Congratulations on the launch! It is a very aesthetically pleasing web design. Seeing the product kind of gives me an itch to start gardening :)
Here's some general feedback I had:
1. Instead of "SproutRobot tells you when to plant & sends you seeds," how about an even more general statement, like "We'll give you everything you need (to grow your own groceries)!" I feel as if the current statement limits your what your product does in people's perceptions. Personally, the main reason I don't garden is because I'd have to do a lot of initial work. If you positioned yourself as making the initial work a lot easier, I'm sure a lot more people would be encouraged to sign up. Sure, gardening may mainly be knowing when to garden and having the seeds, but I think most people perceive that there's a lot more to it than that - of course your product will change that :)
2. Why not change the "Get Started" button to "Get Started, It's Free!" and remove the small banner underneath it that says "It's free to sign up!" I like the button, and I think it's one of the more prominent features on the page, so why not use it to induce trial by incentivizing clicking on it?
3. I also agree with one of the comments above me talking about how the actual sign-up page is a bit wordy. While having illustrations like you mentioned would certainly help, you should try to greatly shorten the sentences. It made me think it would take too long to read the page.
4. Sorry for jumping around, but on the homepage, you should also put in some client-side validation to ensure that the zip code field is already filled in before they click "Get Started!" Receiving that error page would be enough to discourage me from continuing.
Other than that, I'm liking the website. I can't comment too much on the actual product itself because I haven't tried it out yet, but I will certainly give you my feedback when I do.
Thanks, David