I just found what looks to be a useful online plant palette design tool associated with a website called Landscape Resource. It has an obvious URL (http://www.landscaperesource.com/) that I can't actually make an active link since I'm bound by the site's overly broad terms and conditions which state in part,
8. You may not create a link to this website from another website or document without Landscape Resource's prior written consent.
Maybe I'm the only one who reads those things. I emailed them and had a speedy and friendly reply from Rob Maday who told me that he agreed and that the Terms and Conditions were under review.
The site looks promising enough that you should check it out. Hopefully, you'll someday soon be able to refer traffic directly to their site.