I just launched my bootstrapped startup. Pray for me.

I always wanted to plant something, but I never get around to it. Even now I am book marking your site. I guess my lazy ass is going to be the greatest threat to your business. How are you going to effectively combat my/user laziness. How are you going to get the user excited enough - feel this need enough? These are the questions you have to ask yourself going forward.

If you could extend this to Australia, I'd sign up in a heartbeat

Awesome idea, and a great MVP at that, congrats!

I don't live in the US and I don't have any place for gardening right now, but other than that, I'd sign up in a heartbeat.

Congratulations on completing and launching--the site looks great and it's a cool idea.

A totally non-snarky recommendation: require the zip code! I got your "Oops" page with your phone number on it. ;)

Definitely a clean and nice looking entry website. I agree with many others, don't overdo it. Feedback link at landing page is important & missing. Hear good things about uservoice, but have not used it, just an example. 
Good luck with your endeavor.