Is there a good reason why we cannot use lights for 14-18 hours for DSM? I believe longer light periods are used in agriculture.
If a longer period does work, would additional fert dosing be beneficial - such as spraying with a NPK solution?

I do not think the spray will harm anything, as long as it's dilute.
Sure, you can go longer, but just recall, that the plants will do best with the normal ranges, tropicals will do best at about 12-13 hours, but the last and the starting 1-2 hours do not really count too much unless they are marsh weeds etc.

Regards,
Tom Barr

thanks for responding Tom, but I am not sure I understand what you mean by "do their best"?
are you saying the extra hours wont add to growth or are you saying they wont grow correctly?
Also, how deluted should it be? (how many miligrams of each would you add per 1 liter of liquid?)

Diluted as tank water ppms
Their best -= best growth rates.
You are wasting time and electric messing with much more than those time frames.

Patience.

Just wait for nature to do it's thing.
You'll be plenty busy soon enough.

Regards,
Tom Barr

I dosed more, I did 0.2 kno3 0.05 PO4 0.1 pottasium per 750ml. figured that its not a tank and it shouldnt hurt sparying with 60-80ppm nitrates.

I was just asking because I got curious... wanted to know how botanic stores grow them.