To overwinter herbs spiral plantings well, you sort each herb by how much cold it can take. Leave hardy perennials in place under mulch, protect or pot up the borderline ones, and bring tender herbs inside before the first hard frost. The big risk is not cold air. It is wet roots and frost heaving as the ground freezes and thaws.
I knelt at my back-corner spiral the morning after the first hard freeze. I scraped a few inches of shredded leaf mulch around the thyme crown with my bare hands. I noticed the shallow roots sat right at the soil line, half exposed where the surface had buckled overnight. I packed the mulch in until the crown was tucked and dark, then moved on to the sage beside it.
Mulch timing matters more than mulch depth here. You want to wait until the ground freezes before you pile it on, then add about 4 inches over your shallow-rooted herbs. Mulch laid down too early keeps the soil warm and soft, which invites the heaving you are trying to stop. Laid down after the freeze, it holds that frozen layer steady through every thaw. Shredded leaves, straw, or pine needles all work. You just want a loose, fluffy layer that traps air rather than packs down into a wet mat against the crowns.
Before the first hard frost, move tender herbs like basil indoors or treat them as annuals to replant in spring.
Keep rosemary's roots sharply drained and protected, since it is often lost to freeze-and-thaw and wet soil rather than cold.
Once the ground freezes, mulch shallow-rooted herbs about 4 inches deep to limit frost heaving through winter.
Let reliable perennials like thyme and sage stay in place under mulch, and resume normal care in spring.
Rosemary trips up most spiral gardeners, and the reason is simple. Overwintering rosemary fails far more from the freeze-and-thaw cycle and soggy roots than from the cold itself. A rosemary plant that would shrug off a dry frost will rot in soil that stays wet. So your fix is drainage, not a thick blanket. Plant it high on the spiral where water runs off fast. Or pot it up and move it to a bright, cool spot for the winter, then water it just enough to keep the soil from going bone dry.
Basil sits at the other end of the range. It dies at the first touch of frost, so treat it as an annual and pull it before the cold hits. You can grow a small pot of it on a sunny sill through winter, but plan to replant basil each spring rather than fight to keep the old plant alive. Thyme, sage, oregano, and chives are the reliable ones for your spiral. They stay put in the ground under their mulch and bounce back on their own once the days warm up. Sort your herbs into these three groups in fall and the rest of the work gets easy.
The spiral shape gives you a head start when you overwinter herbs spiral style, because it sorts your plants by drainage already. Mediterranean herbs go up top where the soil dries fast, and your thirstier herbs sit at the cooler, damper base. That layout is the same trait that helps them survive winter. For the full year-round plan, including spring cleanup and feeding, see the maintenance section of the main guide. Good winter herb care mostly comes down to one habit. Keep the hardy ones mulched, and keep the wet-sensitive ones dry.
Read the full article: How to Build an Herb Spiral Garden
