Ask Mr. Smarty Plants

Interesting question!   Mr Smarty Plants actually had a talk with the Wildflower Center management about a year ago with respect to that question and Habiturf™ ! [Please insert here the proper disclaimers about this being personal opinion and not legal advice or direction!]

For a concept to be patentable, it needs to be a novel concept, not already in practice, and not immediately obvious to the average practitioner. So, if you really have a special new mix, and it has not been documented as being in use anywhere [an article in a magazine or a journal] - Then, technically:  Yes - You can patent it.  If somebody else has done it, and your documentation precedes theirs [or yours exists and theirs doesn't], then you are still good for it.

There is also the question of "Do you want to?"  It costs money to file and to maintain a patent.  If you are wealthy enough, then of course, a "vanity" patent might be fun, but most patents are filed for commercial purposes.
In this case, once again we get back to dollars - are you prepared to pursue legal defense if you find your patent being violated? - and worse yet, if somebody wants to copy your idea - since you have documented what it is in the patent - all they have to do is modify the mix just a little bit to make it legally different and thus not covered by your patent - Arghhh!

That takes us to the next lower level of protection.  If your invention is easy to hide, many of these are held as "Trade Secrets" [Think - the recipe for Coca-Cola] - or - since a Wildflower mix is pretty easy to figure out, then you can hold the name of it as a Trademark, and have the "real thing" be represented by the name on the product [Think Habiturf™]

Hope this helps!   MSP