Despite the abundance of plants near and far, the green, growing world hasn't inspired many menaces for Labyrinth Lord. But this is a world ripe with potential for the truly terrifying. Monstrous vegetation might sprout from many places and causes: manipulation by spells or curses, contamination by alchemical pollution, meddling by the divine, or even unholy hybrids. The first three possibilities are likely obvious, but the last involves plants or spores transplanted from other planes via plane shift or gate spells, which then cross-pollinate with local flora.
Although fungi are different from plants, they are included here because they fill the same game niche mobile character-eating background scenery.
Bestiarum Vocabulum: Monstrous Plants provides you with detailed accounts of a variety of deadly vegetation for your next encounter.