The speaker has no land left, or at least none worth the name. Outside is a "rank yard," overgrown and untended, and the only real greenery is the plants indoors, in the parlor. The poem sits in that small gap between loss and acceptance.
What stands out is the last line. Someone who is "out of land" might be expected to sound bitter, but the speaker is "pleased / to house the dirt." There's a gentle joke in that phrase, since dirt is what people usually keep outside, and here it is welcomed in like a guest. The title plays on the same idea: a greenhouse, but also a house that has turned a little green.
The mood is quiet, wry, and content. Judging by the wording, the speaker has settled for less space and found a modest satisfaction in it. The poem is short and sparse, and that smallness matches the shrunken world it describes, while still leaving room for warmth.