The poem, Strange Fruit by Abel Meeropol, was written in reaction to seeing the dead bodies of lynchings of black people in the southern states.  The bodies hung in the trees for a long time unless someone was brave enough to cut them down.  The last stanza of the poem which contains the lines you are asking about,"For the rain to gather, for the wind to suck", refers to both the bodies swinging in the trees where rain can gather on their faces and clothes and the wind can blow them around sucking out the moisture and leaving them a dry husk, as well as the crops in the ground.  The last line reminds the reader that this is a strange, bitter crop to grow both in the trees and the ground.  The poem was made famous in a song sung by Billie Holiday.