Tuesday, February 22, 2011

Overwintering Rosemary in Zone 6


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.

32 comments:

Tovar@AMindfulCarnivore said...

Oh, for the balminess of Zone 6. Zone 4 here. It was 10 below zero last night. Keeps populations down, though: ticks, humans, and more. ;-)

Wendy said...

I'm a little cooler than you are in hardiness zone 5b. I love rosemary and have often lamented the fact that it won't overwinter ... I guess it's not necessarily a "fact", though, is it?

I think your little experiment will be on my "to try" list for next winter. Imagine if we could overwinter a plant like Camellia Sinesis in Maine? Ah! Homegrown tea ;).

fullfreezer said...

I may be game to try this next year. I'm in 5a. I think our coldest this winter was -18. I'm not sure how much the window wells would give against that. I've always had terrible luck bringing my rosemary in for the winter. I always seems to die on me. Strange because I do well with everything else.
Judy

denimflyz said...

I have your experiment on this year's list. I am in zone 5 Nebraska. I have never been able to keep Rosemary going, inside or outside.
I look forward to what I can do. Thank you for your insight on this project.
We are still quite cold here. And now we are getting changes with the winter/spring and now we are dealing with ice.

Mitzi G Burger said...

Warm wishes from substropical Sydney. Your rosemary plant must be thanking you for the warm winter palace.

Paula said...

I think the idea of the snow insulating it is right on. Rosemary is a mediterranean plant, so it can withstand some cold. I should think that with snow over most of it (in your mini greenhouse or some sort of cloche), and maybe a tiny spot at the top kept clear so that it would act like a solar tube, you could keep it alive just about anywhere.

El Gaucho said...

I'm in Zone 3, North Dakota, not sure it would work here as we get snow on the ground and it stays for 3-4 months. Could be a fun experiment though, there's nothing like fresh rosemary.

The Mom said...

I'm also 6b. While I've considered doing that, I'm nervous about when it gets really big. I would imagine that just keeping it out of the wind is the biggest thing.

cookiecrumb said...

I LURVE your mini greenhouse. Yeah, it's a bit spendy. But it opens you to thinking. Streetlamp globes? Do you have an architectural salvage place to shop at?

Bureinato said...

I'm in zone 5 and keep it in a pot as a houseplant. It's survived several years this way. It's in front of a south facing sliding glass door in the winter, and out on the deck in the summer. This way I have fresh rosemary all year long :)

meemsnyc said...

This is such a great experiment!!

Amy @ Homestead Revival said...

This is a great idea! I could see it working in lots of applications. I'd have to figure out how to keep it from blowing away.

I have rosemary in my yard, along with lavender, and a few other perennial herbs and they do fine uncovered with snow, but we're only in zone 7.

Hazel said...

We don't use hardiness zones here, probably because there's so little variation between areas other than north = colder and by the sea = windy.

I've just found a zone map of the UK though, and I'm zone 8!
No wonder my rosemary survives the winter! My bay tree looks a bit unhappy after the very cold (for us) temperatures we've had this winter, but it's in a sheltered spot by a south facing fence, so I think it'll be ok.
Hurray for the Gulf Stream!

Oh, and I agree with the Mom about the wind being the main problem.

Did you know the Japanese use a brushing technique to harden off seedlings instead of putting them outside? You probably did, but I'd never heard of it until I read about it the other day.

I've found this online http://www.gardenguides.com/3024-bracing-up-hardening-off-transplants.html

If you scroll down almost halfway he starts talking about thigmomorphogenesis, which is the same principle; movement of air over the plant strengthens it. You can have too much of a good thing though, so lots of cold wind would probably finish your rosemary off.

Kate said...

Tovar, balminess is a relative quality. See El Gaucho's comment.

Wendy, I tried this particular experiment in my garden, which has little shelter from wind. I'd like to play around with the idea in a much more sheltered location next winter. I'm thinking I might have at least one spot which is effectively a zone or more warmer than the rest of the property. Have you got any microclimates to work with at your place?

fullfreezer, I had bad luck bringing rosemary indoors too, until this winter. (I hedged my bets, you see.) A friend recommended I mist my indoor rosemary plants frequently with a spray bottle, and it really has seemed to help with all my indoor plants this winter. You might see if that works for you.

denimflyz, cool! See my tip to fullfreezer for the indoor plants, but I'd love to hear back from you about the outdoor experiment. I'd be happy to link to anyone who tries the experiment, whether successful or not.

Mitzi, I just want to put you on notice that the northern hemisphere is coming for your warm weather. Real soon too. :) Thanks for the good wishes.

Paula, that was pretty much my thinking too. It's nice to have proof of concept, but I don't really consider it all that impressive to have succeeded here in zone 6. I'm curious to see how well it works for those in colder places.

El Gaucho, brrr! I'm not sure it would work there either. Please let me know if you try this experiment, even if not with rosemary, and even if not successful. It would be a very useful service to link to people trying this in various places.

The Mom, I suspect that rosemary can take some serious pruning. But I also suspect that you need to keep at it regularly, rather than letting it get huge and then doing a radical hack job. If you use rosemary a lot for cooking, it should happen naturally.

Cookiecrumb, hiya. I can't take credit for the greenhouse idea. All praise goes to Kevin and Tamar. But yeah, it's pretty nifty. As for streetlamp globes, I'm just remembering an amusing episode from my Berkeley high school days. If we had an architectural salvage place to shop at, we certainly would do. I like to frequent dumpsters on construction sites though, so a girl can hope.

Bureinato, if it ain't broke, don't fix it!

meemsnyc, wanna give it a go yourself and report back?

Amy, I had thought it might blow away too, but the shape of the greenhouse itself seems to be reasonably aerodynamic. You can sort of align it with the direction of the prevailing wind. I didn't do anything to hold it down, but there is a bit of edging that could be buried in mulch if need be.

Hazel, yes, I remember looking at a zone map for the British Isles, and it's almost entirely one zone, as I recall. Though Cornwall was significantly warmer. I had never heard of the Japanese hardening plants off by brushing. But it makes sense to me. The Japanese seem like they put a lot of thought into horticulture, so there's much to be learned from them, I'm sure. Thanks for the link.

lori said...

here in Michigan we are in a zone 5(a? b?,I don't know). Last fall I covered the entire garden with leaves. Imagine my surprise when I went outside during one of those rare 45 degree days last week to discover Rosemary, chamomile,and parsley! Of course they are now buried under a foot of snow again this week......

Kevin said...

I am an avid dumpster diver and I prefer to hit universities at the end of the semester when everyone is moving out. Last year I got ten water cooler jugs with no specific plan they just seemed useful at the time to throw out. I cut the tops off some and used them overwinter plants and next year when I anchor them down better I bet they will work great and for free.

Ottawa Gardener said...

Love it. When I lived in the UK rosemary and bay laurel all grew outside. I think I"d need to do more to keep rosemary alive overwinter as I'm Z. 4 but at least I get to harvest over winter from my inside plant. But I do miss just going outside for some sprigs.

Kenny Point said...

I'm in zone 6 also, didn't even attempt to over winter rosemary this year but I am waiting to see if my globe artichokes survived.

eatclosetohome said...

Mine still seems to be alive (or is it just dried out?) after a winter in the cold greenhouse, which was often down into the single digits this year. I plan to cut most of the stems off when spring finally arrives and let it put all its energy into new growth instead of reviving the old.

Kate said...

lori, I've heard of people doing that with leeks and other crops left in the ground to overwinter. I tried it once though and had a real problem. Rodents loved the leaf piles over the leeks. They went in and ate the roots. I din't know because they were all still standing upright, looking fine. Until I went to pull a few out. Hope your crops fare better than mine did.

Kevin, good score! I keep meaning to go sniff around the campus dumpsters just at semester's end. But I've never managed it so far. Ah, well. There's always this year.

Ottawa Gardener, oh yeah. You'll need to do much more to pull those plants through a zone 4 winter. If you try it with rosemary, be sure to pick a variety that's as hardy as possible. I found quite a range when I was looking.

Kenny, I've never tried artichokes. I like them, but my impression is that they're fussy, take up a lot of space, and don't produce all that much food. What's been your experience?

Emily, that sounds like a good strategy. Thanks for the tip. I'll try that.

Tamar@StarvingofftheLand said...

Kate -- I only just now saw this post (it's been a busy week), and I'm delighted that the rosemary survived!

We put rosemary in our hoophouse in the fall, and it seems to have survived as well. I think we're on the cusp of the rosemary survival zone (my climate is similar to yours - I'm a cold 7), and one layer of something makes the difference between life and death.

I'm going to make sure I have robust rosemary -- along with sage and anything else that might survive -- in the hoophouse next winter.

Rural Revival said...

Very tempting to try indeed! Zone 5 is where it's at here. I'll be thinking of this come this fall.

Kate said...

Tamar, a hoop house is on our agenda for this year too. I know I'm going to have a terrible time deciding what perennials get space in there. Ours will be tiny. Minuscule by most standards, so every inch of space will be at a premium. That said, I'm sure some herbs will be given space. Sage is one of my absolute favorites. Do you know of any that are particularly hardy? I may have to look into that...

RR, I encourage you to give it a try. I'd be very happy to link to any post about the success or failure of this technique in various climates and hardiness zones. Please get back to me if you try it and write about it.

suburban farm girl said...

hmmmm this makes me think, with the Australian winter on its way (WAY WAY milder than anything you guys get). We do get frost and as cold as -4 C haha please dont laugh at me!
However the plants still wont like the frost and our version of freezing temps. Might try something like this in a month or two

Kristi in Maine said...

I put rosemary in a pot in my unheated greenhouse last November. So far it is in that dull flexible state that you described. Still smells great though. I will let you know more later.

Kate said...

Suburban Farm Girl, hey, below freezing is below freezing. That's colder than some parts of the US see on anything like a regular basis. I hope this technique works for you if you try it.

Kristi, thanks for the info, and I look forward to hearing anything else you have to report.

Anonymous said...

I was able to overwinter rosemary into a biannual for several seasons with different store bought rosemary plants. And no protection here in NJ. They did well with protected growing conditions near oregano. They all lasted through the first winter, but never the second. Maybe some protection would extend their lives

Liz said...

I have had a rosemary plant in the ground in zone 6 for about 7 years now. It is planted right up next to the house on the south side. We prune ever year mostly to give rosemary as gifts. Good luck with your little plant I have grown quite attached to mine, especially when it flowers!!!

Anonymous said...

I overwinter rosemary in zone 5. Unheated hoop house - I sink the pots into the soil (hoop is for winter greens so no tables, just good soil. If it gets really cold I cover them with remee. I have also overwintered it in a cold frame, in a sunny semi-protected spot. Rosemary will also overwinter in the house if you put it in a plastic pot, very sunny window and water well weekly - don't let pot sit in a saucer of water- raise it up with gravel.

Anonymous said...

I overwinter rosemary under a wood frame covered with plastic that i lean against the south side of my house. I am zone 6 and my rosemary does very well like this. I dig up my plants in early fall and put them in pots so i can move them easily to harden them off for winter because they don't get much sunlight once they go under the plastic. Then in early to mid December I replant them against the house and keep them covered for the winter under the lean-to, watering once every two weeks or so

craig hoffman said...

Brilliant. I am in zone 6b and have been searching for rosemary success stories. This cold frame sounds like a perfect solution. Thanks!

AshleyShell@gmail.com said...

I know this is an old thread, but I am excited about my success and am hoping it will be helpful to someone.

I searched in vain for a way to keep rosemary alive through the winter, and after two years in a row of bringing a plant inside in the fall only to watch it die, I stumbled across a way by accident.

Last fall when my rosemary plant died, I took a cutting from my friend's healthy plant and put it in a jar of water on a sunny south-facing windowsill to root. After it grew roots, I thought it looked so happy, why take it out and stick it in a pot?

So I just left it...all winter. I didn't do anything to it other than top off its water, which became quite green with algae. It grew a massive root system in its jar, and it even blossomed purple flowers. It's "parent," my friend's plant, did die over the winter, incidentally. Now it's nearly April and it still looks wonderful, and as soon as all danger of frost is past, I will plant it in the ground, then take another cutting from it next fall!