Answer: Your St. John's Wort, or Hypericum, can and will develop tuberous roots called rhizomes and I'm pretty sure that's what you've been digging up in your garden. Rhizomes develop just below the soil surface from the crown and can extend outwards. Although you say yours were not connected and you dug them up after you removed the original plant, I suspect they were connected by roots at one time and those roots died out once the rhizomes were able to store adequate carbohydrates to sustain them. Fragmented rhizomes can develop new plants. You can certainly replant the rhizomes or segments of the rhizomes if you want more plants.
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