I'd say it's not. Undead specifically are immune to all effects requiring Fort saves unless they may be cast on objects or are harmless. In this way, undead are really less like "creatures", and more like animated objects (or that's how it feels to me). Because of this, they are not susceptible to flesh to stone, because it can neither target objects nor is harmless.
As for whether or not this works on plants, the statement could be up to interpretation, but since the last ine of the spell's effect specifically states that it only affects creatures made of flesh, it could be easily interpreted that Plant type creatures cannot be affected by it. It really should be errataed to state something more along the lines of "this spell affects all creatures except Plants and others which are otherwise immune" if it is not intended to work on them. Of course, it seems to be a common sense type ruling, which means that if a spellcaster in your game is interested in it, you let him know which way you're ruling. After all, plant matter could, it seems, be considered flesh.