Ray, I have not been able to find direct information on this, but all of the sites agree that it is critical not to let the seeds dry out during their long germination period. Knowing that, I would cover them, though just in case they need light to germinate, I would use something translucent, such as quartz-based sand. A clear cover over the seed tray could also be used, as in a propagator, but in my experience with other seeds, that increases the chance of fungus damage.