The things that Willie used to plant gave him something concrete that he grew from seed. I don't think it's too big a stretch to suspect that when Willie says this, he is lamenting the fact that his own "seed" has not yielded the fruit that he had hoped for. Neither Biff nor Happy measures up to what he had hoped for them and, no matter whose "fault" this is, it's the reality of what he perceives.
I think this could also be interpreted as Willie's statement about what he could/should have done with his life. One of the things that Willie was good at was working with his hands. It was remarked that there was more of him in the stoop he built than in all the sales he made. Yet, at the end of his life, he has no place to plant seeds; the world has changed; his house is surrounded by "modern" buildings that keep out the sun, the source of his garden's growth. And like this, the world of sales has changed. Willie is an anachronism, no more likely to grown than the seeds he futilely plants.
In "Death of A Salesman," we learn that Willy is an unsuccessful salesman who is having difficulty making money. He brags a lot and acts like everything will be okay because he has bought into the American dream. However, Willy is 63 years old and when he is not dreaming he does have his break downs. He admits that "he has nothing planted," and he has "nothing in the ground to say that he has no leads. A salesman earns his living off leads of companies or people who are interested in buying their product. Sometimes the leads are cold calls, meaning you have never met the people before, and often the sales come from leads that you have been working on for awhile. The fact that Willy has nothing planted means he has no leads, and the statement of nothing in the ground means he hasn't even had leads in the past to begin working with to sell later.
This is very disturbing for a salesman because the seeds to plant (leads) and the seeds already in the ground ( leads that have been worked) are the fruit of the sale. With out these he can't make any money. These two phrases also lend the reader to believe that Willy is saying he hasn't done anything with his life that is important or will bring him greatness. He also has nothing concrete in the works to do so this late in his life. This can also be disturbing for a man in his 63rd year.