Learnt my lesson with 3000k for vegging: DO NOT use it at this stage.

I’ve vegged with 3000k cobs without a issue. Sounds like user error on your end.

PinPin said:

I was thinking it would be faster considering it's QBs but it was not. May be it is not because 3000K does not have a strong blue spectrum. However it does not explain why they have started growing quite noticeably only after I added extra light with more blue.

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It could be entirely coincidental. They may have taken off right right around then without you adding the other light. It could be a number of issues that could have caused the delayed growth rate you witnessed. Especially too much light too early will do it. Slow intial root growth due to slight overwatering can do it too. Happens to the best of us. I just would not jump the gun and announce to the world 3000k is trash for vegging. Again many, many folks with hands on expeience will tell you otherwise.

If you think its just the lack of blue, you would have hell of a time explaining the incredible veg growth hps users can get. And ofcourse hps offers very, very little to no blue at all.