Beats me. I think it depends partly on the type of sword, partly on tank
conditions. Some people have noted that swords are more likely to propagate
via runner when you first get them. That was my experience.

My swords get immense, but don't produce runners or baby plants on stalks.
Some don't propagate at all, they just get bigger and bigger. Others split
into daughter plants, but it's not really visible until you dig the plant up.
Once you're aware that they can do this, you can tell when it's happened
without digging up the plant: you can see that the leaves are growing from two
or more "crowns."



Leigh

http://www.fortunecity.com/lavender/halloween/881/