Perennials are plants that live for more than two years, returning season after season. The desire to start spring gardening early is understandable, but planting before the risk of freezing temperatures has passed requires careful consideration of both air and soil conditions. The “last frost date” is a statistical average, representing the final date in spring when temperatures typically drop to 32°F or below. While waiting for this date minimizes risk, understanding the dangers involved allows for a calculated, early start.
Understanding the Dangers of Cold Soil and Late Frosts
Planting into cold soil presents a significant risk because it severely inhibits the metabolic processes necessary for new root establishment. Roots thrive in a temperature range generally between 50°F and 65°F. Temperatures below 40°F can slow growth considerably, even bringing it to a halt for some species. When root growth is stalled, the perennial cannot properly anchor itself or absorb sufficient water and nutrients.
Cold soil also hinders a plant’s ability to take up nutrients like phosphorus, which is essential for energy transfer and root development. The activity of beneficial soil microbes, which convert raw nutrients into forms usable by the plant, also slows dramatically in cool conditions. This reduced nutrient uptake, combined with sluggish root growth, leaves the perennial weakened and susceptible to disease and transplant shock.
A more physical threat comes from the freeze-thaw cycles that cause frost heaving, particularly in moist, fine-grained soils like silt or clay. As water in the ground freezes, it expands, pushing the soil and newly set roots upward. When the soil thaws, the plant’s crown may remain lifted out of the ground. This process can sever feeder roots and expose the remaining root system to drying winds and cold air. Finally, an unexpected late hard freeze (below 28°F) can destroy new, tender leaf or shoot growth, wasting the plant’s stored energy reserves.
Strategies for Successfully Planting Before the Date
Mitigating the risks of early planting begins with assessing the physical environment, focusing on the soil rather than the air temperature alone. Before placing any plant in the ground, use a soil thermometer to confirm the temperature at a depth of four inches, which should preferably be consistently above 45°F for several days. This measurement is important because warmer soil encourages the root growth necessary for anchoring and nutrient absorption.
Nursery-grown plants must undergo a gradual process called “hardening off” before being permanently set out. This transition acclimates the plant to harsher outdoor elements, including direct sunlight, wind, and fluctuating temperatures. The process involves progressively increasing the plant’s exposure over seven to fourteen days, starting with a few hours in a shaded spot. Hardening off changes the plant’s growth from soft to firmer, making it more resilient to transplant shock and temperature dips.
Proper site preparation is important to prevent water pooling, which exacerbates the risk of frost heaving and root rot. Amending the planting area with organic matter improves soil structure and drainage. If a late frost is forecasted, temporary physical protection can be deployed, such as covering the plants with row covers, cloches, or inverted buckets overnight. A thick layer of mulch applied around the plant crowns helps stabilize and moderate soil temperature, reducing damaging freeze-thaw cycles.
Choosing Perennials That Handle Early Planting Best
Success with planting early often depends on selecting species with a natural tolerance for cold, damp conditions. Gardeners should choose perennials that are rated for hardiness zones colder than their own location; for example, planting a Zone 4 perennial in a Zone 6 garden provides a built-in buffer against unexpected cold. These hardier varieties are inherently better equipped to handle the stresses of early spring weather.
Dormant or semi-dormant plants, such as bare-root divisions or small container plants with minimal leaf growth, are generally better candidates for early planting than those already actively growing. Plants in a state of dormancy experience less transplant shock and possess a greater ability to divert energy toward root establishment in cool soil. Actively growing plants with soft new foliage are highly susceptible to frost damage.
Specific types of perennials are known for their ability to thrive in early spring conditions. Cold-tolerant varieties will begin growth even when temperatures are low and are often the first to emerge. Hardy choices that tolerate early planting include:
- Hellebores
- Pulmonaria (lungwort)
- Certain varieties of Hostas
- Dianthus
- Creeping Phlox
These plants are known for their resilience and ability to establish roots before the summer heat arrives.