Growing rosemary successfully outdoors on the edge of zone 5
posted byon 05/13/08
Rosemary is a delightful herb best suited to warm dry climates and sandy soil. Our family has been impressed with the world of difference a little rosemary makes in our stew recipe. I know that there are many people who would like to grow rosemary but their climate is not favorable. Being on the edge of zones 5 and 6 in the mountains of West Virginia, our climate is the opposite: cold and damp with clay soil. Each winter the temperature drops below zero F once or twice. If I had known about others’ failures in attempting to grow rosemary in my kind of environment, I might not have attempted it. But since the family enjoys using rosemary so much, when I heard about a variety that was “hardy to zone 5” named ‘Madeline Hill’ I decided to give it a shot. It is important when trying something new to keep detailed records. Only then can all the factors be considered for helpful conclusions. I hope the details here will be helpful to someone.
I planted it in April ’06 in front of a small south-facing brick wall but the soil was heavy clay that had been amended with peat moss and gypsum. Because neither clay nor peat moss is recommended for Rosemary, I amended it further with sand. The wall is 7 feet from the outside wall of the house so I do not believe it receives any warmth from that source in the coldest days of winter. However, the eave of a porch roof helps keep the soil somewhat dry and that is a good thing for Rosemary.
It seemed to grow happily the first summer. The following winter I insulated the plant if the temperature threatened to go below 15 degrees. I did this by turning a crate over the plant and wrapping it thickly with burlap bags. “Thickly” means about 15 layers. Since a bag is two layers, that means a 7- or 8-bag thickness all the way around. In a 12-day period in February 2007 the temperature fell to single digits seven times, the coldest temperature being -2 degrees F. The temperature rose
above freezing only twice and the plant was covered the entire time. There were times when snow contributed to the insulation. At other times clouds, wind, and long nights seemed to render any “natural” means of warmth like the brick wall completely useless. I always uncovered the plant to provide light and circulation when the temperature was expected to stay above 15 degrees for awhile.
At the end of the winter the plant showed very little signs of any harmful effects and grew larger in 2007. It was never watered during the drought and never showed any sign of stress.
Since the crate no longer fit over the plant, the next winter I used a piece of fencing that was supported well enough to sustain the weight of burlap and snow. On December 6, 2007 the temperature was expected to fall to 15 degrees F and I almost didn’t cover the plant. But I was glad I did when the next morning the temperature on two new thermometers read 6 degrees F! During the winter of ’07 to ‘08 the cold weather came in short spurts: 2 degrees on January 4; 7 degrees on January 21; 7 degrees on February 11; and -1 on February 21. Since long term wetness is also bad for rosemary, I would keep a piece of burlap over the plant during prolonged rain or snow to shed water and help keep the leaves dry. The plant came through the winter a second time with only a few of the outer shoots killed. This could have been due to the dog making her bed in the burlap too. It put out a few flowers for the first time and seems to be making some seeds. I do not know what the pollination requirements are for rosemary, but if it makes any viable seed I’ll attempt to plant it and see what we get.
Perhaps it would be simpler just to bring the plant indoors for the winter. But it is important to properly “harden off” the plant before putting outdoors in the spring. The year I tried that, I lost all the rosemary plants that I had brought inside after planting them out in the spring. They were “normal” rosemary plants from seed. I really don’t know if the measures I have taken to protect the hardy plant are really required. Ideally, the plant should be able to remain outdoors all year with no protection from winter cold. But I do not plan to experiment with that until I have enough plants that I could afford to lose some. If the Lord is willing there will be an update next year.