They'll produce seed at the slightest opportunity without
your assistance. D. capensis will flower throughout the
growing season. The only reason for removing the flowers would
be if you don't want seed to get into all the neighbouring
pots. I usually remove mine after flowering, but before the
seed ripens. Beware though - the seed can ripen at the bottom
of the spike before the top flowers open!
-- Clarke Brunt (clarke@brunt.demon.co.uk)