There's no reason why you couldn't model desert plants out of geometry, but anytime you can get away with mapping something onto a card in a production setting, go for it.
A simple, flat poly plane works fine. You can use either a photo map, or paint something yourself. Just make sure to create an alpha channel or a separate map to use for transparency. And if you want, once the card is mapped, there's no reason why you can't model out some definition. Sort of like a relief sculpture.
Just keep in mind that if you want them to cast shadows, it'll need to be raytraced to pick up the transparency map.