Early propagation with added light.

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Hi again everybody. I don’t know if I posted this already on GOY but it is that time of year again when gardeners are all itching to get get going and growing for the new season so I’ll pop this on and if it seems a bit simplistic to all the expert gardeners here I don’t actually apologise. It is meant as a simple guide to planting and growing, mainly annuals, not a doctoral thesis. If anybody wants to add to it or detract from it then feel free. It is a few years old now so someone may know better with advances in technology.

There are special grow lamps and systems available. But these can be expensive, are not always available in this country, and are not necessary. Ordinary fluorescent lights will do just as well for the average user.

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 very 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.

The marmalade jar and ashtray (no I don’t smoke, it is just the right height) are there to lift the seed trays ’cos this box is too high for small seedlings.

• 25 Dec, 2010
• 5 likes

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