The Winter of My Herbs' Discontent

October 31, 2011 6:58 AM

How do I not kill my herbs this winter? Container gardening in NYC.

I have several herbs growing well in containers. I brought the plants inside for the unexpected snowfall this past weekend. They're now back outside, but I'd like to keep them growing through the winter. How?

Herbs: basil, mint, chives, sage, rosemary
Containers: 3 plastic containers
Sun: north-facing terrace on 4th floor, full sun for 1-2 hours
Gardening zone: 6B (NYC)
Indoor options: extremely small window ledge with only partial sun

Due to lack of indoor sunny options (and space) I'd like to keep them outside, if possible. Is there a way to construct a cheap-ish greenhouse that I can put the containers in that would keep them warmer? I have about 10 square feet of space that can be used for this purpose. There is an outlet, so I could conceivably put in an additional light source for part of the day.

If my only option is to move them inside, I will need to transplant them into different pots (due to space constraint). The window ledge is roughly 18" wide by 6" deep, with the window being 5-6' high. Will the transplanting combined with moving inside be too risky? If I have no other choice, how do I minimize this risk?

Any help would be greatly appreciated.

posted by melissasaurus to home & garden (12 answers total)

Though freezing temps for any prolonged period will most certainly kill about anything, something as simple as throwing a towel over your herbs at night will help them out. If you can wire a light bulb out there, you can keep them that much warmer. Basil and mint are both warm weather herbs, so you might end up losing those anyway.
posted by Gilbert at 7:10 AM on October 31, 2011

Basil is very sensitive to frost, so I doubt it will make it. The others would probably survive but not be very productive.

I don't think an additional light source would be useful outdoors. What might help is some extra heat. You can get soil heating cables that have a built-in thermostat. You'd probably use the heating cable to keep a sand bed warm, and sink the pots into the sand. A simple frame supporting a tent of heavy plastic sheeting over the sand box would help, too.
posted by jon1270 at 7:12 AM on October 31, 2011

From experience last year, I think the mint will be fine with both the transplanting and the living inside. There's a reason mint has a rep as hard to kill.
posted by yarrow at 7:26 AM on October 31, 2011

You may be better off moving everything indoors, and getting some kind of light for them rather than relying only on the sun. And make sure everyone's in their own pot; because you've got different growing conditions for each.

Basil's going to be hard; it's very sensitive to cold. Even indoors in a draft and it's not going to be happy. Mint can deal with shade, bit it likes it wet; fortunately it's kind of tough to kill. The sage and rosemary could also do okay with enough light and a bit of a draft; however, they like it dry as opposed to the mint liking it wet.

If you can't bring all four inside, here's what I'd try:

1. Leave the rosemary and sage outside. They can cope with SOME cold. Be ready to bring them in overnight when it gets really cold, though.

2. Bring the basil and mint inside -- the basil will want the extra warmth, and the mint being inside will make sure you can water it often enough. Get a grow light for the basil (a desk lamp with a "grow light" bulb would do).

3. Put the basil on the windowsill inside, and make sure your window is sealed well so there's no draft. Put the rosemary and sage on the opposite side of the same window - so they're sort of looking at each other through the window. Set up the grow light right on the basil, but angle it so some of the light gets to the sage and rosemary too. Turn it on during the afternoon and evening.
posted by EmpressCallipygos at 7:36 AM on October 31, 2011

I'm near Boston and leave my mint outside year-round in its ~15" diameter pot. It dies off after frost and comes back in fresh green shoots in the spring that's tall and leafy by midsummer. After 2 years of this cycle it got rootbound and I had to throw out 2/3 of the plant to get a good summer crop.

I think of basil as being an annual - I've never tried to overwinter it, just eat it all in fall and replant in spring.

Other herbs don't necessarily benefit from being old plants instead of young plants, but rosemary is a great plant to nurture for years - it can turn into a really gorgeous woody-stemmed shrub. I've seen some that look like little topiary plants or large bonsai trees. This is a good one to bring in for the winter, or at least into shelter.

My 2 long windowbox-style planters of thyme, sage, chives, oregano, and rosemary I hang on the fence in summer and bring onto my enclosed porch in winter. It does freeze on the porch, but only when it's super-cold outside. Then in spring, new fresh shoots grow (continuing where last year's growth paused) and the old leaves wither to varying degrees (thyme, rosemary not at all; sage somewhat, oregano turned borwn and fell off by late spring). The cold air doesn't bother them (frost on the leaves), the danger is if the soil freezes, that could damage the roots enough to kill the plant.

So, for you... if there's a way to hang them on your window-ledge outside, and tent the whole thing in clear plastic, you'll get the added bonus of trapping some of your indoor heat that escapes as well as trapping sunlight heat. Basically any frame-ype structure with a sheet of clear plastic over it would be fine. Hoops like this are a classic.
Another piece of advice: thermal ballasts: spray paint some empty milk jugs black and fill them with water. Set them under the tent in the sunlight - they'll warm up during the day, and the large volume of water will release the heat overnight, reduce the effect of any low-temperature spikes.
posted by aimedwander at 7:36 AM on October 31, 2011 [1 favorite]

The phrase you are looking for is "cold frame." They are pretty simple to build, and a google search will easily turn up different styles for building. There are also some other options though - here's a good overview of winter gardening techniques. I'm not sure that you can expect a lot of growth from your herbs in winter (many need a dormancy period), but it should help in keeping them alive.
posted by susanvance at 7:37 AM on October 31, 2011

The mint and sage should be fine -- I live in zone 5 and they die back every year but re-emerge the next spring.

Rosemary is more borderline ... you can try leaving it outside, but first snip off a few cuttings just in case the plant doesn't make it. You can root the cuttings in water to make new plants.

Basil needs to be brought inside since it is sensitive to cold. Basil is also VERY easy to grow from seed, so it would not be difficult to start new plants in the spring. You could harvest all of the leaves now, use them over the winter, and establish new basil plants once the weather warms up.
posted by Ostara at 8:59 AM on October 31, 2011

Use your basil to make up a big batch of pesto and can it or freeze it in individual servings. It will give you a blast of late summer tastiness when you're deep in February that way. You can start seeds for next year's crop on your windowsill in Feb-Mar and have transplants ready to go outside in April.

Rosemary in the ground does fine over winter in zone 7. For rosemary in pots, I cover with two layers of clear plastic bags and make sure the 2" deep saucer underneath is filled with rocks and kept moist outside. Works like a greenhouse, and concentrates those meager winter rays.

I agree that mint and sage pretty much make it through everything, including late spring frosts after they start budding out.

The best thing is, herb plants are inexpensive, grow like weeds, and will look exactly like they do now in four weeks' time next summer. Try adding dill so you get some spectacular foliage and flower heads for your end of summer treat next year.
posted by halfbuckaroo at 10:44 AM on October 31, 2011

I agree with the gardenign advice above - but please do not try to home can your pesto. There aren't any USDA-validated procedures for safe home canning of oil-based foods like pesto. Freeze it instead (I freeze mine in half-portions in ice cube trays and then repackage the cubes into more rational storage.
posted by janell at 10:50 AM on October 31, 2011

Thanks, Janell for the canning wisdom. I just figured if they could do it commercially, there must be a way to do it at home. Freezing definitely works great!
posted by halfbuckaroo at 2:54 PM on October 31, 2011

Basil is generally an annual. There's not much point in overwintering it. If you have a perennial variety it is probably too cold outside even with some shelter, and not enough humidity inside.

Keep the pots watered, do not let them dry out. A greenhouse-type shelter is probably your best bet. Herbs don't do well indoors in dry houses. Googling DIY mini greenhouse brings up this, among other results.
posted by oneirodynia at 3:54 PM on October 31, 2011

Thanks for all of the tips!

I decided to leave the mint outside to fend for itself. The rosemary and basil are inside on the windowsill, with a light on them for several hours a day. The sage and chives are outside under a makeshift greenhouse/cold frame that I constructed out of plastic sheeting and duct tape (hoping to make a better cold frame before the real cold hits). So far all the herbs seem to be coping with the changes, though it has been relatively warm in the past week.
posted by melissasaurus at 7:49 AM on November 8, 2011