I have two of these plants in one of my gardens this year and so far I have zero tomatoes growing. Like you mentioned, the plants look great, growing to about 5 feet tall with little branching and lots of flowers. And although the weather is hot now, it's been relatively cool until July. I have several other varieties of tomatoes growing and producing okay in the same garden. I did wonder if it was the fact that a tree grew near this garden, blocking the much of the sun, except some filtering until noon. Now that I've read your post, I'm having second thoughts - maybe German Johnson is just a bummer.
