Sorry, I forgot to mention, this is actually something we've fixed in version 12 of KeySCAPE. Prior to that the 'Mix Name is <Blank>' filter wasn't working, causing the single species hedge group to not show. If you wanted to upgrade to v12, you can download from our Support Site.
If you need to remain on v11, then you can do as Rob has mentioned above (Many thanks for the suggestion Rob). In the attached screenshots, you can see that I've set the notes field (Header works too, but notes is a relatively unused fields, so is more likely to be free for this) of the Hedge mix to say 'HedgeMix', then used that identifier in the schedule style to filter that in or out of the desired groups.