Cedar trees, including popular varieties like Arborvitae (Thuja), are common landscape features that offer year-round greenery and privacy. Seeing the foliage turn brown or the tree appear thin can be alarming. Understanding the specific cause of decline is the first step toward successful revival, as improper diagnosis can lead to treatments that worsen the problem. This guide focuses on identifying the issue, correcting environmental factors, managing biological threats, and recognizing when a tree is beyond saving.
Identifying the Root Cause of Decline
Determining the reason for a cedar’s distress requires careful observation, as browning can signal various problems. A natural process called “flagging” occurs in late summer or fall when the tree sheds its older, interior needles, which turn yellow or reddish-brown before dropping off. This natural shedding only affects the inner foliage, leaving the tips and outer canopy healthy and green.
Signs of genuine distress include browning that starts on the outside tips of the branches, patchy discoloration, or a uniform color change across the entire canopy. Environmental stress, such as drought or excessive moisture, often presents as uniform browning or needle tip discoloration. Pests, like spider mites, may cause speckled yellowing, webbing, or browning on the foliage. Diseases can manifest as distinct cankers on the bark or tip dieback. A sign of root trouble, such as root rot, is the entire tree turning brown quickly or a distinct white fungal growth at the base.
Adjusting Water and Soil Conditions
Improper soil moisture is a leading non-biological factor causing decline in cedar trees. Cedars prefer well-drained soil, and both drought and waterlogging cause serious stress. Drought stress causes needles to become dry and brittle, often resulting in dieback that progresses from the top of the tree downward, as the highest points are the most difficult to hydrate.
Conversely, overwatering or poor drainage can lead to root rot, where roots suffocate and are attacked by water-borne fungi like Phytophthora. Root rot symptoms can mimic drought, as damaged roots cannot absorb water, causing the foliage to turn gray-brown or the entire tree to decline. To check the moisture level, insert a finger or a soil probe 4 to 6 inches deep; if the soil is dry at that depth, the tree needs water.
For a tree suffering from drought, deep, infrequent watering is the most effective solution, ensuring the water soaks the root zone, typically 12 to 18 inches down. Established cedars only need supplemental water during extended dry spells, sometimes only once every two to four weeks. If the tree is in a waterlogged area, improving drainage or diverting water away from the root zone is necessary to prevent further root damage. Applying a 2 to 4-inch layer of organic mulch around the base, keeping it a few inches away from the trunk, helps retain moisture and regulate soil temperature.
Treating Common Pests and Pathogens
When environmental factors are ruled out, the decline is likely due to an infestation or infection requiring targeted treatment. Bagworms are a common pest, recognizable by the spindle-shaped silk bags they construct on the foliage, which can completely defoliate a branch. The most effective non-chemical control is physical removal and destruction of the bags, especially during the fall and winter when they contain eggs.
Spider mites are another frequent culprit, causing fine stippling or bronzing of the needles, often accompanied by fine webbing. A strong jet of water can dislodge light infestations, but for more severe cases, horticultural oil or insecticidal soap can be applied. Fungal blights, such as those caused by Didymascella thujina, result in affected leaf scales turning brown and dropping off, leaving a sparse appearance. Treatment involves pruning out and destroying the infected branches to improve air circulation and reduce the spread of spores.
Recognizing Irreversible Damage
Not all declining cedars can be saved, and recognizing when damage is too extensive prevents wasted effort. A tree with a compromised main trunk, such as a large canker or significant bark loss, is unlikely to recover because the vascular system is severed. If the root system has been destroyed by root rot or severe drought, the tree has no mechanism to support new growth.
A general rule suggests that if 60 to 75 percent or more of the tree’s canopy has turned brown, the damage is likely permanent. The brown foliage will not turn green again, and cedars do not generate new growth from dead wood. In these cases, safe removal is the recommended course of action, followed by planting a replacement species better suited to the site conditions.