My vegetable seeds from Burpee arrived last Friday (Jan. 4th) and I immediately germinated after I arrived home from school that night. I ordered Scallions, Nantes Carrots, Cilantro and Catnip (for the cats, naturally).
I sowed the seeds into Grodan rockwool starter plugs because they are inert. I planted these directly into the right most grow bed the next morning just beneath the surface of the hydroton. That happens to be where the maximum height the water reaches as it fills and drains. (See below)
I then covered the rockwool with about 1/2" of hydroton. The cubes keep warm from the water and have hastened the germination. It has only been 6 days and I already have carrot and catnip sprouts. I didn't expect to see sprouts in less than a week!