Arborvitae (Thuja genus) are popular evergreens widely used in landscapes for their dense foliage and natural screening capability. While generally low-maintenance, they can benefit from supplemental feeding, especially when planted in poor soil or high-traffic areas. Fertilization supports their deep green color and vigorous growth. Understanding when and how to apply nutrients is crucial for maintaining the health and appearance of your arborvitae.
Recognizing Signs of Nutritional Deficiency
Before applying fertilizer, determine if your arborvitae is experiencing a nutritional deficiency. The most common indicator of nutrient stress is generalized yellowing of the foliage, known as chlorosis. If this yellowing affects older, inner needles first, it suggests a lack of mobile nutrients like nitrogen or magnesium. Stunted growth or a lack of new shoots during spring and summer also indicates insufficient resources. Browning of inner foliage is often normal shedding or a sign of drought stress. Rule out other issues first, such as inconsistent watering, root damage, or pest infestations, which can mimic deficiency symptoms.
Choosing the Right Fertilizer Formulation
The most effective fertilizer for arborvitae is typically a balanced, slow-release granular product designed for evergreen shrubs and trees. A slow-release formula is preferred because it meters out nutrients over several months, providing sustained feeding while reducing the risk of root burn. This avoids the sudden flush of growth caused by quick-release liquid fertilizers.
Look for a complete NPK (Nitrogen-Phosphorus-Potassium) ratio that is either balanced (e.g., 10-10-10) or leans slightly toward nitrogen (e.g., 10-8-6 or 12-6-4). Nitrogen promotes deep green color and foliage growth. Providing a complete formula ensures the plant has phosphorus for root development and potassium for overall stress tolerance.
The inclusion of specific micronutrients can be beneficial if the plant is showing signs of localized yellowing. Magnesium deficiency causes older foliage to turn yellow with reddish-purple or brown edges. Iron deficiency, common in high-pH soils, is visible as yellowing on the newest growth with the veins remaining green. Organic alternatives, such as aged manure, compost, or cottonseed meal, can also be used to slowly improve nutrient availability and soil structure.
Seasonal Timing and Proper Application Techniques
The optimal time to apply fertilizer to established arborvitae is early spring, just before the first flush of new growth begins. This timing ensures the nutrients are available to the root system as it begins its active growing cycle. Applying a slow-release product once annually in the spring is typically sufficient.
Avoid fertilizing from mid-summer onward. Late-season feeding stimulates soft, tender growth that cannot harden off before the first frost. This un-hardened growth is highly susceptible to winter damage and dieback.
When applying granular fertilizer, spread the recommended amount evenly across the root zone, extending out to the drip line. Keep the granules several inches away from the base of the trunk to prevent caustic damage. After broadcasting, watering the area thoroughly is mandatory. Deep watering dissolves the fertilizer, moving nutrients into the soil for root absorption and mitigating fertilizer burn.