Arborvitae (Thuja species) are popular evergreen trees and shrubs often used for hedges and privacy screens due to their dense, scale-like foliage. Fertilization corrects soil nutrient deficiencies, promotes robust growth, and ensures a vibrant green color. The timing of nutrient application is the most important factor determining the effectiveness of the process and the plant’s overall health. Applying fertilizer at the wrong time can cause harm, stressing the plant and leaving it vulnerable to damage.
Optimal Timing for Feeding Arborvitae
The most beneficial time to fertilize established arborvitae is in the early spring, just before the first flush of new growth begins. This timing allows the plant to access the necessary nutrients precisely when it is preparing to expend the most energy for the growing season. Providing a fresh supply of food then supports the vigorous development of new stems and foliage throughout the summer months.
A secondary window for feeding established plants is in the late fall, after the current season’s growth has fully matured but before the ground freezes solid. This application strengthens the root system over the winter and prepares the plant for the following spring’s growth spurt. However, newly planted arborvitae should avoid fertilization for the first year, as their shallow root systems are highly sensitive and prone to fertilizer burn.
Avoid fertilizing in the late summer or mid-fall when the plant’s natural cycle is winding down. Fertilizing too late stimulates a tender burst of new growth that will not have time to harden before the first severe frost. This immature tissue is highly susceptible to winter burn and dieback, damaging the plant’s health. Also, avoid applying fertilizer during periods of extreme summer heat or drought, as this stresses the tree and increases the risk of root damage.
Selecting the Right Fertilizer
Arborvitae benefit most from a complete, slow-release granular fertilizer that provides a consistent supply of nutrients over several months. The ideal formulation should have a slightly higher proportion of nitrogen (N) compared to phosphorus (P) and potassium (K) to support dense, bright green foliage. Common balanced ratios like 10-10-10 or 12-6-4, or higher-nitrogen formulas such as 20-15-15, are effective for these evergreens.
Nitrogen is a fundamental component of chlorophyll, the pigment responsible for the plant’s green color, making it the most significant nutrient for maintaining visual appeal. A slow-release product is preferred because it prevents a sudden, detrimental surge of growth, instead offering a gradual feed that aligns with the plant’s steady growth cycle.
In cases where the foliage shows signs of yellowing, a condition known as chlorosis, the issue may be a deficiency in micronutrients rather than the main macronutrients. Specifically, a lack of iron or magnesium can cause the needles to lose their deep green color. A soil test can confirm any specific micronutrient needs, allowing for the application of a targeted amendment rather than a general fertilizer.
Application Techniques and Frequency
For established arborvitae, a single annual application of a slow-release granular fertilizer in early spring is sufficient to sustain the plant through the entire growing season. The most common application method involves spreading the measured amount of granular fertilizer evenly across the soil surface. The application area should extend from a few inches away from the trunk out to the plant’s drip line, beneath the outermost branches.
It is important to keep the fertilizer granules away from the trunk and stem tissue to prevent chemical burn, concentrating the product over the expansive root zone. Alternatively, a deep root feeding method can be used, which involves drilling or punching several six-inch-deep holes around the drip line and pouring the granular fertilizer into these openings. Immediately following the application of any fertilizer, the area must be thoroughly watered to help dissolve the granules, activate the nutrients, and carry them down to the roots while preventing potential root burn.