Getting Rain Lilies to Bloom
Monica Swartz (Sat, 23 Nov 2013 16:25:29 PST)

I grew twenty or so species of rain lilies successfully in a place
even hotter and dryer than Leo's Phoenix, AZ (the blast furnace of
California's Coachella Valley) and learned a few things. The most
relevant was flowering was NOT triggered by rain, but instead by the
changes in pressure that accompany weather fronts. In most places,
rain accompanies pressure changes so I imagine the observation that
the lilies bloom after this trigger is what led to their popular
name. The town of La Quinta, California has three inches of average
annual rainfall, and many years see no rain at all. My rain lilies
bloomed without fail in the days after pressure changes, and many had
never seen water falling from the sky in all their lives. The naming
of "Rain lily" has been confounding correlation with causation. It
would be interesting it know if growers on the coast have fewer rain
lily flowering events than interior continental growers with the
interior's more frequent and greater magnitude pressure changes.
I have also found that rain lilies and many other amaryllids
will be kicked into flowering with a pulse of Phosphate. I have a
salt shaker filled with decomposed bat guano and a big dash of that
often does the trick, though it can take a year to see a flower from
bud initiation in some species.
On another rain lily issue, I now live in Austin, TX where
two species are very common. Some older homes have rain lily lawns
that they mow like grass and mass flower periodically from spring
through fall. Other posts have mentioned taxonomic confusion with
rain lilies, but I have never seen such a muddle as with these two
very different species. I asked Scott Ogden to sort it out for me and
took notes so you'all can go relabel your pots. The giant prairie
lily, Zephyranthes drummondii (aka Cooperia pedunculata or
Sceptranthes) has very broad grey leaves, perhaps the broadest leaves
of any Zephyranthes so it is easy to ID on sight. It flowers most
often in the spring. The other common rain lily from this area is
also white night flowered (probably shares a pollinator) but it has
narrow, more typical leaves and is properly named Zephyranthes
chlorosolen (aka Cooperia drummondii). It flowers most often in the
fall but both species will flower together, especially in summer. I
hope this clears up some confusion, it did for me.

Monica Swartz shivering in a week-long freezing rainstorm