Hello reader! I'm sure you've come here to scroll down the the part where I tell you how to get your trollweed to grow, so I'll make it easy for you.
Step #1: Cross-Breed several different plants to eventually get a Ferru plant.
Tip: I've found that mixing reeds + melons work well. You can
also try stickreed + stickreed. Your best bet is to lay a bunch of
crossbreeding crops down between several plants until you see a tiny pale green
stalk pop up. Also make sure that once you've successfully grown a
Ferru(Trollweed) plant, remove all other crops from around it in order to give
it more air.
Step #2: Wait until the Ferru plant reaches this stage:
Tip: Don't worry about fertilizing it until then.
Step #3 Place an iron ORE block directly underneath the soil patch containing your Ferru plant like so:
Step #4 Repeatedly fertilize and hydrate your crop every 10 minutes or so. You don't have to, but it will take a hell of a long time if you don't.
Step #5 Your crop will reach the final stage and will be ready to harvest. Now just cross your fingers and hope you get a seed drop!
Ferru Plant Stages:
Stage 1:
Stage 2:
Stage 3:
Stage 4 (Finale Stage):
because if you wasted your time trying to get ferru from regular reeds then... sorry 