pocketmine\level\generator\populator\TallGrass::canTallGrassStay PHP Method

    private function canTallGrassStay($x, $y, $z)
    {
        $b = $this->level->getBlockIdAt($x, $y, $z);
        return ($b === Block::AIR or $b === Block::SNOW_LAYER) and $this->level->getBlockIdAt($x, $y - 1, $z) === Block::GRASS;
    }