Diagnosing Arborvitae Decline: Neighbor Damage vs. Natural Causes

A gardener suspects a neighbor deliberately poisoned their arborvitaes. They want to know what chemicals could kill arborvitaes and how to remediate contaminated soil.

Natural Causes to Rule Out First

Before assuming foul play, investigate these common arborvitae killers:

Spider Mites (Most Common)

• Tiny red/brown mites on foliage undersides
• Bronzing, stippling, then browning from inside out
• Test: Shake branch over white paper; look for moving specks
• Treat: Miticide (horticultural oil, insecticidal soap, or specific miticide)

Environmental Stress

• Drought: Brown from top down or inside out
• Winter burn: South/west sides brown in spring
• Poor drainage: Root rot, gradual decline
• Salt damage: From de-icing runoff (brown near pavement)

Diseases

• Phomopsis blight: Tip dieback, cankers
• Kabatina blight: Similar, different timing
• Root rots (Phytophthora, Armillaria): Progressive decline

If Chemical Damage Is Confirmed

Herbicides that kill arborvitaes:

• Non-selective: Glyphosate (Roundup), glufosinate
• Broadleaf-specific: 2,4-D, dicamba, triclopyr (drift damage)
• Soil sterilants: Prometon, tebuthiuron (Triox)—long-lasting
• Accidental: Motor oil, gasoline, bleach, excessive fertilizer

Soil Remediation Steps

1. Document: Photograph damage pattern; note dates.
2. Soil test: Submit samples for herbicide residue analysis (expensive, specialized labs).
3. Excavate: Remove 12-18 inches of soil around root zone. Dispose properly (hazardous waste).
4. Flush: Flood area repeatedly to leach water-soluble chemicals.
5. Replace: Backfill with clean topsoil + compost.
6. Activated charcoal: Incorporate 1-2 lbs per 100 sq ft to adsorb residues.
7. Replant: Wait 4-6 weeks; test with sensitive seedlings first.

Legal/Practical Advice

• Confrontation without proof escalates conflict.
• Install security cameras if pattern continues.
• Contact extension office for neutral diagnosis.
• Consider mediation services for neighbor disputes.

Preventive Care

Healthy arborvitaes resist stress better: mulch 3 inches, water deeply in drought, fertilize lightly in spring, monitor for mites monthly.