Lupines (Lupinus spp.) are, by their fundamental genetic and physiological nature, plants that prefer cool to moderate temperatures and well-drained soils. Many popular garden varieties, such as the Russell Hybrids, trace their ancestry to species native to the Mediterranean basin and North American regions with excellent drainage and relatively low humidity. Their deep taproot system is an adaptation for accessing water in drier, often rocky or sandy soils, but it is poorly suited for waterlogged conditions. High heat, particularly when coupled with high humidity, creates a stressful environment that pushes the plant beyond its ideal physiological range, leading to a host of health and growth issues.
Prolonged exposure to temperatures consistently above 85°F (29°C) places significant stress on lupines. Their metabolic processes accelerate in the heat, increasing water demand. However, the root system may be unable to draw up water fast enough to keep up with transpiration losses through the foliage, leading to wilting. Furthermore, high heat can interfere with the plant's reproductive cycle. Lupines are classic cool-season bloomers, and intense heat often signals the plant to cease flowering and go dormant or to set seed prematurely, drastically shortening the ornamental blooming period that gardeners value.
While heat is a primary stressor, high atmospheric humidity is arguably the greater challenge for lupines in many regions. Humidity creates the perfect environment for the proliferation of fungal pathogens. Lupines are highly susceptible to powdery mildew, which appears as a white, powdery coating on leaves and stems, and to root rot diseases caused by fungi like Phytophthora. Humid, still air prevents foliage from drying out quickly after rain or watering, allowing these fungal spores to germinate and infect the plant. This constant moisture on the leaves and around the crown can quickly lead to decline and plant death.
Given these physiological constraints, success with lupines in hot and humid regions requires careful planning and mitigation. In the American Southeast and Deep South, lupines are extremely challenging and are often treated as short-lived perennials or even annuals for spring display. In the Mid-Atlantic and parts of the Midwest, success is possible with strategic care. The single most critical factor is perfect drainage. Planting in raised beds or amending soil heavily with grit, sand, or perlite is essential. Afternoon shade is crucial to protect plants from the most intense heat. Maximizing air circulation by providing adequate spacing helps leaves dry faster, reducing fungal disease pressure. A strict watering regimen that waters the soil directly (avoiding the foliage) and only when the top inch of soil is dry is vital. Applying a thin, light-colored mulch can help keep root zones cooler but must be kept away from the plant's crown to prevent rot.
