Nasturtiums are popular, easy-to-grow annuals prized for their vibrant, edible flowers and peppery leaves. Despite their low-maintenance nature during warmer months, these plants are highly sensitive to cold temperatures. They are tropical in origin and thrive in mild conditions, meaning they are considered tender annuals in most climates. Even a light frost is the primary threat to the longevity and health of nasturtium plants.
Nasturtium’s Vulnerability to Freezing Temperatures
Nasturtiums are unable to tolerate temperatures that drop to 32°F (0°C) or below. This temperature threshold is where physiological damage begins within the plant tissue. When a hard freeze occurs, ice crystals form in the spaces outside of the plant cells, known as extracellular freezing. This action draws water out of the cells, causing severe dehydration. The resulting water loss leads to the collapse and rupture of the cell walls, a process that is irreversible. This internal damage manifests externally as wilting, blackening, and a mushy texture on frost-killed foliage and stems. The plant’s tropical heritage means it lacks the internal mechanisms to harden against prolonged or severe freezing.
Immediate Steps for Frost Protection
Gardeners expecting a sudden cold snap can take proactive steps to protect their nasturtiums from damage. For container plants, the simplest solution is to move them to a sheltered location, such as a garage, covered porch, or indoors near a sunny window. This relocation provides immediate insulation against dropping temperatures.
For plants rooted in the ground, temporary physical barriers must be used to trap ground heat and shield the foliage. Materials like floating row covers, old sheets, or large cardboard boxes should be draped over the plants before dusk. Ensure these covers extend all the way to the ground and are secured to prevent warm air from escaping. Additionally, thoroughly watering the soil before a light frost can help, as moist soil holds and slowly releases heat more effectively than dry soil, insulating the roots.
Assessing Damage and Seasonal Planning
Assessing Damage
If a frost event has already occurred, the extent of the damage should be assessed by observing the foliage. Nasturtium leaves and stems that have turned black or dark brown and feel soft or waterlogged are likely dead and will not recover. While the plant may sometimes survive a very light, short-lived frost if only the outer leaves are affected, a hard freeze typically kills the entire plant down to the root.
Seasonal Planning
Because of this extreme sensitivity, nasturtiums are overwhelmingly grown as annuals in most regions. To maximize the growing season, seeds should be sown directly outdoors only after the danger of the last expected spring frost has completely passed. This timing ensures that the young, tender seedlings are not exposed to lethal temperatures.