Time to report on my experiment in keeping a rosemary plant alive through a zone 6 winter. Last fall I assembled one of Tamar and Kevin's instant mini-greenhouses, made from two window well covers, for my rosemary plant. I had previously done a little homework to find a variety of rosemary noted for its hardiness, relative to other rosemary varieties. I settled on the un-euphoniously named Arp rosemary, said to be hardy in zone 7, or only half a zone off our bit of earth.
I drilled a few small holes for ventilation at the top of my greenhouse and began covering the rosemary in November. It came through the hard frosts of late fall just fine, retaining its green leaves fresh and ready for the picking. I was pretty sure the truly cold temperatures of winter would send it into dormancy, and they did. I could see no new growth, and the leaves took on a somewhat dull tone.
The question was, would protection from the wind and direct contact with snow be enough to let it survive? With such a small space protected, there wouldn't be much advantage, if any, in terms of temperature. A greenhouse large enough for a person to walk around in would certainly do the trick. But this greenhouse was essentially a flimsy cloche; not thick enough or big enough to hold heat overnight. All I could do was wait out winter's harshness and see how the rosemary fared.
Our big dump of snow came towards the end of January, and we've had snow on the ground ever since. The snow covered the mini-greenhouse completely for several days. I went out and scraped off some of the snow, to allow a little light in to warm up the space inside. I suspect the snow that had built up around the sides then acted as insulation.
We're nearly to the end of February now. Historically the coldest month of the year here is January, though we often see more snow in other months. We're still seeing overnight temperatures substantially below freezing. But we should only be headed into temperatures that trend warmer. Today I checked the plant under there and found it looking fine. It still has a wonderful scent, and the thicker stems are supple under my testing fingers. They bend without breaking. The color of the leaves is still dull green. But green they are. I'm pretty confident saying that the rosemary has survived with the help of this protection. The plant will probably need the shelter of the mini-greenhouse for at least another six weeks though.
Now that I know I can keep rosemary alive through a zone 6b winter, I wonder how much farther north this would work. Any northern type gardeners out there tempted to try?
To keep this particular plant over the long term I'll have to keep it pruned such that it fits under the cover. Or else start new plants each year. There's enough room under there right now to accommodate another plant. I may add some early peas in the next couple weeks. But after the peas are done I might try planting some flat-leaf parsley alongside the rosemary, and see if parsley can also make it through next winter with a bit of shelter. Home grown, nutritionally dense fresh parsley would be mighty welcome through the winter months.
Of course, a little success gets me scheming about other things I could plant, other non-hardy stuff I could drag into my hardiness zone by adding a few more shelters. While it was ridiculously easy to make this mini-greenhouse from two window well covers, it wasn't exactly cheap. Not by my standards anyway. I bought the heavy-duty ten-year covers, and I think it ended up costing about $30. I expect they'll last even longer than ten years, since I'll store them in the shade for most of the year. But I'm still going to keep an eye out for any other materials that might be repurposed for the cause. I'm thinking an old skylight or the globe of a street lamp might do the trick, if I ever came across something like that in a dumpster. I could also experiment with straw bales again. I have plenty of salvaged storm windows to work with as lids for straw bale frames, and overwintered straw bales make such nice mulch in the spring.