Posted: 09/05/2016 at 10:54
I think you may be trying too hard. We have a lavender hedge of alternate Hidcote and Edelwiess along the top of a retaining wall. The ground below is beaten earth topped with brick chippings and is a parking area. It is well covered with lavender seedlings that I am now planning to lift and replant elsewhere.
The seeds fall in late summer and then cope with everything the winter throws at them which can be temps down to -20C in a normal year plus rain, snow, wind, frost and thaw. They germinate and grow in spring. They are all a uniform blue which is paler than Hidcote and are all bone hardy.
Try mixing your seed compost with some perlite or vermiculite or fine grit to improve drainage and leave them outside in a sheltered spot, but uncovered, for winter. However, if you want guaranteed colour and form, it's best to buy small plants of a named variety and grow them on.