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We have pink rain lilies (Zephyranthes carinata) in the garden. In my personal experience, I've found that they always flower, without exception, shortly after rains. However, there are roughly ten totally dry months each year. During this time, even though I water them regularly with well-water, they never flower. I've tried copious amounts of water, and frequent watering, but to no avail. They seem to depend on rain.

The rains here come at different times each year. We just had a totally unexpected rainstorm in May, causing them to flower. We are inland in southern India. It's very hot here, and the climate is quite dry in the periods when it's not raining.

How can this be? What's going on inside the plant, biologically speaking? How can it "know" whether it's getting rain water or well water? I'm looking for a description of the biological processes inside the plant which cause these different responses to rains versus watering.

Also, (minor, secondary point) is there any evolutionary reason why a plant would develop this trait?

I prefer answers that cite their sources for the information.

Edit: A user added some quite relevant personal observations in the form of an answer. I'm adding it here, as the observations are quite relevant to the question.

I live in central India which is marked by a heavy rainy season from mid June to September and almost dry rest of the year. This behavior of rain lilies has been a mystery to all of us for generations.

No matter what season, what temperature, the lily bush will give you at least one bloom even if rains for a mere few minutes. Rest of the time, any amount of watering does not help. I've seen the plant bloom with just a bit of drizzling in summers when temps cross 40 degrees celsius.

Whereas, a friend is growing them in a pot in a water lily pond. Basically, in shallow water where the rain lilies are just dipping their feet in water all the time. So basically, for a large portion of the year, the plant is meeting the low temperature, high water criterion. It doesn't work. The plants bloom only and only when it showers.

By the way, the behavior is not limited to pink rain lilies. It's the same with yellow and white lilies.

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  • $\begingroup$ These are seasonal rains? Does any rain cause the plant to flower? Plants do have flowering cycles. $\endgroup$
    – WYSIWYG
    Commented May 26, 2016 at 15:09
  • $\begingroup$ @WYSIWYG The rains can come at any time during the year. It depends on a complex set of conditions in the Indian Ocean and Bay of Bengal. It's pretty random. There are some months when it's more likely to rain, though. Yes, it seems the rain causes the flowering. I've seen it flower every single time after rains, but never without rains, even though I water it a lot with water from a well. It should be noted that rain generally tends to be pretty heavy here when it does come. $\endgroup$
    – Fiksdal
    Commented May 26, 2016 at 15:19
  • $\begingroup$ Perhaps they bloom just after it rains because many bees come out of their hive at this time, which would allow the flower to reproduce more effectively and thus get the evolutionary advantage. I don't have any information to back this theory up (couldn't find any), and I don't know much about biology. Can somebody confirm or deny this hypothesis? $\endgroup$
    – DivideByZero
    Commented May 31, 2016 at 5:40
  • $\begingroup$ @DivideByZero We could test this hypothesis by having several plants, and surrounding some of them with insect netting. $\endgroup$
    – Fiksdal
    Commented May 31, 2016 at 6:08
  • $\begingroup$ @Fiksdal - I don't think that's what DBZ meant. I think what they meant is that perhaps the plant flowers because they are expecting (because of past experiences) more bees to come around, not in response to more bees coming around at a particular time. $\endgroup$
    – Kevin Fegan
    Commented May 8, 2017 at 0:06

2 Answers 2

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It looks like sustained or consistent moisture might be (at least part of) the phenological cue for flowering:

The wikipedia article mentions that consistent humidity will induce flowering in at least some species and cites Fernández-Alonso & Groenendijk (2004), which says:

It generally flowers after the rainy periods, but in humid pastures and under cultivation conditions, colonies show staggered flowering during the whole year.

There's a reference in Lange et al. 2013 to Zephyranthes flowering after "watering of at least 10 h[ours]", but I'm not able to find the original citation in the google books preview (which is given at "Nitsch 1965").

Edit: I tracked down Nitsch 1965, who cites 2 sources on the topic. He reports Kerling 1949 as showing that Zephyranthes rosea after a certain period of development goes into dormancy, which is broken when two conditions are met: low temperature and high water, which must be extended over at least 10 hrs. He adds that Holdsworth (1961) presents evidence showing that Zephyranthes citrina and Z. tubispatha are primarily triggered by soil moisture.

Fernández-Alonso, J. L. & J. P. Groenendijk (2004). A new species of Zephyranthes Herb. s. l. (Amaryllidaceae, Hippeastreae), with notes on the genus in Colombia. Rev. Acad. Colomb. Cienc. 28: 177-186

Holdsworth, M. (1961) The flowering of rain flowers. Journal of the West African Science Association 7︰28-36.

Kerling, L.C.P (1949) The gregarious flowering of Zephyranthes rosea Lindl. Annals of the Botanic. Gardens. Buitenzorg 51: 1-41

Nitsch, J. P. (1965). Physiology of flower and fruit development. pp 1537-1647 in Encyclopedia of Plant Physiology Vol XV Part 1, W. Ruhland (ed.)

O. L. Lange, P. S. Nobel, C. B. Osmond, H. Ziegler (2013). Physiological Plant Ecology III: Responses to the Chemical and Biological Environment. Springer.

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    $\begingroup$ Thanks for this answer! Does this suggest that it's the mere duration of the watering that's the key here? This would make sense, rains are typically heavy here and last for a long time. Maybe I could test this hypothesis by setting up some sort of automatic sprinkler that watered it constantly for a few days? $\endgroup$
    – Fiksdal
    Commented May 26, 2016 at 17:36
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    $\begingroup$ @Fiksdal sounds like a great experiment to me! For +1, have a control patch that you don't water / water normally! :) $\endgroup$
    – Oreotrephes
    Commented May 26, 2016 at 17:47
  • $\begingroup$ There's a new answer (which isn't really an answer, but includes very relevant observations). I have added it to OP in case it gets deleted (as not an answer.) After considering this information, I have concluded that we still need to look for different hypotheses here. $\endgroup$
    – Fiksdal
    Commented Jul 3, 2016 at 6:25
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It's a nice question, I've tried looking for research papers to no avail. But I will add a few things that I hope will help:

  1. Firstly, tap water's composition is quite different from rain water- two criteria for distinction that come to mind would be pH and TDS, details follow:

    • Tap water has a higher TDS (total dissolved solids)than rain water, making it harder(1). Additionally it has a higher pH(2). Rain water has virtually no dissolved solutes in it that affect it's hardness and it's pH is acidic(3)(4). Thus while tap water may have a TDS in the hundreds, rain water has a TDS in the tens- a stark difference.

  2. Have you considered that 'Photoperiodism' may be at play? Having grown up in India, I can attest to the over-cast sky after heavy shower, sometimes for hours. This plant may be a short-day plant i.e. it flowers only when the duration of daylight has fallen below a certain threshold(5).

References:

  1. http://www.water-research.net/index.php/water-treatment/tools/total-dissolved-solids
  2. https://www.watersystemscouncil.org/download/wellcare_information_sheets/potential_groundwater_contaminant_information_sheets/9709284pH_Update_September_2007.pdf

  3. http://www3.epa.gov/acidrain/education/site_students/phscale.html

  4. http://ei.lehigh.edu/envirosci/watershed/wq/wqbackground/tdsbg.html

  5. http://extension.oregonstate.edu/gardening/what-are-short-day-and-long-day-plants

Some of the resources aren't as reliable as I would want them to be, but it's the best I could find. Number 4 has a great run-down of TDS for different water sources.

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