Plants require light in order to engage in photosynthesis, converting light, carbon dioxide and water into sugars which feed the plant and allow it to grow. Each plant has a different minimum daily light requirement for growth. Short winter days slow plant growth to a crawl. Temperatures remaining below freezing also inhibit growth. Although many plants survive over winter, few will actually grow outdoors during the darkest, coldest months at northern latitudes. A few more can grow with protection from the cold, and even more can thrive all winter indoors under artificial lighting.
Many plants are adaptable to indoor winter growing, inside the home or in a heated greenhouse, provided that adequate artificial light is provided to make up for lack of sunshine.
Flowers like amaryllis, hyacinth and daffodils can be forced to bloom in winter. Shade-loving flowers like impatiens and African violet will thrive through the winter under lights.
Herbs for indoor winter growing include any of the mints, creeping thyme, patchouli, parsley, cilantro, lemon verbena, oregano, rosemary and bay.
Winter edibles that grow in the indoor garden include any of the leafy salad greens: spinach, lettuce, mizuna, endive, arugula and mesclun mixes. Indoor winter-grown salads can also include edible blossoms like nasturtium and violet, and microgreens sprouted from kohlrabi, sunflower, radish and broccoli seeds.
Cold Frames and Hoophouses
Unheated cold frames and hoophouses do not increase the amount of light plants receive during the winter, but they do allow the plants to get above-freezing temperatures on most days in all but the coldest, northernmost latitudes. Plants that can tolerate freezing and thawing will continue to grow through the winter under this protection, although their growth will be slower than it is in months with more sunshine.
Plants for cold-frame or hoophouse growing through the winter include spinach, kale, chard, mizuna, mache, endive, radicchio, arugula, sorrel, kohlrabi, broccoli raab, kale, radishes, beets, parsley and small varieties of Asian stir-fry greens. Many other plants go dormant through the darkest few weeks in a cold frame or hoophouse, but resume growth as soon as daylight hours begin stretching again. These include sprouting broccoli, carrots, cauliflower and winter varieties of cabbage.
In the coldest northerly climates, only mache and miner's lettuce will actually grow outdoors in winter, even under the snow. Kale, chard, spinach and radishes will sit dormant but still edible under snow and during deep freezes, then spring to life and grow during any winter thaw.
Gardeners in more moderate climates, where temperatures do not often fall below freezing, can grow outdoors any of the plants grown by more northerly gardeners in cold frames as well as other cool-weather plants like peas, beets, turnips and cold-tolerant lettuces.