Despite bearing the name Herrscher of Death, the Authority of the Herrscher of Death encompasses far more than merely causing biological death; its true essence lies in manipulating life—or, more precisely, controlling cells, nerves, hormones... It can manipulate anything that constitutes the very essence of life.
Compared to complex animal cells, simple plant cells are much easier to manipulate, which is why Obito most frequently employs vines and thorns—plants that combine crowd control with lethal damage.
Of course, this does not mean Obito cannot control living creatures; it is simply somewhat more troublesome.
Chapter 91