Season summer, cacti obviously in bloom, but when picked, only dropped cactus flesh, no flowers. I had two mods enabled when I generated the world, so it seems likely it was a mod bug, except they were only interface mods (map and status) so how could that affect gameplay? On the other hand I was crashing when I tried to pick the cactus (and a lureplant) until I disabled the mods. I leave this here for completeness sake, if others without mods start reporting cactus issues then you know it's the game, otherwise let's put it down to modding issues!
Summer; picking cactus plants.