I am a new hydroponic grower using rockwool cubes. I seeded tomato seeds in three-inch rockwool cubes, and now just 20 days later, the seedlings are already starting to send roots out below the cubes and some roots are already around two inches out the bottom. Is this normal? I don’t plan on transplanting the plants to rockwool slabs outside for another month or so.

In nature, seedlings will project their roots as fast as possible into the ground to anchor the plant and start searching for nutrients in the soil. In rockwool cubes, the task is greatly facilitated because the fibers lend themselves to root penetration, and water and nutrients are immediately available.

If you have roots coming out the bottom, if they are suspended in the air, they will air prune. Otherwise, you can give them a trim to promote branching inside the block. Also, if you observe runner roots that are already two inches outside the block, it means the block is sitting in a puddle of water, which is not recommended for a few reasons.

First, if you have those runners, it means you don’t have roots branching in the block and the plant is expending its energy on creating roots to go after the water. Second, when roots are sitting in water, you are creating perfect conditions for root rot to occur, given that the block is probably saturated with water and not letting roots breathe.

The best growers are the dry growers. You could elevate the blocks above the surface where they are sitting. This way, gravity takes care of any excess water and the roots will prune themselves.