Horticultural Oil on Arborvitae: Benefits, Risks, and Proper Application

Your landscaper's proposal to spray horticultural oil on arborvitae and other shrubs raises important cautions. Oil sprays effectively control scale, mites, and aphids by suffocation, but they can damage certain evergreens—especially blue spruce, junipers, and arborvitae—if applied at excessive rates or during hot weather.

There is no "greening oil" for arborvitae. Any color improvement comes from adding liquid iron to the tank mix, which risks foliage burn if overapplied. Healthy green color comes from proper nutrition, not topical sprays.

If oil treatment is warranted for pests, insist on a 1% concentration (1 gallon oil per 100 gallons water) applied as a fine mist with wide coverage. Time applications for cool, overcast days—never when temperatures exceed 85°F or drop below freezing. Avoid spraying drought-stressed plants.

Better yet: identify the specific pest first. Many "preventive" oil sprays are unnecessary and harm beneficial insects. A targeted approach based on actual infestation protects both plants and the garden ecosystem.