The Green Giant Arborvitae (Thuja ‘Green Giant’) is a fast-growing evergreen prized for its dense, pyramidal shape and use as a privacy hedge. When the foliage begins to display yellowing, known as chlorosis, it indicates the plant is experiencing environmental or physiological stress. Understanding the cause of this color change is necessary to restore the plant’s healthy appearance.
Water Management and Soil Issues
The most frequent cause of chlorosis in Green Giant Arborvitae relates directly to water availability and soil quality. Yellowing results from a disruption in the plant’s ability to move water and nutrients from the roots to the foliage. This process depends on the balance of air and moisture within the soil structure.
Over-watering is a common culprit, especially in heavy clay soils or areas with poor drainage. Excess moisture displaces the oxygen necessary for root respiration, suffocating the root hairs responsible for water uptake. When roots die back, the plant cannot absorb water, leading to yellowing that mimics drought stress even in saturated soil. This root suffocation can quickly progress to fungal root rot, which further hinders the plant’s function.
Conversely, under-watering, particularly during the first year after planting, also triggers chlorosis. Newly planted arborvitae have a root system that requires consistent moisture to establish. If the soil dries out excessively, the plant conserves resources by shedding interior or older foliage, which yellows before browning. Monitoring soil moisture several inches deep is more reliable than simply wetting the surface.
Improper planting depth also impedes the tree’s health. Planting the root flare—where the roots transition to the trunk—too deeply invites moisture retention around the trunk base. This promotes bark decay and restricts gas exchange, leading to root stress and yellowing similar to saturated soil. The root flare must be level with or slightly above the surrounding grade for proper development.
Nutrient Deficiencies and Chemical Imbalances
If water and soil drainage issues are ruled out, yellowing may point to mineral deficiencies or chemical imbalances in the soil. Chlorosis is insufficient chlorophyll production, which requires micronutrients and macronutrients. The pattern of yellowing across the foliage often provides clues about which nutrient is missing or unavailable.
A lack of magnesium, a mobile nutrient, typically causes yellowing of the older, interior needles first, while new growth remains green. This occurs because the plant relocates magnesium from older tissues to support newer foliage. Conversely, deficiencies in non-mobile nutrients like iron or sulfur appear as chlorosis on the youngest growth tips, as these elements cannot be easily moved from older leaves.
Sometimes, nutrients are present in the soil but are chemically inaccessible to the roots, known as nutrient lockout. This commonly occurs when the soil pH is too high (alkaline) or too low (acidic), affecting the solubility of minerals like iron and manganese. For example, in highly alkaline soils, iron becomes chemically bound and unavailable for uptake, even if it exists in abundance. A soil test is the most accurate way to confirm pH levels and nutrient status.
Remediation may involve applying fertilizer or, if the pH is high, adjusting the soil chemistry with amendments like elemental sulfur. This brings the pH into a favorable range for nutrient absorption. Addressing the underlying soil chemistry is more effective than relying solely on fertilizer.
Identifying Pests and Pathogens
Beyond environmental and chemical causes, living organisms can induce stress and yellowing in Green Giant Arborvitae. Insect pests and fungal pathogens damage plant tissues, interfering with water and nutrient transport. Visual inspection of the foliage and stems helps distinguish these issues from other causes of chlorosis.
Spider mites, which are tiny arachnids, are a common pest whose feeding causes stippling or bronzing of the needles that resembles yellowing. Closer examination may reveal fine webbing spun between the twigs and needles, particularly on the plant’s interior. A strong spray of water can dislodge these pests, or horticultural oils can be applied for control.
The bagworm is another pest whose larvae spin spindle-shaped silk bags covered with foliage pieces. These insects strip needles from the branches, causing affected areas to turn yellow and eventually brown. Manual removal of the bags during fall and winter, before the eggs hatch, is the most effective control method.
Fungal diseases, such as blight, can cause localized yellowing and branch dieback. These pathogens require prolonged moisture on the foliage to establish and infect the plant. While root rot is often secondary to poor drainage, other fungal issues may require a targeted fungicide and improved air circulation.
Seasonal Changes and Environmental Damage
Not all yellowing requires intervention, as some instances are natural physiological processes. The Green Giant Arborvitae, like many evergreens, undergoes interior needle shedding. This seasonal event usually occurs in late summer or early autumn, where the plant sheds its oldest needles located closest to the trunk.
The interior foliage turns yellow or brown before dropping off, allowing the plant to conserve resources. This shedding is distinct because the yellowing is uniform across the interior and does not affect the green growth at the tips of the branches. If the yellowing is concentrated on the outer, newest growth, it suggests a stressor rather than natural shedding.
Environmental stressors can also inflict damage that results in chlorosis or browning. Winter burn occurs when the sun and wind desiccate the foliage during winter while the ground is frozen, preventing water uptake. This often results in yellowing or browning on the side of the plant most exposed to the elements or the sun.
Localized damage can be caused by de-icing salts used on roads and walkways, or by animal urine. Both salt and urine introduce high concentrations of soluble salts near the root zone, which can burn the roots and interfere with water absorption. This damage typically appears as yellowing or browning on the lowest branches or along the edge nearest the contamination source.