to be a bedroom community for the London middle class.