Browning, often referred to as dieback, signals that the Mediterranean native is experiencing significant environmental stress. While lavender is known for its drought tolerance, it is particular about its growing conditions. Browning is almost always a direct result of improper care, soil composition, or the onset of disease. Unraveling the cause requires examining the plant’s environment, starting with the soil and its moisture level.
Moisture Management and Soil Drainage
The most frequent cause of lavender browning is an imbalance in soil moisture, as the plant is adapted to lean, gritty, and dry conditions. This issue presents in two distinct ways: overwatering, which leads to root suffocation, and underwatering, which causes desiccation.
The symptoms of overwatering and subsequent root rot, caused by fungal pathogens like Phytophthora, are the most destructive. When roots sit in wet soil, they cannot absorb oxygen and begin to decay, turning black and mushy. This root failure prevents water movement, causing the foliage to droop and turn a mushy gray or brown, often starting at the base of the plant. In contrast, underwatering is rare but causes the tips of the leaves and flowers to turn a crispy, dry brown as the plant conserves moisture.
Lavender demands a soil structure that drains exceptionally well, ideally a sandy or gravelly loam, and will not tolerate heavy clay or rich organic matter. The optimal soil pH should be neutral to slightly alkaline (6.5 to 7.5), which mimics its native habitat and ensures proper nutrient availability. Amending heavy soils with coarse sand or fine gravel, or planting in a raised bed, prevents water from pooling around the roots. Allowing the soil to dry out completely between waterings is the correct practice, as waterlogged conditions are fatal.
The Impact of Light and Temperature Stress
Beyond moisture, environmental factors like light exposure and temperature extremes can trigger browning and dieback.
Lavender requires full, direct sunlight (a minimum of six hours per day) to produce the energy needed for robust growth. Insufficient light causes growth to become weak and spindly, leading to a decline and browning of the lower, shaded foliage as the plant sheds less productive leaves.
Temperature plays a significant role, particularly during the dormant season. Severe cold, strong winter winds, or unseasonal late frosts can cause “winter kill.” This damage manifests as the browning and death of outer branch tips and exposed foliage, especially on less cold-tolerant varieties like French or Spanish lavender. While the plant’s woody structure is generally resilient, the soft, younger growth is susceptible to desiccation from freezing winds.
Improper pruning is another form of stress that results in browning. Lavender should not be cut back into the old, woody base of the plant, as this section often lacks the dormant buds necessary for regeneration. Cutting too deeply prevents new growth from emerging, causing the pruned stem to die back and turn brown, leaving a permanently dead patch. The best practice is to prune only into the soft, leafy green growth, removing about one-third of the plant’s height after the main flowering period.
Diagnosing Pests and Fungal Infections
When environmental factors are ruled out, browning is often the result of biological threats, primarily fungal diseases that thrive in high humidity and poor air circulation.
Among the most common is Botrytis cinerea, or gray mold, which affects the stems, leaves, and flowers. This fungus appears as a fuzzy, grayish-brown mold that causes the infected parts to wilt and turn brown, often starting at the base of the plant.
Another serious fungal issue is Shab, caused by the pathogen Phomopsis lavandulae, which results in the sudden wilting and browning of entire shoots. Affected stems may exhibit tiny black specks called pycnidia, which are the spore-producing structures of the fungus. Both Botrytis and Shab require the removal and disposal of infected plant material to prevent the disease from spreading to healthy plants.
Pests, while less common than fungal infections, can also cause localized browning by damaging plant tissue. Spittlebugs, or froghoppers, are sucking insects that leave a distinctive white, frothy mass on the stems to protect their feeding nymphs. Damage from their feeding can lead to the browning of affected stem tips. Additionally, scale insects or mealybugs attach to the stems and suck out plant sap, causing yellowing and browning.