Turning Christmas Rosemary into a Full-Time Plant
Q. I try every year to grow rosemary. It does great but does not return after winter. Is there a certain type of rosemary that would be more hardy over the winter, or should I pot it for the first year or two, and then transplant it permanently into my herb garden? We'd like to have a really tall and bushy rosemary.
----Dan in Sellersville, PA
A. Rosemary is a Mediterranean plant, like the fig trees that are such a hot topic on our show. Unfortunately, unlike figs whose roots survive cold winters so often I'm tempted to call the plants 'herbaceous perennials', rosemary just plain dies if left outdoors over the winter in regions roughly North of Washington, DC. It can't 'come back' or 'return' from its roots like a fig. And it doesn't matter how big or old the rosemary plant is.
The legendary variety "Arp" has been bred to be more cold hardy, and it may provide the little edge people in 'almost hardy enough climates' need. Years ago, I tested it against a whole slew of other varieties and it did last the longest into the winter. (But that just meant it was the last one to die in my garden.) But again, in a region just a bit South of my Northeastern PA, it might well make it—especially in soil that drains well.
…Because drainage may be the key here. Many experts feel that rosemary might be able to survive even my winter temps—especially if the plants are growing in a protected location—but not my typically high levels of rainfall. 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.
So—are we then chasing the impossible dream?
Let's call it 'the difficult dream'. But this is the time of year you can realistically jumpstart that dream with one of the little foot-high rosemary Christmas trees you see for sale at garden centers and upscale supermarkets. These are live plants with impressively large root systems that have been aggressively pruned into those conical shapes by the growers from shrubs that are several feet tall.
And as we warn every holiday season: because of that big root system, they won't make it to New Year's unless you get then into a bigger pot right away!
This is absolutely essential. Growers prune really big shrubs down into those festive shapes and then squeeze their giant root balls into the smallest possible pots. That's great for the growers' ease of shipping, but almost impossible for a home owner to keep well-watered.
So take them out of their cute little pot (notice how little actual soil there is in there?), and put them into a container that's at least twice as large and that has great drainage holes in the bottom. Use the loosest, lightest bagged potting soil you can find to fill in the sides and bottom; not dirt from the garden; it's just not going to drain well enough.
Use the biggest pot that you'll be able to move in and out of the house easily. Remember—this is a plant you want to grow into a shrub! (In a very good online article, Terry Ettinger, a garden writer based in Syracuse, strongly suggests that the pot be made of real terra cotta, because it wicks its moisture into the air and keeps the roots drier.)
Sit the newly-potted plant in a few inches of water for an hour or more and let the drainage holes suck up enough to totally saturate the soil. Feel the weight of the well-watered pot. Let it drain in the dishrack, then put it on a plate to protect the surface it'll sit on and place it in a spot that gets your absolute brightest light (but not near a radiator or forced hot air).
Then don't water it again until the pot feels much lighter—which could happen in a few days or a few weeks, depending on your indoor humidity. (It'll probably be a few days if you do use terra cotta. Plastic pots hold their moisture better, but with rosemary, that's often not a good thing.) When watering time does come, repeat the sitting-in-water routine. Saturating the soil from below and then letting the plant dry out completely before watering again is your best bet for long-term survival.
So: North of DC, does it have to stay inside all winter? Should it stay inside all winter?
No. I bought one of these rosemary 'trees' three seasons ago, and my 'tree' turned out to actually be two different plants jammed together in the pot. One of the pair died that winter but the other survived, and I put it outside—in its bigger pot—in early March, well before my last frost date. Rosemary can take light frosts; it's really the combination of extended hard cold and water-logged soil that gets them.
In fact, if you're willing to pay attention to your weather, your potted rosemary can be outside anytime the temps are 40 degrees F or above. On nice days, it would prefer to be outside, but if we drop below 40, bring it back in. Yes, that's conservative; but this is a sport where cowards win. Three years of such treatment later and my tree is still alive. But it's not shaped like a Christmas tree anymore; it's a shrub that's around three feet tall.
And it's still outside for the season as we 'speak'.
I have moved it close to the house—both to offer it the protection of nearby buildings and to make it easy to snag and bring inside during the worst stretches of extended cold and ice storms that are sure to come. (That should give our Southern and Western listeners a good laugh. They grow rosemary plants outdoors that are the size of a small car!)
So whether you start with a 'Christmas tree' or regular starter plants in the Spring, the keys are going to be:
a)     Your climate. USDA Zones 7b and above should have little trouble keeping rosemary alive over the winter. Chances are even good for a 7a gardener if you plant in a protected area, near a wall or windbreak of some kind (not exposed out in the open, or in a low-lying area, where frost and water will collect).
b)     Super loose, well-draining soil. No matter where you live, your rosemary requires excellent drainage. If your soil is clay, plant your rosemary in a raised bed with lots of potting soil and perlite. If it's going to be in a pot, use only a high-quality bagged potting soil—NOT any of your awful garden dirt.
c)     And finally, no matter where you live, if a fiendishly cold night or ice is predicted, cover the plant with a heavy cardboard box for the duration of the cold snap if it's outdoors or bring it inside if it's in a pot.