The Aeonium genus encompasses a group of striking rosette succulents celebrated for their dramatic, sculptural forms. These plants, often called tree houseleeks, are native to the Canary Islands and thrive in Mediterranean-like climates. Unlike most common succulents, the successful cultivation and survival of Aeoniums depends on understanding their unique temperature tolerance and corresponding growth cycle.
The Optimal Temperature Window and Dormancy Cycle
Aeoniums are designated as winter-growing succulents, which fundamentally dictates their temperature needs. Their period of active growth occurs during the cooler, wetter months, typically from late fall through spring. The optimal temperature range for this vigorous growth is consistently between 65°F and 75°F (18°C and 24°C).
When temperatures consistently rise above 80°F to 90°F (27°C to 32°C), the plant’s metabolism slows down, triggering a necessary period of summer dormancy. This dormancy is a survival mechanism where the plant conserves moisture and energy to endure hot, dry conditions. During this time, the plant significantly reduces its water uptake and growth, pausing its life cycle until cooler temperatures return.
Understanding Cold Stress and Frost Damage
Aeoniums possess a low tolerance for freezing conditions, making cold stress a major threat to their survival in many climates. While they thrive in cool winter temperatures during their growing season, they are generally only hardy down to a range of 25°F to 30°F (-4°C to -1°C). Even temperatures consistently below 40°F (4.5°C) can cause significant stress, and light ground frost can damage tender varieties.
The damage from a hard freeze is immediate because the water stored in the succulent tissues expands when frozen, rupturing cell walls. Signs of frost damage include leaves that appear water-soaked, quickly turning dark, mushy, and blackened. If the temperature drops below 23°F (-5°C), the entire plant structure can suffer necrosis and collapse, often resulting in the death of the stem.
Managing Extreme Heat and Summer Dormancy
The upper temperature limit for Aeonium tolerance is around 100°F (38°C), but stress begins well before this point. Prolonged exposure to temperatures exceeding 80°F to 90°F exacerbates natural summer dormancy and heightens the risk of secondary damage. Heat stress is often visible as the rosettes tightly closing inward.
This rosette closure is a protective measure to minimize sun exposure and reduce transpiration, preventing excessive water loss. Other signs of heat stress include shedding lower leaves, pale coloration, and visible sunburn appearing as bleached or brown spots. In this dormant state, the plant is susceptible to desiccation, and high temperatures can damage the root system if the soil is too dry or too wet.
Actionable Steps for Temperature Protection
Protecting Aeoniums involves intervention during both the cold and hot extremes of the year. To mitigate cold threats, potted plants must be moved indoors or into a frost-free environment, such as a garage, before nighttime temperatures drop below 40°F. For in-ground plants, temporary protection can be provided by covering them with horticultural fleece or a frost cloth when a freeze is predicted.
During summer, the primary protective action is providing shade, particularly during intense afternoon hours. Moving potted plants to a location that receives only morning sun, or using a shade cloth for in-ground specimens, prevents leaf scorching and overheating.
It is necessary to reduce or completely withhold water during peak summer heat to support the plant’s natural dormancy. Since the plant is not actively growing or taking up moisture, watering can lead to fatal root or stem rot. Good air circulation is beneficial year-round, as stagnant air combined with high heat can contribute to fungal issues.