Any! I've written them independently. With books two and three of Cascadia Fallen, I'd introduced new characters, which caused some timeline jumping (as we learned how they dealt with the sudden disasters.)
Blades of Grass and Conner's Mission is (and will be) linear. There are a handful of crossover characters that help it all tie together, but you won't be missing anything to read Blades first. In fact, I'm putting out several in rapid order, so if you're a fast reader, that's probably the way to go.