Planting iconic spring-flowering bulbs, such as tulips, daffodils, and hyacinths, is traditionally a fall activity. You can successfully plant these bulbs in December, though it comes with important caveats concerning weather and soil conditions. Successfully planting this late requires a deliberate shift in technique and an understanding of the biological process that must occur beneath the soil surface. This late planting window remains open only until the ground becomes completely frozen and unworkable.
Why Soil Temperature Matters Most
The success of a spring-flowering bulb depends on a process called vernalization, which is the necessary exposure to a prolonged period of cold temperatures. This chilling period changes the bulb’s internal chemistry, signaling it to break dormancy and prepare for spring flowering. If the bulb does not receive this cold exposure, it will not bloom, or the resulting flower will be short and stunted.
The primary goal of planting bulbs in December is to get them into soil that is cold enough to satisfy this requirement but warm enough to permit initial root growth. Ideal soil temperatures for hardy bulb root development are consistently below 50°F, often ranging between 40°F and 50°F. This temperature range encourages the formation of a robust root system before the ground freezes solid.
A strong root system anchors the bulb and allows it to absorb the necessary water and nutrients needed to survive the deep winter and fuel spring growth. Planting when the soil is too warm might cause the bulb to sprout foliage prematurely, exhausting its stored energy. Conversely, planting just before a sudden, deep freeze may prevent root development, leaving the bulb vulnerable to desiccation and damage.
Preparation Techniques for Late Season Planting
Late-season planting success depends heavily on adapting standard planting practices to colder conditions. Site selection becomes even more significant, as well-draining soil is crucial to prevent the bulb from rotting. Bulbs planted late are more susceptible to standing water, so choosing a location that drains freely or amending the soil is necessary to ensure survival.
To quickly improve heavy clay soil, incorporate coarse materials like sand or compost into the planting bed to a depth of at least ten inches. This amendment loosens the soil structure, preventing waterlogging around the bulb. Applying a low-nitrogen, high-phosphorus fertilizer, such as bone meal, during this preparation helps support the necessary root formation.
Adjusting the planting depth provides a measure of insulation against extreme cold. While the general rule is to plant two to three times the bulb’s height deep, planting slightly deeper—an extra inch or two—can offer better protection from severe cold snaps. This extra soil layer helps to moderate temperature fluctuations, especially in regions prone to freeze-thaw cycles.
The most important post-planting technique for December installation is immediate and heavy mulching. Applying a four- to six-inch layer of organic material, such as straw, shredded leaves, or wood chips, immediately after planting helps to regulate the soil temperature. This insulating layer delays the deep freeze and prevents the alternating freezing and thawing of the topsoil, which can physically heave the newly planted bulbs out of the ground.
When the Ground is Impassable
If the ground is completely frozen, snow-covered, or otherwise impossible to dig, planting directly into the garden is no longer an option. However, unplanted bulbs can still be saved and prepared for spring through alternative methods. One reliable technique involves potting the bulbs for chilling, essentially mimicking the natural cold period in a controlled environment.
To do this, plant the bulbs in pots filled with potting mix, ensuring the pot is deep enough for the bulbs to be covered with an inch or two of soil. These containers must then be stored in an unheated location where temperatures remain consistently cool, ideally between 35°F and 45°F, for ten to fifteen weeks. An unheated garage, cold frame, or basement is suitable for this purpose.
For bulbs that cannot be potted immediately, temporary storage is necessary to maintain their viability until the ground thaws or a more permanent solution can be arranged. Store the bulbs in a cool, dark, and dry environment, with a temperature range of 35°F to 45°F. Place the bulbs in a ventilated container, such as a paper bag or a mesh sack, and cover them with a dry medium like peat moss or vermiculite to prevent moisture loss.