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Greenhouse lighting

Posted: Tue Nov 01, 2011 6:17 pm
by maritimesbob
After giving my plants a watering and putting a couple of new additions in the greenhouse, it dawned on me how little sunlight I will be getting in there in the winter months.

I never realised how low the sun will truly get, our bungalow roof is going to shade a good 70% of the greenhouse from around 11am then it will be completely shaded by 3pm.

It is too late and too inconvenient to move the thing into a sunnier spot.

I have the greenhouse rigged up with a fan/frost beater that should keep temps about 5C in the winter, however the lack of sunlight bothers me a little.

Is it worth having some artificial lighting of some kind installed and if so can I get some tips and ideas on this? I have a small 8x6 polycarbonate setup, insulated with bubble wrap.

If not, then will be plants and seedlings be ok for a few months as I am providing a frost free environment despite the limited sunlight?

I have various palm and citrus seedlings, along with larger potted palms, cactus, agave etc. Plus I'll be bringing in my tender container plants on colder winter days/nights.

Re: Greenhouse lighting

Posted: Tue Nov 01, 2011 7:01 pm
by Kristen
maritimesbob wrote: Is it worth having some artificial lighting of some kind installed and if so can I get some tips and ideas on this?
Have a read of the Cannabis growing forums - they have a huge wealth of information about growing under lights.

My opinion is:

Using low wattage, low temperature, florescent tubes requires that the tubes are within an inch or two of the plants, and is only really suitable for growing on seedlings or cuttings etc. Otherwise you can't get the plants close enough to the lights, and the "benefit" of the light drops off with distance by the "inverse square law" - if you remember your Maths from school? :).

More expensive lights, such as Metal Halide, have good "canopy penetration", indeed the lights need to be a good 24" away form the leaves otherwise they burn them, but they are high wattage (400W does about a square metre or two) so use more electricity. In practice that equates to about £100 capital cost and £50 running cost for a 400W lamp used 7 hours per day between November and February, inclusive.

Alternative is fluorescent lights used just to keep plants "ticking over". I'm sceptical that this does much, if any, good and could just be wasting money on capital and electricity cost, but other's here disagree.

Re: Greenhouse lighting

Posted: Wed Nov 02, 2011 10:31 am
by Leigh
Most morocan woodbine growers use lighting to promote fast growth as they want to get to the flowering stage as fast as possible.
With the right Fluorescent tube set up and the use of reflective mylar on the surrounding walls, most plants can easily be kept ticking over during winter very cheaply with the bonus that some grow quite well under fluorescents :wink:

Re: Greenhouse lighting

Posted: Wed Nov 02, 2011 11:47 am
by Kristen
Leigh wrote:... and the use of reflective mylar on the surrounding walls ...
I read somewhere (maybe here ...) that Polystyrene reflectivity is not much less than Mylar - whereas Tinfoil / Aluminium foil (which I have used, for years!, behind my seedlings on windowsills in Spring to stop them "stretching" towards the light) is actually a relatively poor reflector of light :(

Thus, FWIW, I'm planning to use Polystyrene sheets around my light to keep the heat in, and provide a decent side-wall reflector.

Re: Greenhouse lighting

Posted: Wed Nov 02, 2011 3:33 pm
by Vagetarian
Flat white paint reflects 80%, Mylar 90%+

Re: Greenhouse lighting

Posted: Wed Nov 02, 2011 7:47 pm
by Kristen
Good old Chrome - remembered what page I had looked at with the relevant keywords in it :)

Flat white paint would be fine (if insulation benefit not needed)

Code: Select all

Material                   Reflectivity
========                   ============
Black                      Less than 10%
Aluminium Foil             55-70%
Semi-gloss White Paint     60-70%
White / Black / White Film 70-85%
Flat White Paint           75-80%
Polystyrene Foam Sheeting  75-85%
Mylar Sheeting             90-92% ... index.html

Re: Greenhouse lighting

Posted: Wed Nov 02, 2011 11:18 pm
by GoggleboxUK
I wonder what percentile score a mirror would get?


Re: Greenhouse lighting

Posted: Thu Nov 03, 2011 2:25 am
by Leigh
GoggleboxUK wrote:I wonder what percentile score a mirror would get?

Trouble with mirrors apart from they absorb light is that the light they do reflect is too directional and can cause hot spots, even scorched plants if certain bulbs are used, think using a mirror to burn an Ant (didn't we all do that when we didn't know better)

As pointed out above flat white paint and mylar are by far the best, easiet and most important cheapest options, but there are other options out there that are more money.

As this was about a greenhouse i would of thought that painting would be a pain.

My personal set up is to line with Bubblewrap and then cover with Defused Mylar (Google it icon_thumright ) which works like the light reflectors used in the horticultural trade, so that light is bounced round evenly with no hotspots.
I have hung mylar lower than the roofline to keep down waisted heat rise, and im running 2, 5ft, 58 watt and 2, 4ft, 36 watt tubes (what type of tubes are your choice but aim for the blue end of the spectrum and 5000 lumans up, there are even gro-lux tubes for plants ) and at the moment when the lights are on there is a 7c to 8c increase in temp from the outside with no heating on and when lights are off the temp stays about 2c higher.
I also run a small fan sucking warm air down from the roof and giving air movement which is just as important as light or heat.
Other options would be using HID lamps but as said in my other post im not looking for rapid growth just to keep the plants as happy as i can through winter on the cheap

Pay a visit to your local hydroponics shop theres bound to be one close to you, not all the guys who run em are dope smoking hippies :lol: in fact the bloke who runs mine was in the horticutural trade for over 20plus yrs and is a wealth knowledge
also cheapest place i know for perlite,clayballs and other goodies