Tree fertilizer spikes are a convenient method for delivering pre-measured nutrients directly to a tree’s root system. Unlike broadcast granular fertilizers, these spikes are driven into the ground, ensuring the plant food is released slowly into the subsurface root zone. This targeted approach minimizes nutrient runoff and provides a steady supply of nitrogen, phosphorus, and potassium over an extended period. Proper application requires a clear, quantitative approach to ensure the correct number of spikes are used in the right locations.
Calculating the Required Number of Fertilizer Spikes
The number of fertilizer spikes a tree requires is directly proportional to the size of its trunk, which indicates its total mass and root volume. To determine this size, you must first measure the tree’s Diameter at Breast Height (DBH). This standard measurement is taken at a height of 4.5 feet (1.37 meters) above the ground level.
To accurately find the DBH, wrap a flexible measuring tape around the trunk at the 4.5-foot mark to determine the circumference. Divide the circumference measurement by pi (approximately 3.14) to calculate the diameter. Most fertilizer spike manufacturers base their application instructions on this diameter measurement, often recommending one spike for every one or two inches of trunk diameter.
If a tree measures 10 inches in diameter, it would need 5 to 10 spikes, depending on the product’s concentration and instructions. The final quantity should be rounded up to the nearest whole number to ensure adequate coverage. Always consult the product packaging for the exact ratio, as nutrient concentrations vary between brands and tree types.
Strategic Placement for Maximum Nutrient Absorption
Once the required number of spikes is calculated, the next consideration is placement to ensure maximum nutrient absorption by the tree’s fine feeder roots. These roots are primarily located within the top foot of soil and extend far beyond the trunk. They are mainly concentrated near the edge of the tree’s canopy, which is known as the drip line.
The drip line is the optimal application zone because it is where the majority of the active, nutrient-absorbing roots are found. Placing spikes too close to the trunk is ineffective, as the woody roots near the base are meant for support and transport, not nutrient uptake. Spikes should be inserted evenly around the drip line, following a circular pattern to distribute the fertilizer uniformly.
A common recommendation is to space the spikes approximately 2 to 3 feet apart along the established drip line. The spikes should be driven or hammered into the soil to the manufacturer’s specified depth, typically just below the soil surface, ensuring they are entirely covered. This subsurface placement protects the fertilizer from surface runoff and directs the nutrients downward to the root zone as the spike dissolves.
When and How Often to Apply Tree Spikes
The timing of fertilizer application aligns nutrient delivery with the tree’s natural growth cycles. The best times to apply fertilizer spikes are in the early spring, just as the tree begins its new growth period, and in the late fall, after the leaves have dropped but before the ground freezes solid.
The spring application supports the emergence of new leaves and vigorous shoot growth throughout the growing season. Applying in the fall helps strengthen the root system and build up energy reserves before winter dormancy. This two-part schedule is often recommended for established trees to ensure continuous nutrition.
For mature, healthy trees, a single annual application is often sufficient, though a twice-yearly schedule is common for trees showing nutrient deficiency or those in high-stress environments. Avoid applying spikes to newly planted trees for at least the first year, as they require a different type of root-accelerating product. Also, refrain from using the spikes during periods of extreme drought or heat, which can stress the tree and prevent efficient nutrient uptake.