Blight

Necromancy [Death, Evil]
Level: Death 4, Sor/Wiz 5
Range: Touch
Target: Plant creature touched, or plant touched; see text
Spell Points: Death 7, Sor/Wiz 9

The plant withers and dies.

This spell functions like the Death Knell spell (including Augmentation options), except as outlined below.

Rather than targeting a creature with -1 or fewer hit points, it targets a plant creature with any number of HP remaining.

Plants that are not creatures (most are not) can still be targeted and killed by this spell, but they provide you with no bonuses (just like killing creatures that do not have a full HD). Such a mundane plant receives no saving throw against this spell, it simply withers and dies, regardless of health and size.