I agree that "brightness" and "daylength" are different issues.
If things are growing slowly because the light is too
dim, and you need more intensity to improve growth, then one lighting fixture might need to be fairly close to the plants and only "brighten" a patch a foot or so wider than the fixture - say half of your 6x8 GH.
But if they are growing slowly because
they only have 9.5 hours of light per day (7:30 am to 5 pm), then adding 2 to 4 hours of weak light might be enough to convince them it's Spring, not mid-winter.
I don't know how much light
intensity it takes to affect daylength, but I heard of one crop where the full moon was almost bright enough to mess up its photoperiod. If that's a guide, then four T5 tubes are bright enough to "wake up" a 6' x 8' area.
How long a day is too long to avoid flowering ?
"Spinach (some types) critical day length:
13 hrs."
http://assoc.garden.org/course...
"Most present-day spinach varieties will initiate flowering when the daylight duration reaches
14 hours — as early as mid-May in the northern half of the United States."
Read more:
http://www.motherearthnews.com...
Maybe you should aim for a
"day-length" between 11 and 13 or 14 hours.
P.S. Or you could think of your greenhouse as only "overwintering" a crop of spring spinach - providing plants with established root systems that are poised to grow rapidly as soon as outdoor daytime lengthen and soil and air warm up.
I've read a few people say that their "winter crops" of greens are more like holding plants in a refrigerator. The plants don't grow much, but they stay alive until they are picked.
You might look into different varieties of spinach for a winter greenhouse crop.
I see there are distinct 'spring" and "fall" varieties of spinach:
http://www.johnnyseeds.com/ass...
Johnnys Seeds says:
"In the North, spinach can be grown in a hoophouse right through December. The key is to
have the spinach established before the day length drops below 10 hours, after which growth rate slows to a standstill but tolerance to cold temperatures remains in effect, allowing for winter harvest and overwintering for early spring harvest."
http://www.johnnyseeds.com/t-s...