Haworthia koelmaniorum var. macmurtryi gets my vote as the slowest growing Haworthia.  At 6 years old, this little guy is just starting to grow.  The stones in this photo are about 1/4" (6mm) in diameter.  I hope to see this as a mature plant sometime before I die!

Photo from February 2001
Photo in December 2001

Photo 09/22/03
Page Updated 09/23/03

Plant Data: GM 272

Obtained  10/18/99   DEM254-A

Ex Gerhard Marx