Short of plants? Come now, you fail to realize the possibilities! Dragons have fetishes too, after all.

It's because of a Type priority order. Some types, like Humanoid, are very low priority and thus can receive almost every templates and change Type accordingly.

Others Types with higher priority can become anything ABOVE themselves, but nothing UNDER them. Dragons, for example, can receive the Celestial template or become zombies (Outsiders and Undead), but never Werewolves (Humanoid [Metamorph]).

Finally, at the top of the pyramid, there are Outsiders, Undeads and Constructs, which are pretty much final, given their nature. You can turn a werewolf into a vampire, in theory, but NEVER the opposite, because the Undead type has precedence over everything else. Plants are pretty high up there too, so that's why you don't see half-dragon plants.

"half-dragon, because that template could be applied to pretty much anything short of plants..."

Remind me to talk about that one Xanth campaign I played once...

No plan dragon, but maybe a dragon with a magic that allow him to control plant might work.