Im having a similar issue, waiting for perlite to arrive and have root bound seedlings fuck it all to hell. Wanker at gardening place forgot to send it already!

They will be fine, just transplant them and they wil grow to however big their roots will allow. You dont need stupidly big pots.

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All above comments are for entertainment purposes only , all pictures are from other websites.

You can finish the plants all the way through flower in two gallons just fine, we have plants in half gallon that make it all the way through flower...

Eventually you have to pick when you want them to stop with root expansion.

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had a funeral for one of my plants yesterday. root ball broke in half. down to eight plants now. repotted six and gave them a small dose of fox farm nutes. should have some pics up in a day or two. is there a way I can slow my roots down from growing so fast?

A lot of people don't understand how much & quick the roots can grow..twice the height of the plant when starting..My pots range up to 18" across - 12-15" high...a 4-6ft plant will be packed tight when cut....especially when they're young, they grow like mad. I try to calculate my needs..higher or wider pots..all depends on space...some people plant too deep in pot and lose root space that way...every inch you can get...take it..your plant will pay you back..

i heard that if u water heavily and let it dry out as much as possible or something u dont have to repot as soon

transplanted and hung my 250 mh yesterday and also bought moisture meter. didn't know my soil was that dry. what's a good number to keep the meter at and how often should I nute? p.s. calling my plants tony the tiger. they're looking grreeaatt