The D. neesii germinated last season and quite well for a tuberous. I would
use a coarse sand and peat mix. Sow now and keep just wet and warm. Let
cool with the season. It will germinate with lower temperatures. The D.
neesii is a winter grower, i.e. likes cooler conditons.
The N. vieillardii should probably be sown on sphagnum. I experienced two to
three months for germination. Keep warm, but not hot. You'll have to
experiement. I don't recommend sowing on a sterile medium.
Tom