Could I bump these up to 20 or 24 hours without harm to the plants?

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Yes, you can. But I don't like being on 24/24 for all the veg. Give them 24h on the first 2 weeks and put them on 18/6 after. That is what I do sometimes when I "in a hurry". Tchau, do Brazil! =]

Yes I have read 24 hours lights on is not good for plants it makes sense just like any living thing needs rest to recover. For my clones that I'm making seeds from I have two separate areas one for forcing pollen. One for keeping the female separate & in veg waiting for the pollen to fully mature - they are just both in this tent right now saving electricity while they're small enough.

Firstly, by the time you "force pollen" on one, the other will be too old to produce viable seeds. Better to keep any pollen and use it on your next cycle/run. Pollination needs to take place early, when there are small "budlets" for best results and to leave enough time for seed maturation, about 4-6 weeks. 18/6 is optimal for health, cannabis NEEDS sleep to function properly. Besides, growth tops out around the 18 hour mark, meaning running your lights longer than 18 hours is basically a waste of electricity. 24/0 is bad for plant health and would only be used by people who don't understand photosynthesis or how plants function, or maybe both principles. Better to have healthy and happy plants than be greedy and get second rate flowers! If you want them to grow and flower as quickly as possible, do not defoliate ever .............. Maximum amount of healthy green leaves = maximum amount of energy production = maximum amount of growth. Try and only ever remove yellow leaves, for by the time they are yellow, they have served their purpose. Hope this helps, Organoman.

You will be fine on the lights it’s early enough you shouldn’t see any stunting.

I've run 18/6, 20/4, and 24 (for a time) during veg. The real question isn't whether it's 18/20/24 - but what intensity are your lights at? In my opinion, they do grow better with some dark (I run 20's now). I'd move it up to 20 and see how they like it at your existing light level. If you have room to adjust upwards (in intensity) then you're probably set.

I usually veg my plants under 24 hr light. Definitely grows them faster