How do you fine folks decide when to transplant your seedlings into the larger rock wool cubes?

I've read a bunch of articles that say things like "When your seedlings have outgrown the smaller cube" or "When the seedling becomes root bound".

But nobody says how you tell that the seedling has become root bound.

For instance, I have a seedling with a couple of roots starting to show through. Is this plant root bound?

It certainly doesn't look like some photos I've seen with roots wrapping around the cube like vines.

Please help.


I didn't find the right solution from the Internet.


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