[gremlin]

(I have done a search before asking, and most of the answers about lighting are for hens)

Any advice/ experience on using a single "blue" spectrum compact florescent lamp to help seedlings in my greenhouse through these gloomy days.  i.e. its gloomy enough already before I cover the trays with bubble wrap and fleece.

Plan "A" would be to have the grow light up high to cover a wider area  and just give a slight boost to everything during daytime.  But how high? 4 foot above the bench, 6 foot ?

Plan B would be to use the lamp closer to selected trays, but only blast them for a couple of hours.  If so, would it be better in the morning morning or late afternoon?  I am beginning to think that morning light is better than afternoon light, but have no proof.

Then, which plants need extra light the most?

Plants are the usual toms, aubergines, assorted beans, and brassicas (in the cold corner where it hasn't topped 13 degrees at any time in the last fortnight.) 

Of course I need a light meter to work out what light level does what, but as you guessed, I don't have one.

[lazydog]

I can see you are a bit more south than me,but
A/brassicas could go ut side and need no help they are tough
B/the fleece would only be needed at night to give frost protection
C/Even though the temps have not been high, in the greenhouse and polytunnel temps have soared when ever the sun comes out and I have had to open the doors.

As for the aubergines they may be better off in the house as it is a bit cool for them
I am surprised the only info about lighting you could find was about hens usually another illegal crop comes top of the searches on google!
 

[gremlin]

I am surprised the only info about lighting you could find was about hens usually another illegal crop comes top of the searches on google!

I dont know what you mean    .....man !   The only weed I grow - bindweed, horsetail, dandelions and couch grass only makes me cough when the wind blows back from the incinerator.

(Actually I was referring to searching this forum, because someone usually points out the answer in a previous post and complains about people asking the same question.)

I am trying to expand my rather limited knowledge of lighting for plants.  I know some plants like onions are daylight length sensitive.  Cucumbers need 6-8 hours bright sunshine etc.  Still not sure if seeds need light to germinate, despite what Carol Klein says

I have a greenhouse with heated soil borders, but it is a poor light location. I had no better choice unless I demolish next door's house.   The plants are doing OK, but not exactly thriving so I am wondering if is is to do with the light - not temperature.

[Auntiemogs]

I'm no expert but the recommended distance for my light box is just a few inches above my plants as the amount of lux drops off rapidly the further away it is. 
I read on a hydroponics site that the recommended wattage is 50w per square foot but not sure if that's just to supplement the existing light so the info is out there, it may just take a while to find it...
Mine is on a timer so is on for 12 hours a day (7am-7pm) and seems to be working well so far and has 2 x 24w full spectrum fluorescents.