The Arborvitae (Thuja genus) is a popular evergreen tree known for its dense, scale-like foliage, making it a favorite for privacy hedges and landscape screens. Although relatively low-maintenance, arborvitae often grow in cultivated landscapes where soil nutrients become depleted. Supplemental nutrition is necessary to maintain their deep green color, promote vigorous growth, and ensure a thick, full appearance. Understanding the tree’s specific nutrient requirements is the first step in selecting an optimal feeding strategy.
Arborvitae’s Core Nutritional Needs
Arborvitae trees rely on three primary macronutrients, collectively known as NPK, for robust growth: Nitrogen (N), Phosphorus (P), and Potassium (K). Nitrogen is the most influential element, directly supporting chlorophyll production, which governs the rich, dark green color of the foliage and drives growth. A sufficient supply of nitrogen promotes a dense habit and rapid development, which is often a goal for privacy plantings.
Phosphorus supports strong root systems and aids in energy transfer, though mature trees require less of this element compared to nitrogen. Potassium plays a regulatory role, helping the tree manage water uptake, improve cold hardiness, and strengthen resistance to disease. Arborvitae showing signs of nutrient stress often exhibit visual cues pointing to a specific deficiency.
The most common sign of a problem is chlorosis, or the yellowing and browning of the foliage. If yellowing appears on older, inner needles first, it indicates a lack of mobile nutrients like nitrogen or magnesium. In contrast, stunted or light green new growth with dark veins suggests an iron deficiency. Correcting these symptoms requires a targeted fertilizer application, though a soil test provides the most accurate diagnosis of nutritional imbalances.
Selecting the Right Fertilizer Formulation
The best fertilizer for arborvitae emphasizes nitrogen content to support heavy foliage growth. Experts recommend a complete fertilizer that is either balanced (e.g., 10-10-10) or one with a higher nitrogen concentration (e.g., 16-4-8 or 12-6-4). These formulations ensure the tree receives the necessary fuel for dense, vibrant green growth while supplying other essential macronutrients.
The physical form of the fertilizer is important, with granular slow-release products being the preferred choice for established trees. These fertilizers are coated to release nutrients gradually over several months, providing a steady, consistent supply. This slow delivery prevents the sudden surge of growth associated with quick-release liquid fertilizers and reduces the risk of fertilizer burn.
Quick-release liquid products are beneficial for struggling trees needing an immediate nutrient boost, but they require frequent application. Fertilizer spikes are a convenient option that delivers nutrients directly into the root zone, offering a localized, slow-release effect. Organic options, such as compost, composted manure, or organic granular fertilizers, provide nutrients and improve soil structure. Organic products release nutrients more slowly as soil microorganisms break them down, minimizing the chance of over-fertilization.
Timing and Application Methods
The optimal time to apply fertilizer to arborvitae is in the early spring, just before the tree begins its active growth cycle. Applying fertilizer then ensures nutrients are available to support the flush of new growth as soil temperatures rise. This single annual application is often sufficient, particularly when using a slow-release granular product.
A secondary, lighter application can be considered in early autumn to support root development, but avoid feeding too late in the season. Fertilizing after mid-summer stimulates tender, new foliage that lacks time to harden off before the first frost. This unhardened growth is highly susceptible to winter damage, leading to brown tips and dieback.
When applying granular fertilizer, spread it evenly across the soil surface, starting a few inches away from the trunk and extending out to the tree’s drip line (the area directly beneath the outermost branches). The fertilizer should be lightly worked into the topsoil or covered with a thin layer of mulch, then thoroughly watered to start the nutrient release process. Avoiding direct contact between the fertilizer granules and the trunk or surface roots is crucial to prevent chemical burn.
For trees with compacted soil, granular fertilizer can be applied using the deep-root feeding method. This involves drilling small holes within the drip line and filling them with the fertilizer. Always follow the manufacturer’s recommended application rates, as applying too much fertilizer leads to salt buildup in the soil. Over-fertilization can damage the root system, causing the foliage to turn yellow or brown, a condition known as fertilizer burn.