I've mentioned the Bloom in passing a few times now without ever actually explaining it. Here's that explanation, in two versions.
Here's the short one, the one that fits on a back cover: seventeen years before Project Mycelium opens, the world wasn't destroyed. It was dismantled. Power grids didn't fail — they were consumed, absorbed into something that grew instead of exploded. Humanity survived by learning to build with biology instead of machines.
Here's the version that actually matters to me as the person who wrote it: the Bloom isn't really an event in this book. Events end. The Bloom didn't — it's still happening, quietly, in every graft, every symbiont, every thread of mycelial tissue running under every settlement in the series. Calling it something that happened seventeen years ago is a little like calling a river something that happened upstream.
A graft in this world isn't a transplant. It's a negotiation between your body and something that used to be separate. Whether that negotiation holds is most of what these books are actually asking, in one register or another, from the first page to the last. I've taken to calling it connection without consumption, mostly because I needed a shorter way to say it to myself while drafting.
I didn't fully understand that was the spine of the story until I was well into writing it. I don't think that's unusual. I suspect most books know what they're about before their author does.
— C. W. Lindmere