Mulch on top of herbs over winter
I'm here in eastern Massachusetts in Zone 6b. I have a few thyme, oregano and marjoram plants outside that I'd like to keep harvesting from occasionally during the winter. I've heard that I have the best chance of making that happen if I cover the plants with a thick layer of mulch, so I now have some salt marsh hay for that purpose.
But I'm wondering if I should completely cover the plants with several inches of hay during the winter. I'm sure it would be warmer for them that way, but I'm worried that it would completely block out any winter sunlight that I would imagine they would need to keep the leaves green. I can't imagine that photosynthesis completely stops during the winter, or does it?
Any advice on this? Thanks.