Arborvitae (Thuja) is a popular evergreen choice in landscapes, valued for its dense, scale-like foliage that provides year-round color and privacy. While these trees and shrubs are generally low-maintenance, a consistent nutrient supply helps them maintain their vibrant green color and robust structure. Fertilizing arborvitae aims not to force rapid, excessive growth, but rather to sustain overall health and foliage density against environmental stressors.
Selecting the Right Fertilizer Formula
The best fertilizer for arborvitae is typically a slow-release formula that is either balanced or slightly nitrogen-heavy, formulated specifically for evergreens. Arborvitae relies heavily on nitrogen (N) to produce its characteristic dense, green foliage, while phosphorus (P) and potassium (K) support root development and overall plant functions. Ideal NPK ratios often range from 10-10-10, for a balanced approach, to more specialized ratios like 20-5-10 or 12-6-4, which provide a higher proportion of nitrogen.
The physical form of the fertilizer significantly impacts how the plant utilizes the nutrients. Granular, slow-release fertilizers are superior because they are often coated, allowing a gradual trickle of nutrients over several months. This steady feeding mimics natural soil processes and prevents the sudden surge of salts that can lead to root burn, a common issue with quick-release liquid or synthetic products. Slow-release options ensure a stable supply of food throughout the active growing season, often requiring only a single annual application.
While synthetic fertilizers offer precise nutrient ratios, organic options provide additional benefits by improving soil structure over time. Products such as compost, well-rotted manure, or commercial organic evergreen blends release nutrients more slowly as they are broken down by soil microbes. These organic materials also help create a healthier root environment, enhancing the plant’s ability to take up water and nutrients. Selecting a complete fertilizer that includes the three macronutrients is advisable unless a soil test indicates otherwise.
Establishing the Proper Timing and Frequency
The timing of fertilization is nearly as important as the formula, dictating whether the nutrients benefit healthy growth or cause damage. The most effective window for feeding arborvitae is in the early spring, just before the plant begins to push out new growth. This application ensures the necessary nitrogen is available to fuel the plant’s primary growth spurt as the soil warms and its metabolism increases.
For established trees and shrubs, one application of a slow-release granular fertilizer in early spring is usually sufficient to sustain them throughout the entire growing season. Applying fertilizer too frequently can lead to excessive, weak growth that makes the plant more susceptible to pests and diseases. Newly planted arborvitae should not be fertilized immediately, as the high salt content can damage the still-developing, tender root system.
A common mistake is fertilizing too late in the season, particularly after mid-summer or in early fall. This late feeding stimulates soft new growth that does not have enough time to harden off before the first winter frost. This unhardened tissue is highly vulnerable to cold damage, resulting in browning, dieback, and increased stress on the plant.
Application Techniques for Maximum Absorption
Proper application is essential to ensure the fertilizer reaches the root system efficiently while avoiding common pitfalls like fertilizer burn. Granular fertilizers should be broadcast evenly over the root zone, which extends out to the plant’s drip line—the area directly beneath the outermost branches. This area contains the highest concentration of fine, nutrient-absorbing roots.
It is crucial to keep the fertilizer granules several inches away from the trunk to prevent chemical burn to the bark and root crown. After spreading the product, lightly scratching it into the top inch of soil helps settle the granules. A thorough watering is necessary to begin the nutrient dissolution process, moving dissolved nutrients down into the root zone where they become accessible to the plant.
For large, mature arborvitae, deep root feeding or specialized fertilizer spikes can be effective alternatives to surface application. Spikes are driven into the ground around the drip line, delivering nutrients directly to the deeper roots with less risk of run-off. When calculating the amount of fertilizer, always follow the manufacturer’s instructions, which typically base the dosage on the tree’s height or the trunk’s diameter at breast height (DBH) to avoid over-fertilization.
Recognizing Nutrient Deficiencies and Stress
Not all arborvitae browning or discoloration indicates a need for a general maintenance fertilizer; sometimes, the issue is a specific nutrient deficiency or environmental stress. A general nitrogen deficiency, for instance, typically manifests as a uniform pale green or yellowing of the older, inner foliage first. Because nitrogen is a mobile nutrient, the plant moves it from older leaves to support new growth.
In contrast, deficiencies of less mobile micronutrients, such as iron, usually appear as yellowing in the newest growth, often leaving the veins green—a condition known as interveinal chlorosis. Magnesium deficiency can cause yellowing between the veins of older leaves, sometimes accompanied by reddish or purplish discoloration. These signs require a targeted approach rather than a standard NPK blend.
If a deficiency is suspected, a soil test can provide a definitive diagnosis of nutrient imbalance and soil pH, which affects nutrient availability. For confirmed micronutrient deficiencies, a specific supplement, like chelated iron or Epsom salts for magnesium, is more effective than a high-nitrogen evergreen fertilizer. Addressing underlying issues like poor drainage or inconsistent watering is important, as environmental stress can mimic or compound nutrient problems.