Firstly, there are parasitic plants that use other plants to grow and spread, so plants themselves have enemies.
Secondly, plants literally spread by bearing fruits and vegetables for animals to eat them (and later poop seeds, which also acts as a fertilizer).
Thirdly, what prevents some Triffid offsprings not to be insta-hostile to everyone? Maybe there could be a rarer Triffid heart that actually wants to communicate with others, you know like a “scientist”? Not every “evil monster” is evil (but is still dangerous).