PageRank is such a small part of Google's algorithm that I would not worry about link 'juice'. IMHO I don't really think it's conveyed in the way much of the internet seems to, and even if it was once Google will have changed it by now to stay relevant and deal with the link farms.

The issue you have is that these plant name queries are highly competitive and your site is a garden design tool not a nursery or sale site.

My advise would be to look carefully at your site title, page titles and descriptions and compare them to the top ranked results on google searches you'd like to be in (and to the way stack exchange works), I would also add facebook and twitter shares for each page in addition to your google+ button.

It may be worth introducing a tagging system into your site to make it more easily searchable for users.

PageRank is such a small part of Google's algorithm that I would not worry about link 'juice'. IMHO I don't really think it's conveyed in the way much of the internet seems to, and even if it was once Google will have changed it by now to stay relevant and deal with the link farms.

The issue you have is that these plant name queries are highly competitive (even wiki struggles) and your site is a garden design tool not a nursery or plant sales site.

My advise would be to look carefully at your site title, page titles and descriptions and compare them to the top ranked results on google searches you'd like to be in (and to the way stack exchange works), I would also add facebook and twitter shares for each page in addition to your google+ button.

It may be worth introducing a tagging system into your site to make it more easily searchable for users.