Hi! I recently read a tumblr post (which i then lost but i tested the method and it worked) which showed me how to get tall weeds again.
One thing, this can be kinda time consuming and involves TTing but you COULD gain them by just playing day to day as well it'd just take patience that I don't posess.
So first things first, you have to have ONLY around 60/65 (it was possibly 80/85 but I absolutely decimated my weed population for this lol so) weeds in your town. This number or lower is for some reason the requirement for your weeds to keep growing past their smaller stages.
So for me, who was already using weeds as winding forest paths all over my town, this was bad news. But I wanted some of those really nice tall autumn weeds for the entrance to my town. So I proceeded painstakingly scour my town and pull up the majority of weeds I had. Like 200+ easily. I then placed some weeds where I wanted the tall ones to ideally grow and proceeded to TT a random number of days in advance and voila! A bunch of the weeds grew to their final stage!
I then also made a overgrown wheat farm with this method, but I think i'm hitting the limit and they'll stop growing soon so it's back to the short weed paths now for me