I think this might have been one of the later Wyndham novels, because it has incident, which makes it easier to read than, say The Kraken Wakes, where civilisation is destroyed by some nasty creatures who live in the deepest parts of the ocean but probably arrived from space.
Wyndham was quite keen on destroying civilisation in his novels - in the Chrysalids nuclear war has wiped out most of the population. This time civilisation is destroyed by a bunch of genetically engineered plants!

This book and the Chrysalids contrast with The Kraken Wakes and The Midwich Cuckoos in that in the former pair the troubles are all down to humanity rather than alien invaders, a pessimism that is more realistic and perhaps was quite commonplace in the 1950s. There can be no doubt that this book is heavily informed by the Cold War, though it is more subtly handled than in the Chrysalids.

Despite Wyndham having what must have been a fairly progressive attitude to women for his time, the male protagonist of this novel shows some really outrageously sexist attitudes at times whereas, at the same time the female protagonist is portrayed as woman about ten years ahead of her time and having suffered for it.

This novel has some serious points to make whilst telling an entertaining adventure story - read the end carefully to find out what was really happening at the beginning and discover what the book is actually about.