Sylvan Necromancy (Dual Sphere) states:
You can use your (plant) geomancing abilities on corpses instead of plants. In addition, you can reanimate dead plant life as zombies (or skeletons, if they are made of wood) with the grab special ability. These animated plants are treated as animated objects when determining their statistics, although they do not gain construction points. If you possess Hazardous Terrain, undead plant life and plant creatures you create add damage on a successful grapple attempt as if it were an entangle effect.
Has a ruling in regards to this ever made by a designer or mentioned
somewhere in the book which I missed?
What I am confused about is the line about 'dead plant life' since in theory planks and such are also dead plant life. Would that make a ship a valid target? (Which feels broken since for enchantment sphere animating a ship requires twenty caster levels.) A ruling that came to mind was it targeting individual planks but in that case it would also be valid for single pieces of grass which probably isn't intended. (Since if that was the case a death mage could cast 'mass death magic' and create forty minions with one HD)
Best Answer
Plant life is not a defined game term
The rules refer to plant life in various contexts. As a Verdant bloodline sorcerer you your body can be suffused by it, for a treesinger druid or plant master archtype it refers to a plant companion creature. So there is really no universal definition what is covered by the term "plant life", and what is not, and in extension, there is none for dead plant life.
Given this, I think it is up to the GM to judge how narrow or wide the term is. Some considerations:
It may make sense to require dead plants (or plant parts) that are of matching size to build the effigy body of the undead creature, and use the standard bestiary size for a zombie or skeleton, meaning these undead you create would be of medium size.
This way, you cannot turn a single grass leaf into a zombie, but you may be able to do so with the mound of rotting vegetable matter of a pile of fallen leaves. I think the size approach is also useful for your other question: You could rule for example that a pair of chairs or some planks of the ship animate as a plant "skeleton“, potentially creating a leak, but not transform the entire ship.