Mine (which I assume to be U.tricolor - no label...) does the same thing.
As soon as the scape reaches about 3" high the tip blackens and nothing
else happens. :-( I keep mine in about 1" of water (like all my other
Utrics) - perhaps they do require the "amphibious" conditions Slack
suggests? (Might give it a try this year...)
BFN
+-----------------------------------------------------------------------------+
| John Taylor [Catweasel] | Royal Melbourne Institute of Technology |
| rphjt@minyos.xx.rmit.oz.au | Department of Applied Physics |
| | Melbourne, Victoria, AUSTRALIA |
+-----------------------------------------------------------------------------+