I don't believe that Miller intended to have his audience think that Willy had been planting seeds on a regular basis. I think the basic idea is that he kept thinking about planting a little garden but never got around to it. Now it is too late because the tall buildings that have sprung up all around the Loman property shut off most of the sunlight. Willy does not act like a man who has had previous experience in planting seeds. He keeps reading the instructions on the seed packages and trying to follow them out there in the dark. No doubt Willy had given some thought to what he might do with himself if and when he retired, and one of the things he thought he might do was to have a little vegetable garden. His decision to buy the seeds and start his garden that very night suggests that he is beginning to realize he may have not other choice but retirement. The mortgage is paid off. The boys are gone. He and Linda could live pretty simply, and it would help to grow some of their own food. But he has waited too long. 

This scene of Death of a Salesman occurs after the infamous "restaurant scene" where Happy and Biff leave Willy alone in the restaurant, hallucinating and to his own devices, while the brothers leave with two unknown women, Letta and Miss Forsythe, who are just easy girls that Happy procured for himself and Biff.

This is a poignant moment for Willy who, suddenly, decides to buy seeds and goes straight to the hardware store from the restaurant to get them. The symbolic role of the seeds is that they represent two things: a) Willy's real side: the lover of nature, and the artist of the natural world which he denied himself to become and, b) the fact that Willy, at 63, has still not been able to harvest the dreams of success and riches that he had for himself and his children. Nothing has been planted, so nothing has been achieved. It is all a metaphor for Willy's last chance to "produce" something.

The changing scenery, and the urbanization of New York no longer permit the sunlight to hit Willy's home, so none of the seeds that he has ever planted have quite grown. This is also significant; Willy has succumbed to a rapidly-changing society the way that his intended plants have succumbed to industrialism. However, Willy continues to try because this is his tragic flaw: he invests time and wastes energy on things that will ultimately amount to nothing. Such is the story of his life.

It is toward the end of the play, and while he plants his seeds, that Willy has new hallucinations involving his deceased brother, Ben. Willy discusses his funeral with him, claiming that people from all over the area will come and pay their respects. He compares his future funeral to that of Dave Singleman, and he is sure that Biff will be quite proud of him when he sees how great his father actually is.

...that boy will be thunderstruck, Ben because he never realized- I am known!

The tragic side of these words is that, at 63 years of age, without a commission, and nothing to provide for his family, Willy continues to hang from the fantasy that he is known. Moreover, he continues to place a lot of weight in the erroneous notion that being popular is a marker of success. Willy still has not learned his lesson, and it is clear that he never will.