I grow U. prehensilis and U. tricolor outside, under 40% shade
cloth and with periodical (automated) overhead misting - both
flower very well for me (i.e. at the moment!). Of course U.
prehensilis grows naturally not far from here so it should be
happy, but I think the answer is constant moisture. Overhead
misting is probably not practicable for many amateur growers but
sufficient can probably be applied by using a hand atomiser two
or three times a day. I did try and cultivate U. tricolor under
lights where it did very well but never flowered - the
inflorescences kept dying back. Our winters are relatively mild
with the temperature seldom dropping below 0 deg.C for any length
of time.
I have six different forms of U. livida, two forms of U.
sandersonii, U. prehensilis, U. tricolor, U. gibba, U.
appendiculata (I think!) and a few other unidentified species
flowering in my collection at the moment. They have done very
well this year and are putting on quite a memorable display.
Cheers for now.
Best regards from
Rogan in South Africa.
(ROTH@GATE2.CC.UNP.AC.ZA)