Noob question - but just being thorough.
Under Plant Companion it says: Except for the companion being a creature of the plant type, this ability otherwise works like the druid's animal companion.
So animal companion feats in the UW would apply to the plant companion, as well as spells that specifically name "animal companion" (e.g., the Bleed for Your Master or Shield Companion spells in the Animal Archive).
Meanwhile spells that specifically name animal types - like Animal Growth - would not.
(forgot to ask the question) Is my understanding correct?
Yes. The plant companion is essentially an animal companion but is not an animal.