The Thaumonomicon entry leads one to believe that they spread by making more of themselves, but I've had this one sitting in the same place for real-time days now, and no more have appeared. It's in the middle of a patch of grass, out in the open, in a taiga biome.

Is it intended that they don't spread on their own? Am I just supposed to make more, or am I missing something?

Hindsight is 20/20 lol thank you.

They grow up things. Similar to vines on walls. It's not explained well, but they don't spread 'out'. Use shears, put it on the ground near the base of a wall. It'll start growing up them, and you can shear those vines and place them else where, so the growth is exponential.