"The second drawback is what we call ' low and slow' germination. Lavender seeds have a short shelf life, and therefore the germination rate (how many seeds out of 100 come up) is usually pretty low. They can also take a long time to sprout (two weeks or more) and this invites fungus to the seed tray, often causing the seed to rot before it can sprout. Seeds benefit from light, so cover lightly when sowing. The germination temperature should be around 70 degrees and spring seeding is more successful than fall seeding. Those seeds that do sprout will take one to three months before they have enough roots and top growth to allow successful transplanting. Adding fertilizer to the sterile medium used in the seed tray can help the little plants get off to a better start, but it can also invite fungus in cool, humid situations."

It sounds as though it might be better to start them indoors (and not in the typical higher humidity of a greenhouse). Lavender originates around the Med, so I guess it needs those sort of conditions to germinate.

good luck!

Chris