Evergreen trees retain foliage year-round, so the sudden appearance of brown needles in the summer is alarming. Browning is a physical response indicating the plant is under stress. This discoloration is a symptom triggered by specific environmental pressures or biological factors active during the growing season. Understanding the cause requires looking closely at the tree’s symptoms and its immediate surroundings.
Environmental Stress and Water Issues
The most frequent causes of summer browning are related to the physical environment, primarily involving moisture regulation. Evergreens possess a protective waxy layer on their needles, called the cuticle, which helps minimize water loss through transpiration. During prolonged summer heat, however, water can evaporate from the foliage faster than the roots can supply it, leading to desiccation.
When the tree cannot maintain its water balance, a state of drought stress occurs, forcing the plant into a survival mode. The evergreen will intentionally sacrifice older or outer needles to reduce the total surface area requiring water. This response manifests as browning, often starting at the needle tips or the top of the tree where water delivery is most challenging.
The tree’s location can also expose it to heat scorch. Intense midday sun, especially when combined with heat reflected from nearby hardscape like pavement or brick walls, can injure the needles. This rapid browning, sometimes called sunscald, is a thermal injury distinct from simple dehydration. Needles exposed to this reflected heat can turn brown quickly, even if the soil moisture is adequate.
Environmental injury is often exacerbated by previous seasons’ stresses, as evergreens can take months to show damage symptoms. A tree weakened by drought the previous winter may be more susceptible to browning during a moderate summer heat wave. Deep, regular watering is necessary during dry spells to ensure the root system can replenish the water lost through the needles.
Pests and Disease Activity
Browning can also be caused by biotic agents, particularly insects and fungi that thrive in warm conditions. One common pest is the spruce spider mite, active during hot and dry weather. These tiny arachnids feed by sucking fluid from the needles, causing characteristic stippling (small yellow or bronze pinpricks) that eventually leads to widespread browning and needle drop.
A heavy mite infestation can be identified by the fine, silken webbing left on the needles, or by tapping a branch over white paper to observe the small dark mites. Other invaders, such as bark beetles and borers, cause localized damage by tunneling into the wood beneath the bark. This internal damage disrupts the tree’s vascular system, preventing water and nutrients from reaching the needles, causing the affected branch to brown and die rapidly.
Fungal infections, often grouped under the term needle cast or blight, also cause summer browning, though the infection may have started months earlier. These diseases often become noticeable during the summer when the plant is stressed by heat or humidity. Unlike environmental browning, which is often uniform, disease-related browning tends to be uneven and spotty, affecting specific branches or sections of the canopy.
Needle cast fungi cause lesions on the needles, which eventually girdle the tissue and stop the flow of water, resulting in discoloration. The appearance of small, black fungal structures emerging from the needles’ breathing pores can serve as a diagnostic cue for a fungal infection. When browning is random, spreading, or affects only the newest growth, a pest or disease is likely the underlying cause.
Distinguishing Natural Needle Drop
Not all summer browning indicates a problem; it can sometimes be a normal, healthy process called seasonal needle drop. Evergreens shed their oldest foliage on a predetermined cycle that varies by species. For example, some pines hold needles for only two years, while spruces often retain them for five to seven years.
This natural shedding typically occurs on the inner needles, those closest to the trunk and on the oldest sections of the branch. The new, green growth at the tips remains unaffected, which distinguishes it from disease or pest damage. The browning process is usually gradual, though species like white pine and arborvitae can shed many needles simultaneously, making the process look dramatic.
The cycle often peaks in late summer or early fall, but it can begin earlier in the summer months, especially during periods of environmental stress. If the browning is limited to the interior of the tree and the outermost needles are vibrant and green, the tree is simply self-pruning. This process is beneficial, allowing the plant to redirect resources toward its newer, more photosynthetically active foliage.