I didn't like the fixed-width banner at the top of each page. That meant more side-ways scrolling on a narrow screen than would otherwise be necessary. (This stopped being an issue when I turned my phone on its side.)

I found the text pointsize to be unnecessarily large. Less text per page makes it harder to find stuff by scanning.

I didn't have a problem with speed on WiFi, but some of the picture-heavy pages were over 6 Meg which seems a lot. I appreciate the large pictures are important for identification. I'd suggest using large but lower quality thumbnails in the main page, linking to higher quality images that you can use for positive ID. (This would be less of an issue for people who download the site in advance. You might also consider turning it into a PDF to help offline use.)

Could the database be indexed by both location and season at the same time, rather than by one or the other? Presumably the intended use is for people in the field, who are mostly looking for what they can find "right here, right now". A general search field would be good.

Maybe the "full plant list" should be renamed to "Find by plant name".

Generally the pictures seemed pretty good. I suspect that large width and height is more helpful than high quality. I didn't actually try to make any identifications.

I was using a Nokia n97. It looks like it's already a valuable resource, so well done. I hope this post doesn't sound too negative. It's the information which is the main thing.