[GpsFrontier]

Hello ematt,
Like Stan mentioned, it's fine to start the lettuce seedlings (or anything) in rockwool cubes, but the idea behind the floating raft (water culture system) system your freferig to is to keep the roots continuous submerged. Not the growing media/rockwool cubes, just the roots hanging down out of them. Rockwool cubes absorb too much water and easily becomes saturated, thus can easily suffocate the roots. The roots hanging down in the water are kept from suffocating because of all the air bubbles rising and circulating the water. With some growing medias it's fine to have it touching the water because it wont absorb so much water, but rockwool sucks it up like a sponge. Generally the spray from splashing bubbles 1/2 to 1 inch below the cubes is plenty to keep rockwool cubes moist without saturating it. The air bubbles don't penetrate the rockwool cubes, so any roots inside it can suffocate. That can also lead to root rot. Another issue you can easily wind up with saturated rockwool cubes is stem rot. Stem rot occurs when the stem of the plant is continuously subjected to moisture.