How frost resistant is Rosemary?

The winter is already here and I was concerned by the temperatures which have been as low as – 10 °C last week. I did not know if my Rosemary plants (named also Rosmarinus officinalis, Polar plant, Romarin, Rosmarin, Romere or Rosarine and "dew of the sea") left outside were able to survive this winter frost without damage to their evergreen leaves, stems or roots. Some people said  that the Rosemary are "frost hardy" resistant, but the term "winter hardy" or " frost resistant" depends by the degrees of frost and also by the plant varieties.

Herb Cottage Rosemary branches  in snow 

Is not an universal standard to define if a plant is cold resistant. According with the Atlas of Canada, I'm in  plant hardiness zone 8a, the mildest USDA (United States Department of Agriculture) zone for Canada,  in a climate where  most of the plants can be grown, with temperatures as low as -12°C to -7°C.

Upright Rosemary (Rosmarinus officinalis) branch

A common classification of  plant hardiness as is described in many books is detailed bellow, but are only general guidelines:   
-frost hardy plants can stay alive even when temperatures go down to -5°C;  
-fully hardy plants can resist with short spells as low as -15°C;
-half-hardy plants cannot tolerate frost but can survive even when temperatures go down to 0C and
-tender plants which is good to keep inside, may be damaged when temperatures go down below 5°C.

Rosemary  unharmed by first frost

Rosemary (Rosmarinus officinalis)  harmed by first frost

I let most of Rosemary plants outside over winter and I choose some plants to spend the winter indoor. Last week was unexpectedly a very cold weather, with a lot of snow and the temperatures as low as -10°C. You can see the photos when I was very concerned about the rosemary plants covered by fallen snow blanket.
Before these days I bring some potted rosemary inside, but they are not so happy as the rosemary plants which remained outside. I have some possible explanations  for this:  there is  not enough sunlight or is not enough air movement or maybe over-watering. I'm still thinking how to improve the indoor conditions for them.

Rosmarinus officinalis flower

Over centuries, rosemary was used as a divinatory herb, as a love charm and a wedding symbol and sometimes as a symbol of remembrance for the dead,

"There's rosemary, that's for remembrance"  Hamlet, Shakespeare.

Upright Rosemary  trained as indoor bonsai

The Rosemary plants are from Mint family. Their leaves have a glossy green color on the upper-side and underside have a downy texture. The foliage and the flowers has strong fragrance and a bitter, astringent taste, highly aromatic. Some varieties have different scents as Pine Scented Rosemary which has pine scent and Lemon Scented Rosemary  with a hint of lemon.

Rosemary  is closely associated with the cooking of Europe / Mediterranean area. Very important is that not all the varieties are recommended for cooking, some as  Golden Rain Rosemary (has a golden tinge to its leaves) and Pink Rosemary  (has pale pink flowers) is better to be used only in flowerbeds as decorative accent bushes.
  I'm using Upright Rosemary leaves to flavor foods: fish, chicken, lamb, even bread, tomato sauces or egg dishes and also for homemade bourbon essential oil  and homemade scented  rosemary water for hair rinse: encourage hair growth, recommended as hair loss remedy, strengthen hair roots and participate actively in the excellent quality of hair (see my other post about  homemade Rose water).

Rosemary covered by snow blanket

I had only 3 cultivars in my garden:
-Upright Rosemary, the most common variety of rosemary with light blue flowers, which  it is the standard type used for cooking (my first indoor rosemary bonsai) 
-Trailing Rosemary with fragrant foliage and small, pale blue to white flowers along branches in winter and spring  (which was brought inside 2 weeks before frost) and
-"Herb Cottage" Rosemary - upright plants with light blue flowers, which is still in the garden although I was not so sure it make it through the winter with little or no dieback (I did not cut back them in the fall  to protect them by frost period).

Rosemary unharmed by first snow

The temperatures increase and the risk of frost has passed. Now I'm sure that the cold not caused significant harm to them (the plants doesn't look very scorched after frost).

Trailing Rosemary - Indoor bonsai

Rosemary is one of those plants I'll always love for the smell and how looks. Every winter, when I bring inside the rosemary plants, I'm potting them in medium pots (10 cm high) with well-drained soil, and  I cut the plants back turning them into small edible, fragrant... indoor bonsai, of course, beautiful to look and aromatic to cook.

"As for rosemary, I let it run all over my garden walls, not only because my bees love it but because it is the herb sacred to remembrance and to friendship" Sir Thomas More