I’d set the soil type to what you believe the soil is. Since ferns are pretty high water use, you may want to adjust the evapotranspiration in Advanced Zone Settings from the default – perhaps look up the species and see if there’s an ET0. I’m guessing they’re 80-100% ET. Also set the appropriate root depth – my guess is they’re really shallow! Maybe no more than 6", could be only a few inches – check yourself or google for literature on the species you’ve planted.
Once you’ve adjusted those settings, you’ll find that zone type doesn’t matter anymore. So you could set to whatever and changes the root and ET settings.