On the line: overwintering rosemary
On Thanksgiving Day I received a query from my friend Natalie. “Hi, Pam! How should I winter my rosemary plants? Inside, outside — I’ve read both. (I’m hoping outside, as I don’t have a lot of sunny window spots.) They’re in pots, and sit outside my west-facing kitchen door.”
This is a great question. With its woody stems and sturdy, needle-like leaves, rosemary looks like it should be able to easily handle our modestly cold winters here in the Delaware Valley. However, as I’ve experienced over the years, this is not the case. I replied to Natalie, “I don’t have great advice. I have tried both ways (outdoors and indoors) with indifferent results. Kind of depends on how severe (cold) the winter is going to get. Maybe a garage would be the best?” I also promised to learn more and get back to her.
A clue to rosemary’s winter vulnerability is that it is native to the Mediterranean region, which — unlike our area — has hot, dry summers and relatively warm, moist winters. (Much of California’s coast has a similar climate, and you’ll commonly find full hedges of rosemary in gardens there.)
Rosemary (Rosemarinus officinalis) is designated as a “tender perennial” through USDA Hardiness Zone 7. Below Zone 7, it’s considered to be an annual. The Delaware Valley is right on the edge of this Zone, with some of us potentially warm enough in Zone 7b (parts of Philadelphia) and Zone 7a (Philadelphia, Kennett Square), and some of us too cold in Zone 6b (West Chester, Phoenixville). Inexplicably, Pottstown, farther north than Phoenixville and West Chester, is in Zone 7a.
Note: “Below Zone 7” is a geographic, not numeric reference. As you go farther south on the United States map, the numbers go up. (The southern tip of Florida is Zone 10.) Below Zone 7 takes us north on the map. The Zones are based on the coldest potential temperature for a region, e.g., Zone 7b (5 to 10 degrees F.), Zone 7a (0 to 5 degrees F.), Zone 6b (-5 to 0 degrees F.)
The Saturday after Thanksgiving, still thinking about Nat-
alie’s question, I happened to be in my car just in time to hear that “You Bet Your Garden” host Mike
McGrath would be addressing this very question later that morning. I couldn’t listen then, but found the
podcast and a full article (see below).
In his typical blunt fashion, McGrath goes right to the heart of the matter: “Rosemary just plain dies if left outdoors over the winter in regions roughly north of Washington, D.C. It can’t come back from its roots like a fig. And it doesn’t matter how big or old the rosemary plant is.”
Having said that, McGrath suggests that drainage may be a bigger issue than the cold. “Like its cousin lavender, rosemary can’t stand waterlogged soil. And without summer heat to help dry them out, northern soils stay pretty damp over most winters.” I would add that with our typical pattern of snow-then-melt-then-snow, over semi-frozen earth, the soil here may hold more water in the winter than in the summer.
What about bringing rosemary indoors for the winter? McGrath says that potted rosemary plants may be outside any time that the daytime temperature is 40 degrees or above; in fact, they’d prefer to be outdoors. Just bring them back indoors overnight if it’s going to be really cold.
If you get a potted “Christmas” rosemary, McGrath says that it’s important to immediately repot the plant into a larger container, using “the loosest, lightest bagged potting soil you can find.”
Read all of McGrath’s rosemary-wintering tips at http://www.gardensalive. com/product/ybyg-turningchristmas-rosemary-into-afull-time-plant.
To check your hardiness zone, go to http://www. plantmaps.com/list-of-hardiness-zones-for-pennsylvania-cities.php.
If you want your inground rosemary to make it through the winter, you might experiment with R. officinalis “Arp,” reputedly hardy to Zone 5.
A planting of rosemary spills over a garden wall in Berkeley, Calif.