The short answer
Root a spider plant baby in lightly moist potting mix. A plantlet with small root nubs can also be rooted in water, but soil rooting avoids a later change of medium.
Why this happens
The small plants on arching runners can form their own roots. You can leave a baby attached to the parent while it roots in a nearby small pot, then cut the runner afterward.
What to do
- Choose a healthy plantlet with visible root nubs.
- Set its base against moist mix without burying its crown.
- Keep it in bright indirect light.
- Detach the runner after roots establish and the plantlet resists a gentle tug.
Keep in mind
A tiny plantlet in a large wet pot is prone to rot. Start with a small container with drainage.