The plant, tree, and seed imagry all refer to growth, or at least the potential for growth. When Willie was younger, he was able to plant a garden and make things grow ... clearly there was the implication that Willie was creative and hoped to "grow" as well. Later on in the book we find that Willie's house has been surrounded by tall apartment buildings that block out all the sun and make it impossible to grow anything.
In Act II, part 6, after Willie's final disappointing dinner with his "boys," we find that he has bought some more seeds and is trying again to "grow" something. Ben appears and speaks to him, telling him it is a cowardly solution, but that his insurance would provide a significant amount of money for his family, especially Linda. We know that the flowers/vegetables won't grow, and we also know that his insurance plan won't "grow" either. But, as with many other things in his life, Willie decides to go it on his own.
The greatest irony is that they don't need the money; Linda remarks that she made the final payment on the house the day of Willie's funeral. "We're free,"she remarks ... but Willie isn't there to share the freedom. Once again, Willie tries to "grow" something, but it isn't going to happen ....