To propagate Japanese Aralia (Fatsia japonica), you can take stem cuttings in late summer, dip the cut end in rooting hormone, and plant them in a pot with well-draining soil, keeping the soil moist and the humidity high. You can also propagate from seed, but cuttings are more reliable for maintaining specific cultivars. 

Here’s a more detailed guide:

1. Stem Cuttings:

  • Timing: Take cuttings in late summer. 
  • Selection: Choose healthy, semi-ripe stems with a few leaves. 
  • Cutting: Cut just below a node (where leaves emerge). Remove lower leaves to expose the stem. 
  • Rooting Hormone: Dip the cut end in rooting hormone (optional but recommended). 
  • Planting: Place the cutting in a pot with well-draining potting mix. Ensure the base is covered with soil and lightly tamp it down. 
  • Care: Keep the soil consistently moist. Maintain high humidity (use a humidity dome or plastic wrap).  Roots can take several weeks to develop, so be patient. 

2. Seed Propagation:

  • Seed Collection: Collect seeds from the ripe black berries in late fall or early winter. 
  • Seed Extraction: Soak and crush the berries to extract the seeds. 
  • Sowing: Plant seeds in rich potting soil. Use a warming mat to maintain a temperature of around 80°F (27°C).  Cover the pots with plastic wrap to maintain humidity. 
  • Germination: Expect germination in 2 to 8 weeks. 
  • After Germination: Remove the plastic wrap once seedlings emerge. Gradually acclimate seedlings to less warm and humid conditions. 

Tips for Success:

  • Warmth: Fatsia japonica seeds and cuttings thrive in warm, humid conditions.
  • Light: Provide bright, indirect light.
  • Watering: Keep the soil consistently moist but not soggy.
  • Patience: Propagation can take time, so be patient and don’t give up.