The initial design of the red weed would require contaminated soil or a tree/large mushroom to grow on. So the player would either have to be the one to contaminate the soil to farm it or allow it to spread through very densely packed trees that physically contact one another likely again requiring the player to set up the conditions for that spread to happen. I thought of letting it just spread without those conditions but I think it might be better to have it only spread because of something the player did so it doesnt irreversibly destroy the whole continent it is introduced to. But… if it spread slowly enough, it could still be contained at almost any stage of that infection.