Johnny's Winter Growing Guide
A recent New England growers' meeting revealed a ranking of crops proven successful for winter growing regionally. In order of popularity, these crops fell into three tiers:
Hardiness, adaptability to winter growing, and quality and quantity at harvest time also factor into the ranking.
In addition, there are other crops which thrive in the winter: specialty greens such as Sylvetta, Minutina, and Mâche can be offered to gauge customer interest; and Onions and Leeks provide further winter growing options. Depending upon your experience, methods, latitude and microclimate, you may gain success with others as well.
Here are the top performers in our winter growing trials, crop by crop….
Winter spinach is exceptionally sweet because the plant builds up sugars in response to cold, which protect its cells from bursting in freezing conditions. It's also easy to grow for all skill levels, which makes it a good first choice if you are just starting out with winter growing.
Seed your winter-harvest spinach 35–50 days before the start of the Persephone period. Spinach does not germinate well under the warm conditions that often exist when seeding for winter. To optimize germination rates, irrigate before planting to cool the soil. You can also start spinach in plugs and grow to transplant size (two true leaves) to ensure your desired plant density.
A reliable choice for winter-harvest spinach is Red Kitten. Flamingo and Emperor are two other spinach varieties that can be seeded in late fall for winter harvest. Corvair performed very well in our recent caterpillar trials, and we have seen it overwintered very successfully at other farms. Seed so the plants are one inch tall when the hard freeze begins. They will grow quickly in spring and must be harvested before bolting.
Kale, like spinach, is much sweeter in the coolest months of the year. All varieties of kale can be grown in the winter, but curled-leaf types are a bit hardier, and make for bigger bunches more quickly — you will spend less time harvesting the kale and your bunches will look fuller.
Seed kale in late July or early August for transplanting in September. Cover with lightweight row cover if flea beetles or cabbage loopers are a problem. Harvest late October through March by clipping the leaves from the bottom up. It may not be necessary to protect kale for winter harvest if the crop will be used up by the time the coldest mid-winter temperatures hit.
Widely fluctuating temperatures can, however, result in cold damage to the leaves, and even hardy kale will need protection for overwintering at higher latitudes and more exposed microclimes. Kale plants of varying size can be kept overwinter in low tunnels, for harvest in early spring. But again, the quality of the large leaves remaining in spring will depend upon the severity of the preceding weather. Surviving small plants will quickly begin producing new growth.TIER 1–2
Pac Choi is a great candidate for winter growing due to the thickness of its stems, which can endure a measure of freeze damage. Other Asian Greens, including Komatsuna, Mizuna, and Tatsoi, should not be overlooked, as they too can deliver the winter results you're looking for. Use caution with many of the mustard varieties, however; we have found them to bolt rather quickly following the winter solstice.
While they may not grow significantly during the Persephone period, they can be harvested during those darkest weeks.
Cress makes another great addition to a winter growing program because of its quick growth cycle.
Greens for winter harvest should be planted from August through October for harvest from September through March. Soil temperature when seeding should be 70°F/21.2°C or lower, to optimize germination rates.
All of these greens will survive over the winter as very small plants for extra-early spring growth and harvest.
Cilantro should be sown mid September to early October in a protected structure, such as a high tunnel.
Cover plants with heavy-weight row cover when temperatures dip below freezing in the structure, but try to uncover when temperatures warm back up, to better expose the plants to sunlight and allow for air flow.
Plants will grow very slowly through the depths of winter, but take off in late winter to early spring, with harvests possible from March through May.TIER 3
— Sandy Arnold, Pleasant Valley Farm, New York
Lettuce is less cold-hardy than many greens, and fares best in a partially heated greenhouse or under a low tunnel within an unheated hoophouse. We suggest harvesting the lettuces before they're required to endure the coldest temperatures post mid winter. In addition, the young leaves of salad mix tend to be less susceptible to freeze damage than mature lettuce heads.
Recently, growers in the north have been reporting some success with Salanova Lettuces grown for mini head production in unheated high tunnels. Salanova has also excelled in winter trials at our own research farm in Albion, Maine. Setting out transplants in late September, and covering them with two layers of supported row cover inside the tunnel, provides nice mini heads with minimal leaf damage for winter harvest.
One limiting factor in high tunnels is the filtering of sunlight that decreases the vivid red color of some lettuce varieties. For winter growing, we recommend Five Star Greenhouse Lettuce Mix for its downy mildew resistance as well as the ability of the red varieties to hold their red color. During winter months varieties in some mixes may have varying growth rates. Some growers prefer to plant each variety individually and mix them after harvesting.
Radishes should be sown September through October for harvest through December. They should be covered with lightweight row cover if flea beetles are a problem.
Although radishes are generally quite cold-tolerant, they will become spongy if frozen hard repeatedly.
You can direct seed turnips in the fall. Plant enough seed to provide for a long winter-harvest period, and lay row cover over the crop if flea beetles are a problem.
Be advised that turnips will not hold into the spring, and will bolt by March in the tunnel.
Winter-harvest carrots are super sweet. In addition, they are orange — unlike most fresh winter-harvest crops — adding a welcome touch of color to what you offer for sale.
Carrots should be direct-seeded in early August for harvest from Thanksgiving through Christmas. If grown under row cover, their tops will be protected. Selling bunched carrots with attractive tops signals freshness to prospective customers.
Carrots can also be successfully overwintered as young plants, to grow and reach harvest size in early spring.
Bunching Onions (Scallions), Spring Onions & Leeks
Bunching Onions (scallions) are easy to grow for winter harvest, although some protection may be required if temperatures are extremely cold.
Direct-seed bunching onions in August, and start harvesting when they reach the desired size. Or leave some of your earlier crop in the field to overwinter, divide, and replant in the spring. Evergreen Hardy White is a standard variety for this method.
Select varieties of bolt-resistant onions can be overwintered in low tunnels in northern areas of the country. Onions for overwintering should be sown in late August to early September, and transplanted out in late September to early October with the goal of having them reach the size of a pencil (about 4–5 leaves) before the hard freeze in November. If they make it through the winter, you will have full-size spring onions by late May to early June. In Quick Hoops trials at our research farm in Albion, we found that Bridger is great for this application.
Many varieties of leeks are winter-hardy to varying degrees. Protection in tunnels or by row covers will further enhance their survival. Lexton and Bandit are particularly winter-hardy leek varieties.
Winter growing is not a simple process with a guaranteed return. If you have an interest and the resources required, it can be rewarding in many ways. Your winter harvests can command a premium price, being in higher demand than the same crop in the summer. Many growers find that fresh greens nicely augment a selection of winter storage crops at winter farmer's markets and in CSA distributions.
If you complete construction of your first new high tunnel in late summer, you will likely want to plant something right away. That first winter harvest can be very inspiring. For those with just one tunnel to utilize, the wisdom of ripping out high-return crops like tomatoes, cucumbers, and peppers from the tunnel in late summer may seem misguided, if they are still producing. But as winter draws closer, the quality and quantity of those crops will decrease. Your best decision may be to replace heat-loving summer crops with ones that perform well during the darkest time of the year. With clever succession strategies and the construction of additional tunnels, you can reap the benefits of fresh produce harvested even in the depths of winter.