Triggering flowering by manipulating hours of light
 

Was thinking last night about what causes orchids to flower. Some require drops in temp, some just "know" when to bloom, but I was wondering how they know. I assume it's some kind of shortened period of light to trigger bloom - is that correct or no?

Can you just give same amount of light all year, since most of the orchids are from some kind of tropical area... I assume they get around 12 hours a day all year.

Can you give them more light than that and expect more growth? Is there a point of no extra return for the extra light? Can they grow under 18 hours of light a day fine?

I know with indoor lighting, they receive the same amount of FC all day long, so does that mean they can have shorter "days" indoors (6 or 8 hours) and still receive the same amount of light as outdoor plants receive in a 12 or 14 hour daylight?

Hopefully that makes some sort of sense... had to get it out quick - hahaha.

I don't think it's that simple. There are lots of things to trigger flowering. Haeralla, for instance just keeps flowering over and over with exact same light and water and, etc. Epidendrum ellipticum doesn't flower till the stem gets a certain length. Phals often require a cool-down to initiate spike. Dendrobium anosmum requires a total dry spell and cool period at same time to initiate buds. So it's too complex to give just one reason or even 2 or 3 reasons.

Yeah orchids really are quite complex plants compared to most it seems. A lot have simple triggers, like reduced number of hours of light, or temperature drops, etc... I guess there are just too many variables and too many types when dealing with orchids.

Thanks

Culture aside for triggers and please correct me if I'm wrong here. Flowering plants begin the production of flowering growth due to hormonal levels. If the hormone cytokinins is higher, flowers are produced. If the hormone auxin is higher the plant grows forward growths and roots. To induce higher levels of cytokinins you can add seaweed or kelp to your ferts or sprays. I buy Safers insecticidal soap with seaweed extract and spray my orchids 1 month before they are supposed to flower. Seems to work good and the orchids stay bug free too!!!

It all comes down to the type of orchid you like to grow. There are roughly two effects that trigger flowering in plants generaly, Photoperiodism and vernilazation.

Depending on the orchid specie you grow some require both and some just one of them. With photoperiodism plants will produce a phytochrome that will absorb light at 660nm and deactivates at absorbing 730nm which is emitted by moon light. Thats how plants know how long the nights are. PLants do not measure daylenghts but nightlenghts.

The problem comes when growing in a basement is simply that there may be no deep red light to absorb and thus upsetting the plant thinking there is no night. Using deep red light may help.

Vernilazation is the effect that plants want low temps for a period of time, usually <15 C. How that mechanism works is still debated. I believe cymbydium and dendrobium are one of those.

Phala's are subjective to photoperiodism.

Vanda's are dayneutral and only want lightlightlight.

Just search google with "photoperiodism vanda" for example and you will find the info you need for that parcticular orchid specie.

The problem indeed with artificial light is the constant feed a day. The nightlenght will not change but the amount of energy absorbed will increase drasticly. Plants may easily burn because they get more then they expect. Also it may be usefull to check this link out.
Day Length
If you know where that specific orchid is coming from you know how the daylenghts over there are and you can adapt your lighting to it.

Be also forwarned about artificial light that it may be a frustrating an expensive work to get the right amount of light for your plants. On the other hand you get to play for god which is fun as well ;)

Hope this long story helps.

Orchidaholic & Daemos

Thanks for the great info! Precisely what I was wondering.
Now I need to organize all of this information....

Bob Gordon's phalaenopsis book discusses this. If I remember correctly (and I often don't!), his conclusion was that most phals don't care, a few do, and a lot more research needs to be done. I can look it up if you're interested. He suggests using a cool down period of about 3 weeks starting at Indian Summer, plus increasing the lights afterwards (good time to replace the bulbs to get more light). This replicates the climate conditions that phals grow in naturally.

The amount of light you get indoors is going to be much less than outside, because your bulbs simply aren't as bright as the sun. So a shorter day with lights will be less total light than outdoors, not more. Of course, on overcast days, or with shaded windowsills, the grow lights will give you MORE light than the windows or outdoors would. This is a big plus for those of us living in areas where winters are grey, like the northeast and pacific northwest. Another bonus is that lights make heat, which can help if you're growing warm orchids. The light and heat keep the orchids growing more through the winter than they otherwise would.

I generally run my lights from 12 to 16 hours a day and the phals are happy. Some people adjust them every month or two to replicate the climate of origin, but I'm too lazy.

Did that help?

Sheila,

Yes, thank you. It seems that my house will probably provide the correct cooling period in the winter that many orchids desire to trigger flowering. I was also wondering if some required a shorter photoperiod. I think I now have my answers.

Thanks!

About the only cut-flower plants that are strongly day-length sensitive are catts.

Back when that Easter corsage was a huge market, you would find nurseries covering them with a black cloth in early afternoon, several weeks before. Then they all bloomed in time for Easter.

Most phals are not photoperiod sensitive. Look at where they originate - near the equator where there is little light level variation with the season. Dr. Yin-Tung Wang of Texas A&M has shown that they spike in response to a period of cooler temperatures (and not a day-night chill as we have been taught).

Interesting discussion. Lots of good info here. But I have a question. :scratchhead:

I can see how plants might measure the length of the nights and not the days, but I suspect that it's not just the measurement of moonlight at 730 nanometers. What about cloudy nights or even months? What about moon phases? Also, natural light contains most or all wavelengths (full spectrum).

Isn't it possible that night lengths are measured by the absence of light in the usable wavelengths? Could it be the length of the "dark phase" of photosynthesis or Calvin cycle? What about the duration of some other metabolic pathway? :hmm

I'm not discounting your info, but I have doubts that flowering is so that dependent on the presence moonlight. I would tend to believe it's the absence of sunlight that makes the most difference. I have flowered many photosensitive plants indoors, away from any moonlight or any other light during the night.