A corpse flower is a certain sort of flower. With this meaning, the sentence could be read literally as a question about the flowers the person planted. However, there is a double meaning in this line. Corpse can also mean “dead body”. A dead body is a impurfying entity. A possible interpretation of this line would, therefore, be that mankind even pollutes their self-created nature by burying dead bodies in their gardens.
Gardens are a symbol of men wanting to dominate and control nature. They appear several times in the poem. There is the Hyacinth garden, there are the gardens with the frosty silence and other gardens in the Unreal City. In the form of these gardens, men create their own nature, within their range of control. Gardens are orderly and beautiful and predictable. Nature outside of gardens, real nature, however, is anything but predictable. It can be harsh and cruel and deadly. These features are taken from it, when creating gardens. Gardens are some kind of “fake nature”, that only present the good features of nature. They are not threatening. They should be beautiful and pure.
Pollution, however, does not make a exception for them. Mankind is, therefore, even polluting their own creation. They are not only polluting the wild, untamable nature outside of their control, but also their very own gardens. Pollution is slowly turning all kinds of nature, no matter whether inside or outside of society, into a Wasteland.
The dead body is, furthermore, polluting not only nature but society as well. Gardens are part of domesticity, the household of a person. By burying a dead body in ones household, this household or domestic realm of a person gets polluted and contaminated.
This line is also connected to the very opening of the poem: “April is the cruelest month”. In April the first flowers start to sprout, the snow that covers everything in forgetfulness and hides the secrets and sins of everyone, is melting and leaves behind the cruel reality. Pollution becomes visible in April, because there is no longer snow to cover it and everything that had be buried comes back to the surface.