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.

This is a very good idea, and the hive mind concept is what quite literally happens in reality. Trees in a forest do communicate and send nutrients to each other- I work in forestry so this is how I'd think a sentient forest would play out

Their strategy should be to centralize and "play tall" around nutrient rich lands, building forests and jungles, and relying on natural processes like re-wilding to win. Other races should have incentive to harvest resources from the forests but are held back by the wildlife, the more centralized the forest the harder and longer it should take to harvest for resources.

I'm imagining a human kingdom next to the triffids and they have a balanced symbiotic relationship where the forest lets men harvest it in return for protection. Say the humans get into a big war and need resources and start moving into the forest, which the triffids of course wouldn't like. They would much prefer someone like the wood elves, but at least theyre not orcs. Each race would have a different relationship with forests

It would be cool to see the triffids try to convert the whole planet into a single interconnected rainforest. They should absolutely have a hive mind strategy aimed at this. Also consider multiple separated hive forests that can unite and become a single forest if they are able to terraform the desert between them. And other races can split a hive forest in half and create two separate ones. 

Personally I think triffids shouldn't even be capable of moving around like LOTR ents, but should be stationary trees (at least in early game). In order to "move" they plant more triffids. Also when their forest is being cut down they are unable to immediately and directly respond, but rely on the monsters and natural effects of their forest to wear enemies down by attrition, and to slowly infiltrate and overgrow into cities. 

Midgame startegy for triffids could be their mother tree becomes an ent. In forest has some exceptionally big and old trees, which when they reach critical mass are able to walk around. To kill or control a forest would be to prune and prevent these mother trees from growing big enough. Of course this would be difficult for men, seeing as the forest is deep and dark, and it isn't exactly obvious where these trees are, and it isn't simply a matter of killing these trees means killing the forest.

Those are my best ideas for what a fantasy race of tree people would look like, with emphasis on terraforming their territory into nutrient rich areas, and as a race that can form symbiotic trades with other races.