So how cold-hardy is Rosemary?

My rosemary looks fine after weeks of 20-degree weather

One of the many mysteries of gardening is the contradictory advice we read, especially but not solely on the Internet, about plants.  It can even contradict our own experiences growing plants in our very own gardens, so what’s a gardener to do?  I always go with my own experience – and keep an eye out for why mine was different.

Take this rosemary, for example.  According to everything I’ve read, it can’t survive freezing temperatures – yet here we are in late January after an unusually wintery December, and this specimen on my deck is just fine, thanks.  And don’t tell me that’s because it’s up against the house and therefore in a “sheltered location” because I only moved it here for the photo.  Its real location is out at the edge of the deck, fully exposed to the wind.

But I’ll back up and tell you what some online sources I usually find trustworthy are saying about rosemary.  Garden writer and horticulturist Marie Iannotti tells us, “The three fundamentals for successfully growing rosemary are: Sun, Good Drainage and Good Air Circulation. Provide a sandy, well draining soil and 6-8 hours of full sunlight. ”  Then about winter-hardiness, “If you live in a frost free area, you can grow rosemary in the ground year round.  Where the winter temperatures dip below 30 degrees F., rosemary plants will have to spend the winter indoors.”  See, as though it can’t take frost!  And from Fine Gardening Magazine we learn that: “In the fall, when the temperature dips to 30ºF, it’s time to bring rosemary indoors.”

Source after source repeats this warning that rosemary can’t handle temperatures below 30.   So I posed my burning questions to Kerry Kelley, Homestead’s manager of annuals, including herbs, and she replied:

There are a couple of varieties that we know make it in our zone 7.  ‘Arp’ rosemary is considered the hardiest.  It’s an open, upright plant that should do fine here with proper siting.  Also ‘Hardy Hill’, but that’s not as frequently commercially available.  Some people with zone 8 microclimates (Capitol Hill, inner city Baltimore, close to the bay, or just a warmer, protected spot) may be able to grow other varieties–some success had been had with ‘Tuscan Blue’.

Also consider that the bottom temperature for zone 7 is 0 degrees and for zone 8 I think it’s 10 degrees.  If we have a relatively mild winter, with only sporadic temps below 10, other varieties may last.  But when the REAL Maryland winter returns (if it ever does), the plant will be toast.  In other words, plants may do fine for a few years, and then we have one hard winter and the fun’s all over!  And that’s true of any zone 8 plants, not just rosemary.  A warm spot near the house protected from north winds is the best bet.  Plants really should be in the ground and not in a container, since roots are not as hardy as top-growth.

I love one of the tips one of my customers gave me about using rosemary: she sprinkles the flowers on her family’s breakfast eggs–beautiful and delicious.

Rosemary flowers sprinkled on eggs – what a wonderful image!

But I’ll end with what everyone seems to agree on: that as a Mediterranean plant, rosemary likes it sunny and dry – which means great drainage, something that pots usually do a good job of providing.  And rosemary can be pruned to almost any shape, so it’s popular among enthusiasts of topiary.

Posted by Susan Harris.