Why Are My Arborvitae Turning Brown?
Aborvitae are rugged performers with a stately presence that anchors the landscape all year. Whether the native species or their cultivars, their multitude of forms, sizes, and colors highlight various garden scales. While durable, they are susceptible to disorders that cause browning. Garden expert Katherine Rowe outlines reasons an arborvitae may turn brown and ways to remedy the situation.
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Arborvitae, in their multitude of forms from columnar trees to low-growing shrubs, are versatile across garden spaces. The adaptable evergreens are low-maintenance and easy to grow in the right conditions.
Thuja occidentalis is a North American native tree that occurs naturally in the east. Thuja plicata is native to the western region, with other species native to East Asia. The ubiquitous trees create a quiet backdrop that anchors the landscape year-round, providing attractive foliage and shelter for birds and wildlife.
When we see arborvitae turning brown, there are a few primary causes to explore, including natural leaf drop. The remedy may be as simple as clipping off the damaged leaves after a harsh winter to make way for fresh growth.
About Arborvitae
Arborvitae have flat, scaly leaves arranged in fanning clusters in rich greens and golds. Small cones appear on the ends of branches.
Thuja occidentalis and Thuja plicata are tall, columnar trees that reach 30 to 50 feet high. Hybridization creates numerous cultivars that range from small round globes to mid-size pyramidal specimens.
While the foliage naturally thins and fades as the species age, cultivars improve on density and coloration. If you don’t have space for a giant, more compact options abound. Thuja makes excellent evergreen screens to define a space, create privacy, or shield utilitarian objects.
They also make good windbreaks, though we’ll cover browning that can occur as a result.
5 Reasons Arborvitae Turn Brown
If you notice your arborvitae turning brown, it’s important to act quickly. Identify which one of these causes is most likely to tackle the problem.
Seasonal Drop
Arborvitae and other conifers experience a natural needle drop as foliage ages. Older leaves turn brown and cling to branches for a time before dropping.
They drop mostly in fall, along with deciduous species. The dropping of leaves two years or older makes way for new foliage further out on the stem to perform plant processes.
With seasonal drop, you’ll notice your arborvitae turning brown internally first. These don’t receive the amount of sunlight that newer leaves experience as branches grow. It’s typical for internal leaves to dry and fall.
With natural leaf drop, you’ll notice brown foliage, free of spots, and without yellowing first.
Drought Stress
Thuja are adaptable, and while they prefer organic, well-drained soils, they grow in a range of types, including rocky. They also adjust to varying soil pH, from slightly acidic to slightly alkaline (6.0 to 8.0).
But the hardy evergreens aren’t drought-tolerant. They don’t withstand prolonged dry spells without a physiological response, namely, arborvitae turning brown. Leaves turn yellow and become crispy with a lack of moisture.
Arborvitae benefit from one to two inches of water per week, whether from rainfall or irrigation. Newly planted specimens require even moisture with more frequent watering during the warm season until they establish. Provide supplemental irrigation during dry spells to maintain healthy growth and prevent browning.
Winter Damage
One of the main causes of arborvitae turning brown is winter damage, including sunscald and leaf scorch.
Leaf scorch occurs in drying winds when there’s little moisture available. In climates with cold winters, roots don’t absorb regular moisture in frozen conditions. Coupled with frigid winds, the drying causes dieback. You’ll notice browning from the tip of the branch down, often on only one side of the tree or in an isolated area.
Leaf scorch and sunscald also happen in summer, when high temperatures, dry spells, and hot winds cause the same result. Foliage loses more moisture through transpiration than it absorbs, and leaves can’t keep up.
Scorch results in brown leaf clusters or patches. Intense sun exposure in both winter and summer also impacts sunburned leaves, which brown and crisp before dropping.
Arborvitae are also susceptible to damage from ice and heavy snowfall, where branches split or break under the weight. Entire branches may brown and die as a result of breakage.
With both leaf scorch and snow damage, a few extra measures protect the specimens in winter. Since we often employ arborvitae as wind screens, some of it is a natural expectation in seasonal extremes. Add a three-inch layer of mulch (leaf litter, bark chips, straw) to insulate roots, retain moisture, and protect against frost heaving with regular soil freeze and thaw cycles.
Keep them well-watered through frost to support the roots and branches as they prepare for winter dormancy with less stress. Additional measures like wrapping individual specimens in burlap or frost cloth and snow protection help prevent arborvitae turning brown in the lowest hardiness zones 3 to 4.
Diseases
Fungal tip blights are arborvitae diseases that cause browning that begins at the end of the stem. Phytopthera root rot, too, can cause quick decline as roots are unable to support upper growth.
Tip Blight
Blights like Pestalotiopsis funerea enter stems weakened by winter damage, drought stress, or other injury. Black spores appear on stems and leaves, but the yellowing and dark browning of leaves on the ends of branches is more evident. The fungus thrives in damp, humid conditions, and spreads on the wind, by splashing up from the soil, and on tools.
Clip off and destroy the infected portions to reduce the spread and stop your arborvitae turning brown. Avoid overfertilizing and overwatering, which can cause weakness. Allow proper spacing for mature root development and airflow around branches.
Root Rot
Fungal diseases arise in overly damp situations, the exact opposite of drought-stressed conditions. Root rot occurs in humid environments and wet conditions.
With root rot, the pathogen Phytophthora, among others, impacts feeder roots, which are unable to absorb water and nutrients. Signs of root rot include drought-stressed leaves that turn yellow and wilt as they don’t receive moisture.
Root rot in arborvitae presents with browning from the top of the tree down. Large brown areas develop, especially in hot weather, and the tree quickly declines. Check for rot by inspecting stems and roots slightly below the soil line, when feasible. Rotting roots will be yellow or brown rather than a healthy white.
While it may be too late to save the tree, try reducing irrigation frequencies to allow oversaturated soils time to drain. Keep soil evenly moist but not soggy, and increase air circulation by pruning surrounding vegetation in crowded conditions.
Pests
Bagworms are a common pest of Thuja. They feed in the warm season and begin to web and cocoon late in the season. The caterpillars can defoliate branches and result in arborvitae turning brown before pupating in the “bags.”
The simplest control is to manually remove the bags to reduce future populations. Bacillus thuringiensis (BT) is an organic control that may limit populations in early summer as they feed.
Scale, mealybugs, spider mites, and leaf miners may also damage Thuja through their feeding on plant tissue and as vectors of diseases. Cultural conditions like even moisture, well-draining soils, avoiding excessive fertilizer, and ample air circulation can help prevent stress and increased weakness to pest damage.
With insect damage, you’ll see patchy yellowing, browning, and dropping leaves. Spray leaves and stems with a strong stream of water to displace the pests from the leaves and stems. Horticultural soaps and oils can help defray large infestations.
