Playing around and experimenting with the trees was fun and all, but eventually, it got boring. The trees grew slowly, and Lucy and Mindle needed to have a lot of patience whenever they did something. Scrael also hadn't found much else she could do except make tiny dark purple branches sprout all over the place.
Toz and Nil could create something, and if they left it with enough mana, it wouldn't dissipate before the tree finished growing and replacing it. Lucy and Mindle had to constantly continue supplying mana and keeping their magic active if they wanted to direct the trees' growth.
So after they finished with what they had first set out to do, they didn't continue.
Lucy had created a figurine of dark, almost black, wood for every member of their group.