Making them only produce Tangela might help partially.
Though im still not entirely warm on any sort of exponential growth. Its kinda hard to balance.

Reducing the acid damage might also help a bit. But its also something in cautious about, since its EA.
There are are a lot of light armor nations that will get utterly shredded by this.

I dont think im underselling skeletons by far in this comparison though.
Yeah skeletons are encumberance 0, but thats less of an issue when they are unlikely to survive far enough though a combat for it to matter.
Plants meanwhile use ranged attacks, and so dont care about encumberance themselves before the enemy gets into melee.
Cold resistance is a nice perk, but of limited use in most battles. While the plants are also poison resistant.
And yeah being fearless is great. But if ranged fire devastate the opponent before they get into melee, then thats again less relevant.

In the end i dont think its possible to have viable freespawning pokemon be viable without nerfing that pokemon type into the ground.
Instead i propose making a nation specific N1H1 spell that creates a random spawning of those plant pokemon, for 1 nature gem.
And then make the beauties N1 H1.