One of the problems with talking about something like plants, a type of life that is so far removed from us biologically, is that it’s really hard to conceptualize how they’d actually function if they were on our level. We have barely any understanding of how the dumb plants communicate with one another, so it’s nearly impossible to predict how a smart one would. That being said, I think it’s important to try to understand them from their level, so:
One of the main ways that we know plants communicate is through releasing chemicals into the air and soil. For example, if one plant is under attack by insects, it releases a chemical that warns all other plants in the area, who respond in kind. An intelligent, self-aware plant race could theoretically evolve to mimic and use these chemicals in order to manipulate the behavior of less advanced species and, in essence, quasi-domesticate them.
In this way, a plantoid could manipulate a forest to their will without even moving. If they needed to starve an enemy race out, they could release one chemical in order to have some of the local plants produce more toxins (which is a real thing plants do), or selectively release a chemical that causes food-producing plants like berry bushes to die off. They could release a chemical that attracts harmful insects (again, a real thing plants do) like mosquitos. They could prioritize the growth of plants that are lethal to animals. Theoretically, they could even influence the weather by releasing a chemical that manipulates the local flora into reducing the water vapor they release (again again, a real thing plants do during droughts), thus reducing evaporation and creating a temporarily drier climate.