Plant monsters are among the most rare sort of monsters, and not enough variety of them at low enough CR are present in the official books to have a summoning spell just for plant creatures.
Also, not even close to all other monster types can be summoned. There are no official spells which summon aberrations, constructs, dragons, giants, humanoids, monstrosities, oozes, plants, or undead short of gate which can get you anything so long as you know it's name and it isn't on the same plane of existence as you. That's only 5/14 types that have normal summoning spells in official material so far.
Perhaps awaken will compensate for not being able to summon plant monsters in some way.
Technically you can't summon a creature belonging to the Material Plane, and all the plants belong to the Material Plane. You can conjure only creatures of the other planes. Even Conjure Animals, says that you are actually conjuring fey spirits that take the form of animals.