I hope someone knows
the answer to your question
I've been trying to grow lavender for years here in Arkansas.
Last year I bought seed for four varieties and had almost perfect germination. I had over one hundred plants potted up carefully, in a mix of sand, limestone crushed rock and some compost/soil. It took all summer, but gradually, all but six have died. Those six are a Munstead variety and seem to be wintering over in pots outdoors just fine. ONE might be an english lavender.
I think it's the humidity for most varieties because even with well drained, proper PH soil the leaves will darken and die when there is too much rain or humidity. I've had even less luck if I try to plant in the ground here. I've tried bought plants and many packets of seed...almost obsessive....it's such a wonderful plant.
The varieties I planted last spring were munstead, english, french and czech, a four pkt. offer from Richter's I think. Nothing wrong with the seed or germination, just our humidity. The french variety was the most delicate and the english not far behind.