Thanks for the feedback. No worries about hurt feelings - unfiltered feedback about people's instant reaction is exactly what we are looking for.

First, let's get the tech specs out of the way: the site runs on Ruby on Rails and is powered by a PostgreSQL database. The system was developed by myself and Brian Lambert from scratch.

Make the site "greener" that gardening feel needs to be stronger
I agree. What you see here is my own design, and while I'm reasonably happy, I am not yet fully satisfied for the same reason, i.e. it has to be clear within half a second that this site is about growing plants.

Make it more more obvious when a question has an answer
Working on it as I'm writing this. In fact, what we are implementing is a way for the question poser (and/or the community) to announce when s/he is satisfied with at least one answer. A question could have many answers, but still be "open" in a broader sense because non of the answers seems satisfactory.

Make it more obvious that there are other people participating on the site so that one immediately feels a connection to the community.
Good point. Right now this should be obvious through recent questions, but it could made more obvious (i.e. show when answers have been posted, when votes have been cast etc.)

The gardening images are neat but need to be bigger and more eye catching
Very true and very challenging at the same time! Finding good, eye-catching photography of plants that we can use on the site without violating someone's copyright is difficult. Our hope is that there will be many nice images that are uploaded by users that we could use (because all content on the site is under the Creative Commons license).

The dropdown menu with the plants is not that that great, difficult to explore. Should be a page with images for each plant and a small snippet of info that shows how many questions/answers a given plant has. Just try to explore a few plants right now see how much clicking it would take.
Yes, you are right. There should be a faster way.

Your landing page for a new user lists the wrong incentives. Tone down the creative-commons thing (it is nerdy and gardeners are usually not), same with the reputation gain (it is too premature to promote it, you are actually scaring me by telling me I have to put in work here). These are all in one's face but why is that important for a gardener? Just put the best questions and answers there. That's the only reason someone comes here, I want awesome answers with minimal effort or commitment.
The landing page will eventually just show current activity. Indeed, we would hope that in the future, people won't even come to the landing page in the first place, but get straight to questions and answers, perhaps through Google.

We've noticed that without the banner at the top of the page, a lot of people think that this is just another forum. It is not, though, and we wanted to make sure we get the main points across right in the first few seconds when you are on the home page. Also, once you're signed in, we won't show you the banner.

I actually disagree with you on the notion that gardeners are not nerdy. You and I are nerdy :-) Look at many of the garden forums out there - painful to read (you have to read through a lot of noise before you get to the good answer), painful to look at, and not engaging. Many of the really cool tools I use on a daily basis are created by programmers for programmers. I guess many gardening platforms are designed by gardeners for gardeners, which is why they often lack the technical sophistication, and feel like stuck in the nineties. PlantVillage is to some extent a hybrid, we want to bring the great tools and concepts that we see in our daily technology-driven lives to the food growing community.

But your point is well taken, and we'll keep thinking about how to best convey the message within the first few seconds.

Right now the useful information on your main page is no more than three lines at the bottom. A large number of the potential uses will not immediately register so make sure the site is useful for them as well. This is the audience that will eventually become your registered userbase.
I hope you are right - the community will become stronger with more registered users. I certainly feel strongly that you should be able to read the page without registering. I personally despise the sites where you have to register just to even read content (unless of course that serves a purpose, such as privacy protection).

In the future, we'll have more ways for users to participate, and to get the relevant content to them, e.g. through emails and social media. We're working on these features.

Thanks again for the feedback, and I hope you keep coming back.