Okay, this may be a stupid question but is a Blacklight any good for the plants? Does it emit a usable spectrum of UV? Anybody used one to supplement their grow?

I've read that UV-B is good for resin production (as per the Nepalesian mountain sides), though I don't know of anyone using it in their grow room???

i'm not so sure, but i think that blacklight has a lot of UV A, but you need UV B. i don't know anybody using blacklight for growing, but i know some people, who use UV lamp for reptiles that you can get in pet store. that one has UV A and UV B spectrum (what you need).
the main idea using UV lamp is that UV B radiation is believed to be harmfull to plant. therefor the plant produces some substances on its surfance to protect itself from that harmfull radiation. but it is still unknown what that substance is made of.
but as i said, i'm not sure about that.

on the other hand, UV A (arround 400nm) can be used for photosynthesis. one of 4 clorophyle peaks is at 425 (blue), but the absorbtion is quite good also at 400nm (which is UV A)

Those Canadian Sun-Cell growboxes use some form of UV light in their boxes.......not sure what for, I think it's for killing bugs and spores or something.

possible, but that is not blacklight or UV B. i think UV C is the best thing for that purpose, but be carefull. you can get blind - very harmfull radiation.

This is a old discussion and now still keep unaswered.

There is a lot of evidence that suggest that UVA and UVB, mostly this last one, induces the resin production. UVB is proved to convert THCV into THC directly in lab, but the effect over living plants isnt so clear. But there is a general consensus in UVA-B increasing resin production and THC percentage. But there is a lot of discussion about the grade of this effect. Some says is very slight, while others says it affect a lot.

IMHO, the grade of the effect is strain dependent. A rough generalization is sativas strains notice UV far more than indicas strains.

Anyway, the UVB added must be low. Avoid any UVC, because is very dangerous for both you and your plants. And keep UVB in low levels, because it can be harmfull in high levels, too. UVA seems to be very safe, and have similar effect to the blue light, in photosyntetic effect and other biological processes (stomata conductance regulation, etc).

Some people has hypothesized than the right dosage of UVB is esential to get the desired effect. Too low and the effect is near unnoticiable, while too much and you get undesired effects (ive seen reported a part of a plant reveging while the remaining still flowering) or can harm the plants. And probably the right dosage is strain dependent.

I know is little help, but i havent experimented with UV, although i was very interested in this topic. But as i grow for medical use mainly, im not specially interested in increase THC percentage.

I use UV.

A 300 w Osram bulb,and it kicks out UVA,B and a little bit of C.

I use if for 3 1/2 hrs around the mid-part of the 12/12 day on my very Sat dom AK 47's[5ft plus]and it does make a definite difference to the AK i'm growing.
I've found they produce a hell of a lot more trychs on my AK's than without,but i've yet to try it on anything else yet.

Last grow i used it from the last week of veg just to try and stop a fungal prob with the UV light[i was desperate]and it not only kicked fungal leaf spots ass but it "Activated" far more budsites that it shone upon.

The budsites and the trychs flouresce under the UV light,and i move 'em around to get an even dose of it as the bulbs in a fixed position and i keep it there because of HEAT.

It finishes a few day quicker,but the BIG PROB is HEAT.

I've got it rigged with timers to blow cool air over it when it's ON as it does get very hot and in my limited experience that's the biggest prob with it.

MY AK's are more like xmas trees than a see of green though,and seem very SAT dom so like knna says it might be a SATIVA thing but it works for me.