Here's a look at how the designs evolved over time! The designs on the left are my initial designs from 2018, the middle ones are my redesigns from 2019, and the ones on the right were drawn for promotional material earlier this year (there isn't one for Elena, though). Lily, Amaranth, Moss, and Nettle were actually designed based on rough sketches by Joe - you can see those in the back of the book itself. In general, each sprite is loosely based on their namesake plant.
I think I had initially wanted them to be a little weirder / more exaggerated (and my art was just really different back then), but as time went on I became interested in making something softer and more simplified. I ended up altering their color schemes in the end, too, because it turns out saturated pinks, purple, and blues don't translate into CMYK well at all.
I remember being kind of busy at the time, so I didn't actually draw many pre-book images of the characters aside from Wisteria. I basically just hit the ground running once I started drawing pages and got to know them as I went along, but I think next time I make something I want to get to know the characters better beforehand and save myself some trouble...!
In the end, I'm happy with the designs that appear in the book, but I do have a huge soft spot for Moss's earlier unkempt look.