My wife and I both love to use Rosemary for cooking, so over the years I have tried many different strategies for keeping it happy through the cool, damp, dark Vancouver Island Winters.
Key Points to Consider
Rosemary is considered a half hardy perennial in our climatic zone, so unprotected outdoor overwintering is a bit of a gamble.
Unlike many other perennial herbs, Rosemary stores very little energy in it’s stem and root tissue, it has to produce food for itself by photosynthesizing all year round, so it needs light even in the winter.
Rosemary does not like having wet feet, but benefits from moisture on it’s foliage.
Rosemary is damaged in the winter by cold temperatures and wind that produce low humidity conditions that desiccate and damage it’s foliage. Extreme cold temperature will freezes it’s roots.
The first line of defense is choosing a more winter hardy Rosemary cultivar, the standard recommendation in this area is a variety called ‘ARP’, originally from ARP Texas, it is considered to be considerably more cold hardy than most, however I (personally) don’t like the flavor or aroma of ARP Rosemary. Another variety that overwinters well on the herb farm is Northcot.
Pick a location, with lots of light, shelter from the wind, and excellent drainage. The cold dry winds are going to come from the north and sometimes the west. If you are lucky enough to live near the sea your Rosemary will benefit from the moderated ocean side winter temperatures and higher humidity.
A Note on Mulch
I am not keen about mulching around Rosemary, the benefit would be added insulation over the root zone, the risk is possible root-rot causing moisture retention.
Consider Container Growing
Growing your Rosemary in a large pot is great strategy as you can move it into an unheated or lightly heated greenhouse, cold frame or covered porch. If you want to take it right inside your house and grow it in a bright window be aware that it will require regular misting to compensate for your dry indoor air and your Rosemary plant may trade insect pests with your house plants which can be tricky !
I grow my Rosemary in large pots that are located on a sunny deck by the kitchen door all spring, summer and fall. Then I move them into an insulated unheated shed and put them under a florescent light fixture that contains 1 cool white and one GE “daylight” bulb.
They light is on 12 hours per day and I get to pick fresh Rosemary all winter and have lots of cuttings for propagation in the early spring.
Care Free Solution
The last, and maybe best strategy is ” don’t worry be happy ! ” and grow your Rosemary as if it is an annual plant. Just buy a new plant every spring. I have seen lots of fantastic Rosemary plants at the early spring “Seedy Saturday” plant and seed exchange sales at very reasonable prices.