December is often seen as a time for garden rest, but it represents a strategic period for planting activities that prepare the ground for the following seasons. Successful gardening depends heavily on understanding local climate conditions and adjusting techniques accordingly. Planting in December focuses on establishing roots and initiating cold periods for future blooms and harvests. Activities range from direct outdoor sowing in mild climates to starting crops indoors and installing dormant perennial stock. Careful planning ensures plant survival and vigorous growth once warmer weather arrives.
Outdoor Planting for Mild Winter Climates
Gardeners in regions with continuous mild winter weather (typically USDA hardiness zones 8 and above) can treat December as an extension of their cool-season calendar. The ground remains workable, allowing for the direct sowing of cold-tolerant vegetables and the installation of winter-blooming flowers. These temperate conditions provide an ideal environment for root establishment without the stress of summer heat or hard freezes.
Cool-season vegetables thrive in the mild, shorter days of December, often developing a sweeter flavor profile after light frosts. Leafy greens can be sown directly into well-drained soil amended with compost. The reduced light intensity naturally slows the plant’s growth, preventing bolting and extending the harvest period into early spring.
Root vegetables are also suitable for direct December planting in these warmer zones. These crops benefit from the consistent cool soil temperatures, which encourage steady root development without the rapid growth that can lead to woodiness. Proper soil preparation is important, involving deep loosening and the removal of rocks to allow for uniform root expansion.
For visual appeal, annual flowers such as pansies, violas, and snapdragons can be planted as transplants in December. These varieties are tolerant of cool temperatures and provide consistent color through the winter months. The perennial Christmas Rose, or Helleborus, is another excellent option, as its blossoms appear even when snow is on the ground, adding resilience to the winter garden.
Starting Seeds Indoors for Spring Preparation
December is the optimal time to initiate the growth cycle for many plants that require an extended indoor period before they can be safely moved outside in spring. This technique is beneficial for warm-weather crops, which have a long maturity timeline and cannot tolerate cold soil or frost. Starting seeds indoors grants a significant head start, translating into earlier and more substantial harvests.
Long-season vegetables like peppers, eggplants, and artichokes need 10 to 12 weeks or more of indoor growth to develop into robust transplants ready for the spring garden. These species require sustained warm temperatures, typically a soil temperature between 75°F and 85°F for optimal germination, provided artificially using heat mats. Once germinated, seedlings need a consistent light source, such as fluorescent or LED grow lights, positioned just a few inches above the foliage to prevent them from becoming weak and leggy.
Slower-growing herbs, including rosemary and thyme, also benefit from a December start, allowing them to form a sturdy root system before planting outside. Early tomatoes can be started now, provided the gardener has the space to manage the large plants indoors for several months. The seed-starting mix should be light and airy, often containing perlite or vermiculite, to ensure proper drainage and prevent fungal issues like damping off.
Planting Dormant Bulbs and Bare-Root Stock
December serves as an important window for installing plants purchased in a state of winter dormancy, including spring-flowering bulbs and bare-root trees and shrubs. Planting while dormant minimizes transplant shock and allows the root system to establish itself before the demanding growth period of spring. For this to be successful, the ground must be thawed and workable, not frozen solid.
Spring-flowering bulbs like tulips, daffodils, and hyacinths require a period of cold temperatures, known as vernalization, to trigger the hormonal processes necessary for blooming. Tulips need 12 to 16 weeks of sustained soil temperatures below 50°F to develop properly, making a December planting viable in many climates. Daffodils are more forgiving and can be planted later, as they do not require as long or as cold a chilling period to initiate flower buds.
When planting bulbs, they should be placed pointed-end up at a depth approximately two to three times their diameter, and the soil should be gently firmed around them. Bare-root stock, which includes fruit trees, berry bushes, and roses, should be planted as soon as possible after purchase to prevent the roots from drying out. Before planting, the roots should be soaked in water for several hours to rehydrate the fine root hairs.
The planting hole for bare-root stock should be dug two to three times wider than the root spread, but only as deep as the original growing depth. A small mound of soil can be created in the center of the hole to gently spread the roots over, ensuring the graft union on trees remains several inches above the final soil line. Backfilling the hole with a mixture of native soil and compost, then firming it gently to eliminate air pockets, allows the plant to begin root development immediately.
Cold-Weather Crops for Protected Gardens
Gardeners in colder regions where the ground freezes can still engage in December planting by employing protective structures and focusing on extremely cold-tolerant crops. These techniques allow for the extension of the growing season, even when ambient temperatures drop below freezing. The goal is to moderate the soil and air temperature around the plant, preventing lethal temperature fluctuations.
Specific crops such as garlic and overwintering onions are planted directly into the ground in December, relying on the winter cold to initiate a process called bulbing. Garlic cloves are typically planted a few inches deep and require a thick layer of heavy organic mulch, such as straw or shredded leaves, to insulate the soil and prevent repeated freeze-thaw cycles that can heave the cloves out of the ground.
For leafy greens, a protected environment like a cold frame or a low hoop house covered with clear plastic or a heavy frost blanket is necessary for December planting. Under this protection, crops like mature kale, spinach, and Brussels sprouts can survive temperatures into the low 20s Fahrenheit. The structure traps radiant heat released from the soil, keeping the plants in a state of suspended animation or very slow growth, ready to resume production at the first sign of warmer weather.