There are hardier rosemary cultivars (‘Arp’, ‘Madalene Hill’) that, with a warming climate might live through a Zone 5 winter.
If the plant is healthy, mature and well-rooted in a sheltered spot at the base of a stone wall, with perfect drainage and winter protection – and winter temperatures don’t yo-yo.
Might. That’s a lot of if’s.
I’m not taking any chances with my big beautiful gnarly plant. I grew it from the cutting of a cutting from the rosemary bush outside the Lombardy home my family left in 1903.
The original plant (from a cutting she smuggled in her shoe) was entrusted to me by my great aunt after she turned 100 – and I blew it.
I killed quite a few rosemary plants before the late Joanna Reed, one-time Herb Society of America president, showed me how she overwintered the huge pots of rosemary set out in her Pennsylvania garden year after year.
Here’s the trick:
Joanna’s trick was to keep plants outside as long as possible in the fall, until it got cold, but not below freezing, then overwinter plants in a cool sunny place.
That lets plants go dormant and stay that way until spring – as nature intended.
Lovingly treating rosemary like a houseplant – keeping it warm, watered and misted – is a good way to lose the plant before spring rolls around again.
Joanna had a ground-level screened-in porch with a brick or stone floor and storm panels for winter. She just packed her plants into this bright, protected space, as I recall, just leaving them there untended, although she may have watered occasionally.
I wrestled my plant, now in a very large container, into my southwest-facing walk-in basement on Halloween, along with my now defoliated and dormant fig tree.
I do water – sparingly, when the soil dries out. Rosemary requires more water than expected (I’ve killed them by underwatering too), especially in a cool but not cold space.
One fall I wasn’t paying close enough attention and my great aunt’s plant froze on a cold night.
Fortunately, rosemary roots easily in a glass of water. I had a baby already potted up. I’m taking good care of her now. A dozen years later she’s magnificent.
My son and I love to root and share progeny of this heritage plant. If I ever miscalculate nighttime temperatures again, I know where to beg a cutting.
