A seedling is a young plant that has recently germinated, typically grown indoors before being transplanted into the garden. These young plants are particularly susceptible to cold exposure because they lack the established root systems and hardened tissues of mature plants. Understanding their specific temperature limits is foundational to successful gardening and prevents the loss of weeks of cultivation effort. Controlling the temperature environment directly influences the health and ultimate productivity of the plants.
Defining Critical Temperature Zones
The temperature threshold that proves too cold for a seedling depends entirely on its species, categorized as cold-tolerant or cold-sensitive. Cold-tolerant, or cool-season, seedlings thrive in lower temperatures and can withstand a light frost. This group, which includes cabbage, broccoli, kale, spinach, and lettuce, can survive temperatures dropping as low as 26°F with only minor damage.
Cold-sensitive, or warm-season, seedlings require much higher temperatures and suffer first from cold exposure. Crops like tomatoes, peppers, squash, cucumbers, and basil originate from warmer climates and can be damaged by temperatures well above freezing. For these vulnerable plants, the danger zone begins around 55°F, which can lead to chilling injury.
Chilling injury occurs when temperatures fall into a range between 32°F and 55°F, with the most sensitive species suffering damage when temperatures are consistently below 45°F. Freezing injury is a more severe threat that begins when the temperature falls to 32°F or below, resulting in ice formation that can be lethal to most seedlings.
The Mechanisms of Cold Damage
Chilling injury occurs when temperatures are cold but remain above the freezing point, causing physiological stress without ice formation. This non-freezing cold disrupts the plant’s metabolic processes, leading to enzyme failure and damage to the cell membranes.
The damaged membranes lose their ability to regulate the movement of substances, resulting in cellular dysfunction and a reduced capacity for the roots to absorb water and nutrients. Visible signs of this stress include wilting, yellowing, or browning of the leaves, which can stunt the plant’s growth.
Freezing injury is a mechanical and dehydrating event triggered by temperatures at or below 32°F. When water in the spaces between the cells freezes, it draws water out of the cells, causing dehydration. If the temperature drops quickly or severely enough, ice crystals can form inside the cells, physically puncturing and rupturing the cell membranes. This cellular destruction results in the plant tissue turning black and limp, often leading to the death of the seedling.
Acclimating and Protecting Young Plants
The process of preparing indoor-grown seedlings for the outdoor environment is called hardening off, which significantly increases their cold tolerance. This transition should occur over 7 to 14 days, gradually exposing the plants to cooler temperatures, wind, and direct sunlight. Start by placing seedlings in a sheltered, shaded location for just one or two hours on the first day.
Exposure time is then incrementally increased daily, slowly introducing longer periods of direct sunlight. This controlled stress encourages the formation of a thicker cuticle and stronger stems, which improves resilience to temperature fluctuations. After the full period, the plants are ready for permanent transplanting.
When an unexpected cold snap or frost is forecast, immediate protective measures are necessary. Watering the soil thoroughly the day before a freeze is beneficial because moist soil retains and radiates more heat than dry soil, stabilizing the temperature around the roots. Containerized seedlings should be moved indoors, into a garage, or grouped tightly against a warm structure.
For plants already in the ground, temporary shelters can be deployed to trap ground heat and block frost. Lightweight row covers, garden fleece, or inverted cloches can be placed over individual plants or rows. It is crucial that the protective material does not directly touch the foliage, as this can transfer the damaging cold, so supports should be used to create a small insulating air space.