Arborvitae are popular evergreen trees, prized for their dense foliage and ability to create privacy screens and landscape accents. Homeowners often become concerned when their vibrant green arborvitae begin to turn yellow. This guide explores the various reasons why arborvitae might turn yellow, from natural occurrences to issues requiring attention.
Seasonal and Natural Yellowing
Not all yellowing in arborvitae signals a problem. These evergreens naturally shed older, interior needles, a process often called “flagging.” This shedding typically occurs in the fall, where inner foliage turns yellow before dropping, allowing the plant to redirect energy to newer, outer growth. The yellowing and browning from flagging are usually uniform throughout the inner canopy.
Younger, newly emerging growth on arborvitae can sometimes appear lighter green or even yellowish before it fully darkens to its mature green shade. This temporary discoloration is a normal developmental stage. Additionally, some varieties of arborvitae may exhibit a bronze or yellowish tint during colder winter months as a natural response to temperature changes, typically reverting to green in spring.
Environmental Stressors
Environmental factors frequently contribute to arborvitae yellowing due to imbalances in their growing conditions. Improper watering, both overwatering and underwatering, can cause foliage discoloration. Overwatering leads to saturated soil and root rot, causing yellowing or browning leaves. Conversely, insufficient watering or drought stress also results in yellowing.
Excessive direct sunlight, especially for younger plants or certain varieties, can lead to scorching and yellowing of foliage. Poor soil conditions, such as compacted soil or incorrect pH, hinder nutrient absorption, leading to yellowing. Arborvitae generally prefer well-drained soil with a pH between 6.0 and 8.0. Temperature extremes, including harsh winter winds and cold, dry conditions, can cause “winter burn,” where foliage turns yellow or brown. Exposure to de-icing salts from roads or walkways can also lead to salt damage and yellowed foliage.
Pest Infestations
Certain pests can cause arborvitae foliage to yellow as they damage the plant. Spider mites, tiny pests often indicated by fine webbing, feed on needles, causing yellowish speckling or stippling. Severe infestations can turn leaves brown.
Arborvitae leafminers, small larvae, burrow into leaf scales, causing affected foliage, particularly at the tips, to turn yellow and then brown. Bagworms can also defoliate arborvitae, leading to yellowing or browning as they construct their distinctive bags from plant needles.
Disease Issues
Several diseases can also lead to yellowing in arborvitae. Root rot, often a consequence of overwatering or poorly draining soil, damages roots, impairing water and nutrient absorption, which causes systemic yellowing and decline.
Canker diseases, caused by fungal or bacterial infections, manifest as localized yellowing, browning, and dieback of branches. They create sunken lesions on the bark that restrict water and nutrient flow, discoloring affected foliage. Phytophthora is a soil-borne disease that causes root rot, leading to yellowing and plant decline, especially in poorly draining soils.
Nutrient Deficiencies
A lack of essential nutrients can cause arborvitae foliage to yellow, with specific patterns indicating the missing nutrient. Nitrogen deficiency typically results in general yellowing of older, inner leaves, as nitrogen is a mobile nutrient reallocated to new growth. If yellowing appears on new growth, with areas between leaf veins turning yellow while veins remain green, this indicates iron chlorosis. This condition is common in alkaline soils where iron is present but unavailable for uptake.
Magnesium deficiency can also cause yellowing, often affecting older foliage first, similar to nitrogen deficiency. While less common than iron issues, a lack of magnesium hinders chlorophyll production, leading to pale or yellowed leaves. Soil pH plays a role in nutrient availability; if the pH is outside the optimal range, nutrients may be present but inaccessible, leading to deficiency symptoms.
Diagnosing and Resolving Yellowing
Addressing arborvitae yellowing begins with careful diagnosis.
Diagnosing Yellowing
First, observe if yellowing is confined to inner, older foliage in the fall, indicating natural flagging. If yellowing is widespread or occurs at other times, assess watering practices; check soil moisture a few inches deep to determine if it is too dry or overly saturated. Ensure the soil drains well and consider amending heavy clay soils to improve drainage.
General Care and Prevention
General care practices improve plant health and prevent many causes of yellowing.
- Proper watering involves deep, infrequent irrigation.
- Mulching around the base with a 2-4 inch layer of organic material helps retain soil moisture, regulate soil temperature, and suppress weeds; keep it a few inches away from the trunk.
- A soil test can identify nutrient deficiencies or pH imbalances, guiding fertilization.
- Pruning dead or diseased branches improves air circulation and plant vigor.
- Protecting arborvitae from winter burn with adequate fall watering, anti-desiccants, or windbreaks prevents cold-related yellowing.
For persistent or severe yellowing, especially with other concerning symptoms, consult a local arborist or extension office for expert diagnosis and treatment.