Vegetative growth poor sunlight

probably not. and, they'll go into flower the moment they are old enough to do so (1-2 weeks from sprout? it might vary slightly per plant). So, they'll be very small plants. The next question is what is your regions average Daily Light Integral (DLI) for this time of year. If low, an already small plant will be even smaller with much less potential energy from the sun than it could have made use of. 20-40gram plants, maybe? If you have good DLI, then maybe 40-50grams each. If it's cold out, that'll further reduce yield. also, it's not so much that the plants 'need' 18h. that is just a common number of hours used. You need to avoid 10+ hours of uninterrupted darkness to avoid the plant going into flower phase. Hours of operation x light intensity = DLI. DLI is what matters as far as growth potential and yield. One factor without the other is not enough information. Hours alone is irrelevant. the wiki on DLI gives a good run down. Worth reading to get the gist of it. *not all plants flip to flower at the same threshold of 'long darkness.' This is why people use 12/12 schedule - it covers genetic variety, safely.