Lynn, You got me with that one.
I would think it would not matter too much how you separate them, as long as you give the divisions some time to callous over the wound before planting. As I see it, however, there are likely some pros and cons to how you divide them, which might apply to individual situations.
By pulling apart:
Pro: Should tend to follow natural growth patterns in the plant.
Pro: No risk of cutting oneself with the knife.
Con: Expect that the irregular tear would expose more of the plant and root's divided surface to possible infection because of the increased surface area of the division (due to it being an irregular tear)
Con: Possible risk of greater damage to plant and divisions, due to forces being applied to more of the plant structure during the tearing. The knife wound would be more focused on a smaller part of the plant.
By cutting:
Pro: Better control of lines of division, assuring more equal allocation of roots to rosettes.
Con: Leaves get cut by the knife, sometimes not anticipated. Typically not a problem unless the rosette is very small.
Con: If dividing multiple plants, greater possibility of disease or genetic material transfer between plants if knife is not properly sanitize between plants (because of the physical contact with the knife).
That's how I see it, but remember that I have not tried the tearing method. A lot might depend on the size and health of the cluster, as well as the size of the rosettes involved.
-t
