One of the most important symbols in this excellent tragedy is the seeds that Willy constantly refers to and which he tries to plant towards the end of the play. They symbolically represent Willy's need to have something that he can leave for his family, some kind of legacy, after his death. When he tries to grow some vegetables at night this symbolises his deep embarrassment about not bringing in enough of a salary to support his family, and that he will have nothing to leave his wife and children when he passes. Note what he says at the climax of Act II:
Nothing’s planted. I don’t have a thing in the ground.
Here Willy realises that he has nothing to show for his life and work, and that his death will end up in him being forgotten. It is also important to realise the way in which the setting plays an important part in Willy's inability to grow anything. The big buildings that crowd out the sky represent the American Dream and materialism that drive Willy to his premature grave. Miller is making the point that these actually make growing things, or trying to leave a legacy, often impossible.
At the same time, the seeds also represent his nurture of Biff. Willy recognises that his eldest son, once the apple of his eye and the repository of all of his hopes, is growing up to be nothing more than a "lazy bum" as Willy himself refers to him. This of course reflects on Willy's own failure to raise him to be successful.