I don't the answer to your post, but I'm very curious too.

Lavender 'Otto Quast' was one I was looking at that is supposed to be one of the hardiest of the Spanish Lavenders, to zone 7a. But I'm in 6b, so I doubt for me but perhaps where you live it will survive.

I have Lavandula stoechas (it also goes by 'Stickadove') here in DC that overwinters fine for me. But my back yard is fairly sheltered from the wind, so that might help. It's also planted in a slightly raised bed of poor soil that drains quickly. I've never taken any special steps to protect it.

So great you posted this thread: Very recently I moved from Baltimore, MD to northern CA. I lived in Baltimore for 7 years and gardened extensively with herbs and roses. I did a trial with the Spanish lavender; a few I potted up and overwintered in my old fashioned 4-season sunroom with southern exposure. A few I left in the yard, sheltered by a north facing detached garage with full southern exposure. The lavenders in my sunroom went dormant but did not die over the winter, in fact, when spring came, they began to bloom indoors. The lavenders in the yard however, bit the dust when temps got into the low 20's, never to return. This is not proper science, just something I tried out on my own. Hidcote lavender proved the most hardy for me outdoors in this region, although it must be replaced every 3-4 years.

I have lived in Fairfax and now in northern Maryland. I have a lavender plant purchased at Betty's Azalea Ranch ( so if it is "Spanish" I dunno) that has spent every winter outdoors in a pot since about 1996. It doesn't look great, being rather woody, but it's still alive as I speak and puts out new growth every year.

My experience in southern Frederick county is that English lavenders (woody stems) are quite hardy provided you can avoid winter wet but I've never successfully overwintered a "Spanish" lavender as shown in the lovely picture by Gene.