by Kathy Martin 
About ten years ago my husband built a cold frame tucked up against the south side of our urban house. I had tried for years to overwinter potted rosemary in the house—each time it dried up and died. In my new cold frame, I planted a large rosemary and left it outside for the winter. It was the most amazing plant that year. It overwintered beautifully in the shelter of the cold frame, and I harvested fresh rosemary all winter, even when I had to dig down under the snow to open the frame. After overwintering, the plant was huge. It was very happy the next summer. So was I. I still remember the taste of those rosemary-spiced winter meals!
There are several ways to extend the herb harvesting season. These include fall and winter sowing of annual herbs and overwintering of perennial herbs. I prefer to grow and overwinter herbs outside in the garden.
Fall sowing is the simplest as it requires no cold protection. The timing of sowing is based on the date of the average first fall frost and is set such that plants are mature and ready for harvest during the last few weeks before frost. Winter sowing and overwintering require protection from the winter cold. The timing of winter sowing is based on the date when days become too short to support plant growth. Sowing time is set so that full-sized plants can be harvested all winter. I’m referring to “overwintering” here as an approach to extending the outdoor harvest of fresh perennial herbs by growing them with cold protection—like the rosemary I overwintered years ago in our cold frame
Many herbs can be overwintered inside, but often overwintering outside is the best option. It can provide more space, reduce the costs and time of setting up plant shelves and lighting indoors, and provide a healthier and more humid environment for many herbs. Bringing outdoor perennial herbs inside for overwintering brings the challenges of low indoor winter humidity and controlling any insect pests that may have been on the plant.
Fall Sowing
One approach to vegetable gardening is to plant in the spring, harvest all summer, and then clean up and put the garden away by midfall. However, the crisp fall days can be a great time to garden. Not only are temperatures cooling, but there are fewer bugs, fewer weeds, and less need for watering. Also many herbs and vegetables do best in the cool weather of fall. These crops can be set out in late summer, filling in spaces left by summer crops such as basil, potatoes, garlic, onions, and zucchinis.
Fall growing generally does not require any protection from the weather. Sowing is timed so that crops are harvested before the average first killing frost date. However, nature doesn’t always follow average dates. Some light covering may be needed on chilly nights. Old sheets or blankets or heavy row cover draped over the plants will do the job.
Annual herbs are best for fall sowing. My favorites include cilantro, dill, and mustard greens. I have sown fall salad mixes that include misuna, lettuce, tatsoi, and escarole. Cilantro and dill are great to plant repeatedly, and their leaves become plentiful in the cooler weather and shorter daylight of fall. They can spice up fall meals. Having fall dill and cilantro is also perfect for making end-of-the-harvest pickles or salsa fresca. Other herbs for fall sowing include basil, borage, chervil, and calendula.
Winter Sowing and
Overwintering Herbs Outside
Winter sowing extends the time that fresh herbs are available. Plants are available for harvest throughout the entire winter, plus there is a burst of new growth in very early spring. In areas that freeze, overwintering requires protection from the cold, snow, and winds. Cold frames, tunnels, and greenhouses are the most common approaches. The first two, cold frames and tunnels, are low cost, easy to set up, and flexible for different uses year to year. These are the two discussed here.
The harvest time of virtually any herb can be extended with a cold frame or tunnel. Most can also be overwintered even in the coldest climates, with the exception of tender, warm-weather herbs such as basil.
Annual herbs can be winter sown, while perennial herbs can be moved to or grown in the protected area. Annual herbs to winter sow include cilantro, dill, fennel herb, mustard greens, bunching onions, arugula, spinach, endive, escarole, and parsley. Tarragon and rosemary are tender perennial herbs and, with some luck, the same plant can be overwintered many years. Parsley is a biennial herb and can be overwintered once or winter sown. Parsley is very hardy and in milder winters or winters with good snow cover, it will overwinter with no protection, giving an early fresh spring harvest. Bunching onions, arugula, spinach, endive, and escarole aren’t herbs, but it’s a shame to write about winter sowing without mentioning these, especially spinach. These vegetables overwinter beautifully and can provide homegrown greens all winter.
Hardy herbs will generally survive the winter, but often not in a condition for harvesting during the darkest and coldest time of the winter. By placing these in a winter tunnel or cold frame, fresh leaves can be picked all winter. Hardy perennial herbs that I grow include sage, lots of varieties of thyme, oregano, chives, mint, chamomile, marjoram, and winter savory.
Building Cold Frames and Winter Tunnels
A cold frame is usually a wooden box covered with windowpanes or clear plastic. The frame rests directly over the soil. Cold frames can be purchased premade or constructed with a wide range of materials. A simple cold frame can be made using the following steps:
1. Build a box about 3 feet by 6 feet from untreated lumber. The 6-foot lengths will be the back and front of the frame. The back of the box should be about 18 inches high, sloping to about 14 inches at the front. The slope will position the top at an angle that enables more sun to reach the plants. The angle will also allow the cold frame to shed rain and snow.
2. Hinge an old window frame over the top of the cold frame, attaching it in the back along the highest side. Alternatively, fiberglass or clear plastic can be supported by panes without glass and used for the top. The insides of the cold frame can be insulated with rigid foam. Weather stripping along the top edges will also help to insulate the interior. In extreme cold or snow cover the cold frame with a tarp, heavy burlap, or an old blanket.
3. Position the cold frame so that it faces as close to south as possible. Positioning the frame next to a building will also help by trapping heat and protecting it from cold winds.
The cold frame my husband made was a fairly temporary structure with dimensions of about 6 by 12 feet. In a cold frame this big, I had to be able to get inside to reach all areas. He made the back about 4 feet high and the front about 2 feet. The frame had greenhouse plastic on the top panel, which generally lasted for two years before it grew brittle and needed replacement. The sides were all-purpose, 4-millimeter plastic sheeting from Home Depot supported on a wooden frame. Tears were mended with duct tape. We added rigid foam insulation in the coldest times of winter.
The frame was located in a very protected and warm spot. It was up against our white-painted house, facing southwest. Another house was nearby but not close enough to block the sunlight. Even without a cold frame, this spot cooled down slowly in the fall and warmed up fast in spring. I had to remember that a closed cold frame can build up a lot of heat on a sunny day. When temperatures went above 40 degrees Fahrenheit (4 degrees Celcius), I opened the frame to the air.
I used a remote thermometer to watch the temperature inside the frame since it was close to the house. The lowest temperature I recorded inside the frame was about 28 degrees F (-2 degrees C). This was on a very cold night that went into the teens outside the frame. I had covered the frame with a tarp. During the day, I pulled off the tarp and let the sun warm the soil as much as it could.
I had visions of rebuilding my cold frame someday. Perhaps a brick base and glass panes. Cables for heating the soil. But we moved away before I could rebuild it. At our new house I set up a winter tunnel instead of a cold frame.
A winter tunnel is easier to construct than a cold frame. It is a flexible, removable structure that can be easily repositioned to other locations. A winter tunnel can be made by following these steps:
1. Secure a 10-foot-long, ½-inch PVC pipe to two sides of a garden bed about 4 feet wide, forming an arch about 4 feet high. Repeat, placing pipes about 2 feet apart down the length of the bed. Secure the pipe by affixing a pole or board down the length of the bed at the top of the arches.
2. Cover with plastic, securing the edges with weights such as rocks, boards, or sandbags. For my tunnel, I use standard clear 6 millimeter greenhouse plastic, 12 feet wide, from an online garden supply store.
3. If winter temperatures regularly fall below 25 degrees F (-4 degrees C) or so, an extra layer of protection may be needed. Insulating row cover such as Agribon can be used on short hoops placed inside the plastic tunnel. Use 9-gauge wire to make the interior hoops. The cushion of air between the two layers will add a large degree of additional cold protection.
My winter tunnel is in the backyard of my house. It covers one of my 1-foot-high raised beds. The PVC pipes are secured to the outside of the raised bed with hardware brackets. It has protected my plants during cold snaps as low as -12 degrees F (-24 degrees C). It’s stayed up under snow covers of three feet or more. On warmer days I lift up the plastic from the south side and allow air circulation. I’ve had trouble with critters like voles and chipmunks, so I dug in ¼-inch hardware cloth 8 inches deep around the base of the bed, and this has solved that problem—so far.
Try using a remote thermometer to watch the
temperature inside the cold frame or tunnel. You’ll be able to watch the low and high temperatures and know when to either add layers or open the frame.
There can be a lot of flexibility in setting up a winter tunnel. A friend of mine built a tunnel she called a “tent house.” It was a tent of clear plastic vinyl over a series of two 1-by-2-inch boards that had been connected to form a peak at the top. Like me, she also used insulating row cover as a second layer over the plants inside during the coldest times.
When to Sow for Fall and Winter Harvesting
The trick to planting in the fall is to count backward from the average first fall frost date in your garden. To figure out when to plant a certain herb or vegetable, check the seed packet for its number of days to maturity. Then add about two weeks to account for the fall factor. The fall factor represents the slowing of plant growth as light fades and temperatures cool down. Subtract this number of days from your first fall frost date to find the time when you should sow (see sample table on page 41).
Leafy greens are generally best for fall growing. They mature fast and thrive in the cooler temperatures. Great crops to sow include cilantro, dill, mustard greens, lettuce, spinach, endives, and escaroles, arugula, and Asian greens. If you have space in a window at home, you may want to start seedlings in pots so you can water them evenly while the sun is still hot. Transplant to the garden after about three weeks.
Sometimes local nurseries will put some fall transplants out for sale, but availability is unreliable. If you’re willing to search, you may or may not be rewarded. Lack of availability of fall seedlings is great reason for sowing your own. The goal with winter sowing is to have your plants ready for harvest before the low light of winter stops their growth. In this condition, if plants are protected from the weather (cold, wind, and snow), they can provide a fresh harvest all winter.
The time to sow annual herbs for overwintering outside depends on your latitude, which determines when your daylight will fall below ten hours per day. Most plants don’t grow at all with less than ten hours of sunlight. It’s not a sudden change, but as daylight decreases, plants gradually stop growing. To have a crop that you can harvest all winter, annual seeds need to be sown at a time that allows them to reach their full size before the low sunlight stops their growth. The date when the hours of sunlight fall below ten a day for your city can be found online. The US Naval Observatory offers a tool to generate a table for your area here: http://aa.usno.navy.mil/data/docs/Dur_OneYear.php.
To determine winter sowing times, you’ll need to count backward from the date your day length falls below ten hours per day and then add a winter factor of about three weeks. As with calculating fall sowing times, check the seed packet for the plant’s number of days to maturity, add about three weeks, and then count backward. A table with some winter and fall sowing times is below:
|
Sample Sowing Times for Fall and Winter Planting |
|||||
|
Days before frost or 10-hour day |
Sowing dates for Boston area |
||||
|
Herb |
Days to Harvest |
Fall |
Winter |
Fall |
Winter |
|
Basil |
70 |
85 |
95 |
July 17 |
Aug 7 |
|
Borage |
50 |
65 |
75 |
Aug 6 |
Aug 27 |
|
Calendula |
50 |
65 |
75 |
Aug 6 |
Aug 27 |
|
Chamomile |
60 |
75 |
85 |
July 27 |
Aug 17 |
|
Chervil |
60 |
75 |
85 |
July 27 |
Aug 17 |
|
Chives |
80 |
95 |
105 |
July 7 |
July 28 |
|
Cilantro |
50 |
65 |
75 |
Aug 6 |
Aug 27 |
|
Cutting Celery |
80 |
95 |
105 |
July 7 |
July 28 |
|
Dill |
40 |
55 |
65 |
Aug 16 |
Sept 6 |
|
Parsley |
80 |
95 |
105 |
July 7 |
July 28 |
|
Shiso |
80 |
95 |
105 |
July 7 |
July 28 |
|
Spinach |
40 |
55 |
65 |
Aug 16 |
Sept 6 |
Spinach, generally considered a vegetable, is included here because it is an ideal plant for fall and winter sowing. It is very fast growing and cold tolerant.
Don’t worry if your winter sowing is later than these times. The young plants will perk up in the spring and begin growing once daylight exceeds ten hours and above-freezing temperatures are maintained in the cold frame or winter tunnel. Overwintering these small plants will give an early spring harvest that can’t be beat!
Hardy perennial herbs that are already established plants can be dug into the soil of a cold frame or winter tunnel for overwintering. Their leaves will generally remain tender and ready for harvest for at least a good part of the winter. Try thyme, oregano, and sage. Tender perennials can also be overwintered in a well-protected cold frame or tunnel. Try rosemary or tarragon.
For both fall and winter sowing, there are mobile apps and online sites that can do the backward counting (subtracting dates) for you. There are also apps that will calculate fall and winter planting dates for specific crops and specific locations. They are great fun to use and help you experiment with extending your herb harvest.