December is a month of transition in the garden, presenting a wide range of possibilities that are almost entirely dictated by geography. For many gardeners, the short days and cold temperatures mean December is dedicated to planning and maintenance. However, for those in warmer regions, this is an active planting season for a thriving winter harvest. The decision to plant outdoors or prepare indoors hinges on understanding your specific climate conditions.
Determining Your Winter Hardiness Zone
Successful December gardening requires identifying the minimum cold temperatures your region experiences. The USDA Plant Hardiness Zone Map is the most common tool for this, defining zones based on the average annual extreme minimum temperature. Each zone represents a 10°F difference in this average low temperature, which directly impacts a plant’s ability to survive the winter.
This system differentiates between mild and cold December climates. Mild winter climates, generally corresponding to USDA Zones 8 and warmer, rarely experience sustained freezes, making outdoor planting viable. Conversely, in cold climates (Zone 7 and below), the ground is often frozen or temperatures frequently drop below freezing, making direct outdoor seeding impossible. Localized microclimates, such as those near a large body of water, can slightly alter the temperature profile compared to the general zone map.
Outdoor Planting for Mild Winter Climates
Gardeners in mild climates can treat December as a peak season for cool-weather vegetables. These areas provide the necessary cool, frost-free conditions that prevent crops from bolting. Planting now ensures a continuous harvest through the winter and an early spring yield.
Root crops thrive when planted directly into the ground during December’s cooler soil temperatures. Carrots and radishes are excellent choices, as cold weather enhances their flavor by causing the plants to produce sugars as a defense mechanism against freezing. Radishes mature quickly, often being ready for harvest in a matter of weeks. Beets can also be sown now for winter growth.
Leafy greens are a staple of the December garden in warmer zones, benefiting from reduced pest pressure and cooler temperatures. They can be direct-seeded or transplanted now. Kale and collard greens are cold-hardy, sometimes tolerating temperatures near 10°F, and their leaves become sweeter after a light frost exposure.
- Spinach
- Kale
- Lettuce
- Swiss chard
December is also an ideal time to plant brassicas, although they often benefit from being transplanted as seedlings rather than direct-sown. Broccoli, cabbage, and cauliflower require a long, cool growing period to form their heads correctly. Planting starts now allows them to mature before summer heat returns. These cool-season crops need full sun exposure during the shorter days of winter to ensure adequate photosynthesis.
Alliums require a long cold period to develop properly. Garlic cloves must accumulate sufficient “chill hours” over the winter to produce large bulbs for a summer harvest. Onions, typically planted from sets or small plants, also benefit from a December planting to ensure a spring harvest.
Indoor Starting and Preparation for Cold Climates
In colder regions (Zone 7 and below), December marks the beginning of indoor preparation and outdoor maintenance. Direct outdoor planting is not feasible, so efforts shift to tasks that set the stage for spring. Seed starting indoors focuses on long-maturing vegetables that need an extensive head start before the last frost date.
Onions, which require a long period to bulb up, and peppers are prime candidates for December indoor starting. These seeds should be sown eight to ten weeks before the expected transplant date. Using a heat mat is beneficial for peppers, as their germination rate is improved by soil temperatures between 75°F and 85°F.
Outdoor efforts focus on soil health and planning rather than planting. December is an excellent time to amend garden beds with compost or aged manure. Applying compost now allows the nutrients time to break down and integrate into the soil before spring planting. If the ground is not yet frozen solid, sowing a cover crop, such as winter rye, helps prevent soil erosion and nutrient leaching.
Gardeners can also use December to plan the layout and perform tool maintenance. Cleaning and sharpening tools, organizing the shed, and ordering seeds for desired spring varieties ensures a smooth transition once the weather breaks, maximizing the limited planting time in spring.