light requirements at night

[drew33998]

I have an array of orchids(phal, dend, onci) which i keep on my front porch. It is east facing and offers perfect lighting during the day. At night my front door lights are sensored and stay on all night. I was wondering if anyone thought this might interfere with the lower light requirements for the night phase of photosynthesis of my plants. All the plants look great and have tons of new roots, leaves, and p-bulbs.

[catttan]

I have an array of orchids(phal, dend, onci) which i keep on my front porch. It is east facing and offers perfect lighting during the day. At night my front door lights are sensored and stay on all night. I was wondering if anyone thought this might interfere with the lower light requirements for the night phase of photosynthesis of my plants. All the plants look great and have tons of new roots, leaves, and p-bulbs.

As most of your orchids come from the tropics, which has roughly a 12 hour day:night ratio, the all-night light might interfere with the blooming.

[sarraceniac]

I think that porch lights will be so 'dim' compared with sunlight that they will not affect it. A 40 watt flourescent tube which is 3 inches from a plant will produce only about 450 foot-candles of light and this drops off enormously as the distance increases. At 3 feet it is only about 45 foot candles and at 9 feet only about 10 foot candles fall on the plant. As even plants (chids) with low light requirements need about 1,500 foot candles of sunlight for photosynthesis the smidgeon of light produced by a porch light is neither here nor there even to the most sensitive plant. I wrote the following many years ago for general early forcing of standard garden plants in light boxes. It is a bit simplistic but it may give you some idea.

'Light levels.

To simplify matters, I shall ignore the 'colour' of the light, other than to say that the green area of the spectrum is not useful because chlorophyl does not absorb it which is why the newer LED types are always red and/or blue, for foliage and flowering. In practice, white light (a mix of all the colours) is adequate for the everyday grower.

The key element of a light box is to get enough light for the plants. Light levels are measured in lux. Examples are: -

0.25 Full moon on a clear night
10 Candle at a distance of a foot
50 Family living room lights
400 Brightly lit office
1,000 TV studio lighting
20,000 Sunny day

For plants, an adequate level is usually taken to be about 2000 lux. This can be achieved if the plant receives about 20 watts per square foot of fluorescent tube light. However a 20 watt light over a one square foot area will not achieve this, as most of the light will be lost and only a small percentage will fall on the target area.

This is where the concept of a light box comes in. To prevent light being lost, the plants and the lights must be enclosed in a box that reflects all the light from the source onto the plants. A good light box can be as efficient as 75% or even higher. Ie, 75% of the light produced will fall on the plants. It could be as low as 5% without a box.

To achieve high efficiency, the walls of the box must be highly reflective, and the box must be totally enclosed allowing nothing to escape. The most efficient reflectors are mirrors and metalised melinar film, but a good white surface is nearly as good. In practice 100% efficiency will never be obtained so it is advisable to put the lights as close to the plants as possible. This can be achieved by attaching the lights to a suspended ceiling, which can be raised or lowered. Normal practice is to have the light two inches from the top of the plants. There are systems that allow the lights to be a lot further away, but the lights must be much more powerful and hence wasteful(and expensive to buy and run). That sort of system is more useful for large plants, where the light can’t be two inches away from every part of the plant.

Light sources

There is much talk about special bulbs with the correct spectrum for growing plants. whilst the light from any sort of bulb is adequate for most purposes, the bulb should be as efficient as possible, partly to save money and partly to reduce the heat given out, which could scorch the plants. Bulb efficiency is measured in lumens of light per watt, and typical values are: -

Ordinary light bulb (incandescent) 15
Quartz halogen 24
LED (Light Emitting Diode) 25 – 50
Long life (fluorescent) light bulbs 55
Long fluorescent tubes 70
Sodium street lights (yellow ones) 150

The best practical light source is a long fluorescent tube. Long life light bulbs, which are small coiled fluorescent tubes are not quite as efficient. Tubes are inefficient at the ends, so the longer the tube the better. They are also said to deteriorate faster with time than straight tubes. Long tubes also produce more even lighting. The fastest developing type of light source is the LED and it is predicted it will eventually be as efficient as sodium lamps – but it is not there yet.

One popular misconception is that more efficient bulbs produce less heat. That is not true. No common bulb turns more than 10% of the electricity used into light, so the other 90%+ is turned into heat. A 100 watt incandescent light bulb seems hotter than a 100% watt fluorescent tube only because its smaller and the heat is less spread out. But they both give out virtually the same amount of heat – 100 watts,'

It really isn't as complex as it looks at first sight.'

[AtantaR]

I always assumed that incandescent lights would not effect plants photo period as much as florescent lights would. Florescent lights would be more efficient for growing then incandescent would. Granted we can see the light but some rays we can not see. Such as ultra violet rays. Who knows if this effects plants in such a way in nature. Florescent light emit uv rays. I think its every 8 hours is 1 minute of supposed sun. But grow lamps also usually emit a blue and/or red spectrum. Check to see what kind of light you have on the front door. Florescent lights may save you money, but yes bloom, no blooms, maybe blooms...haha who knows....hmmmm what to do, what to do??

[drew33998]

Yes they are the 40 watt mercury vapor flourescent lights. I assumed that it wouldnt be couter productive to the plants to leave them on given the "low" foot candles. However the shadows that are cast at the site of the plants is very sharp. I guess the only real way to tell is to measure the footcandles at the site of the plants at both day and night and add the total amount of footcandle hours to see if it exceeds the maximum recommeded light in a 24 hour period for each plant species. Or i could just wait until they die. Haha. J/k of course

[Brutal_Dreamer]

Interesting question and discussion.

Cheers,
BD

[Spencer Aloysius]

I would like to note that I grow hybrid Phals at my workplace and when we turn off the lights at night, some of the ballasts do not turn off. I have a Phal directly under one of these fluorescent ballasts and the 24hour light doesn't seem to have affected its blooming at all or the length of time the plant stays in bloom.

[sarraceniac]

The extended version of my contribution is available but can be hard going. As there is no blog section on the forum and also it is a tad dry I won't bore people by posting it here. It was written when I was States-side so is probably more readable there than here but it is not for the faint hearted. Lol. If any masochist wants to see it all though if you PM me I can email it to you. But I suggest it is not for amateur gardeners.

[sarraceniac]

As an addendum to the above post can I just say that I will post the entire treatise in the 'Article' section of the forum after I have got rid of all the trade names and made it more user-friendly. So anyone who has masochistic tendencies can pick it up there in a week or two. Thanks Bruce. I didn't notice this area for 'boring stuff' until you pointed it out but your suggestion is the best idea.