Artificial lighting (again again)

[Lee Martin]

I've been using a few low energy flourescent spiral bulbs for the last year. Two cool white at 6500k and one warm white. Great for seedlings and good for a few adult plants. They also generate enough heat to get my propagator up to around 30C.

[iann]

25C-30C is probably better for most seedlings during the day at least, but a little cooler at night. There would be some that like it a little cooler, also some that would do well warmer. Ariocarpus seedlings seem to crave heat, at least nice warm air although not blazing sun. Probably some of those tropical species that I don't grow would like it very warm too.

[AnTTun]

And one more thing... since electricity here is almost double cheaper during night, do you guys & girls think plants would mind if they get light overnight instead of daytime?

[DaveW]

There seem to be arguments both ways since some claim for vegetative growth you can grow in constant artificial light, whilst others claim for flowers you need the light/dark cycle.

For raising seedlings I would have thought you are trying to extend the day-length in winter to simulate that of later in the season when they normally germinate, not provide continuous daylight. Most plants seemingly still need a period of dark. Probably trying to increase the light intensity to to match that of the middle of the year if it is for more mature plants. So if you light all night it will probably be OK if you keep the plants dark for the required period during daylight. I found these quotes on the Web:-

"Most plants (C3) require time for respiration during dark hours. Some plants (CAM, e.g., cactus) actually carry on photosynthesis during the night to prevent water loss."

"GROWING CACTUS INDOORS, UNDER LIGHTS

It is recommended that a minimal level of illumination to grow Cactus indoors is around 15 watts per square foot (150 watts/ sq. meter). Fluorescent lighting should be placed 12 - 15 inches (28 - 35 cm) from the top of the plants. High Intensity Discharge Bulbs should be placed considerably further away (depending on wattage). Plants do much better if the day length is kept more or less constant, depending of course on the season. Be sure and use a timer set to 12 - 18 hours a day. Most plants grow best if the light, dark period matches that of their native habitats. When using artificial lights, be sure and use reflectors to catch and concentrate as much light as you can on the individual plants. For maximum growth, your plants should be rotated about every two weeks to assure even illumination.

Cactus, like most plants are more sensitive to certain frequencies (colors) of light. This is usually towards the blue and red parts of the spectrum. For best results use a grow light type of tube for fluorescent lights, or for killer results, step up to a metal halide. These kind of lights produce more light in the colors that the plant can use."

An interesting article here for you:-

http://cactiguide.com/article/?article=article17.php

Seemingly when it comes to artificial lighting for plants opinions differ, but obviously it depends on what plants you are growing.

DaveW

[jfabiao]

Plants do much better if the day length is kept more or less constant, depending of course on the season. Be sure and use a timer set to 12 - 18 hours a day. Most plants grow best if the light, dark period matches that of their native habitats.

This is one of those things that puzzles me to no end. The two last statements contradict themselves and it is something that I have read again and again. Some people seem to think that in the tropics the days are longer than in the temperate zones, and they're not. On the equator, days and nights are exactly the same length, 12h. Travelling north or south you start to have differing day/night lengths, but still the differences are small until you travel a long distance. The longest day you can enjoy in London is less than 17 hours long.

True, you can extend day length to make for the lack of intensity of artificial lightning, but it will only work to a point. In my very humble opinion (and based on my limited experience with artificial lighting), making days longer than, say, 14 hours defeats completely the purpose of matching the light/dark periods with those on the plants' habitats.

BTW, the light/dark cycle may be of little importance when growing seedling cacti since you don't expect them to flower before they are 2-3 years old and that's the physiological response that is (mostly) triggered by day (or night) length. However, I wonder if we can write off the importance of day/night cycles when growing mesembs like Conophytum or Lithops, for instance.

[DaveW]

Yes its puzzling as I can find nothing on the Web as to whether cacti do need a dark period for photosynthesis. In habitat they open their stomata at night when it is cooler, but I cannot find if this is dark or simply temperature triggered. I would suspect more the latter since they stop growing in mid summer when nights are too warm, even though it is dark.

I also read some succulents change their type of photosynthesis from seedlings to larger plants, so what form of lighting and temperatures may be appropriate for their seedlings may not be for more mature specimens. Though as said above, it may be more temperature than dark related for their stomata opening at night. Therefore a cool night rest for CAM plants may be required rather than a constant higher temperature day and night.

http://jmbutlerenterprises.com/NewCamSummary.pdf

Certainly stronger less etiolated plants are produced with a proper nights rest, which is after all the conditions they have evolved for.

DaveW

[IanW]

I go for 18hrs lighting and get excellent results, but of course I'm going for growth, not flowering!

[AnTTun]

My question was more like: will they 'know' something is wrong if I give them light when they (somehow) feel there is actually moon above? I.e. to have the lights on during night, and off during day (they are in closed area so it will be dark inside even its daylight outside).

[DaveW]

Evidently they don't know since nurserymen use extended daylight and induced darkness to bring plants into flower out of season.

http://www.nzetc.org/tm/scholarly/tei-B ... dy-d1.html

http://www.gelighting.com/eu/resources/ ... ulture.pdf

DaveW

[AnTTun]

Thanks DaveW. I owe you one. Or two. Few.