Do I need to sink the 6″rock wool all the way into the 8″or just enough to hold and deep enough to cover roots in the 6″? What do you recommend?
What type of system do you run? Top feed drip, flood and drain/ebb and flow, trays, buckets, hand water, etc..? The height of the rockwool will change with the different systems. With top feed you can usually have taller rockwool blocks because the water does not have to be “pulled or wicked” up like with a flood tray system..
No matter which system I always recommend setting the cubes at least 1-2 inches into the larger block. This will protect the roots at the bottom of the smaller cube and give the plant better support before the roots grow into the 8″ block. Let me know what type of system you have, so we can check out what will work best for your application.