The ‘Mr. Bowling Ball’ arborvitae, a dwarf cultivar of Thuja occidentalis known botanically as ‘Bobzam’, is prized for its naturally spherical shape and dense, lacy blue-green foliage. Although it is an evergreen, brown foliage is a frequent concern for owners of this popular shrub. Browning is a sign of stress, and while it can be alarming, the cause is often identifiable and addressable. Determining the specific source of the discoloration is the first step toward restoring the plant’s year-round vibrant color.
Browning Caused by Environmental Stressors
Improper care and external conditions are the most common reasons a ‘Mr. Bowling Ball’ arborvitae will begin to brown. Water issues, whether too much or too little, quickly lead to distress. Under-watering, particularly during dry spells or high temperatures, causes the foliage to become crispy and dry, often starting at the tips of the branches.
Excessive soil moisture is equally damaging, depriving the roots of oxygen and leading to root problems that manifest as dullness and browning progressing from the inside out. The ‘Bobzam’ cultivar prefers evenly moist, well-drained soil and struggles in persistently saturated conditions. Poor soil drainage or compacted soil can exacerbate this issue, stressing the plant’s root system and leading to eventual foliage dieback.
Sun and heat exposure can also cause localized browning, especially in hot summer climates or when the plant is newly established. Arborvitae placed in intense, unfiltered afternoon sun, particularly facing south or west, may exhibit browning on the exposed side due to heat stress. Furthermore, chemical injuries often appear as distinct patches of brown foliage; this can result from salt spray from nearby roads during winter or concentrated exposure to pet waste, which is high in nitrogen and salts.
Identifying Browning from Pests and Diseases
Browning that is not related to environment can often be attributed to biological invaders, such as insects or fungal pathogens. Spider mites, specifically the spruce spider mite, are a common pest that feeds on the plant’s fluids by piercing the foliage cells. Infestation symptoms begin as fine yellow speckling (stippling) on the leaves, progressing to bronzing and widespread browning.
The “tap test” checks for mites: sharply tap a branch over white paper; if tiny specks move, an active infestation is likely present. Bagworms are another threat, whose feeding activity causes localized browning where they consume the foliage. These pests are identified by the silk bags, which look like small cones hanging conspicuously from the branches.
Fungal issues like root rot usually develop in conditions of poor drainage or overwatering. This disease attacks the root system, inhibiting its ability to transport water and nutrients, causing the entire plant to appear sickly and the browning to spread rapidly from the base upward. While the ‘Mr. Bowling Ball’ is generally robust, it is still susceptible to these common arborvitae problems.
Understanding Natural and Seasonal Browning
Not all browning signals a problem; some discoloration is a normal physiological process for the arborvitae species. A common occurrence is “flagging,” where the plant sheds its oldest, interior foliage, typically in the autumn. Because the outer, newer growth is dense, it blocks sunlight from reaching the inner scales, causing them to die and turn brown.
This inner shedding is a natural self-pruning process and is not a sign of poor health, provided the outer layer of the shrub remains green and healthy. Winter desiccation, often called “winter burn,” is another seasonal cause, particularly visible on exposed sides of the shrub. This browning occurs when cold, dry winds cause the foliage to lose moisture through transpiration while the ground is frozen, preventing the roots from replenishing the lost water.
Remediation and Future Prevention
Addressing water management is the most immediate and impactful action to reverse browning from environmental stress. If the soil is too dry, deep, infrequent watering should be implemented, aiming to moisten the root zone without creating waterlogging. If overwatering is the issue, checking soil drainage and allowing the top two inches of soil to dry out between waterings is necessary.
For pest control, small bagworm infestations can be managed by manually picking the silk bags off the shrub before the larvae hatch. Spider mite infestations can be treated with horticultural oil, which suffocates the mites, or with a miticide specifically formulated for mites, as general insecticides are often ineffective. Applying horticultural oil in the early spring or fall when mites are most active can help manage their populations.
To mitigate winter burn, applying a layer of mulch around the base of the plant in late autumn helps insulate the roots and retain soil moisture. In exposed, windy locations, wrapping the shrub with burlap can serve as a windbreak, reducing moisture loss from the foliage. Dead or severely browned material should be pruned out to improve air circulation and encourage fresh, green growth once the underlying cause has been corrected.