I’ve noticed buds on several trees are starting to almost break. I’ve even seen a few blooms on azaleas that were tucked away, protected from the cold weather of a few weeks ago. Warm weather in late February quickly turns our attention to the garden and landscape. Also, this time of year, questions start coming in on how and when to fertilize.
The most frequent fertilizer questions I receive are about vegetable gardens, followed closely by lawns, and then fruit trees and shrubs. Questions about fertilizing trees, especially mature trees, aren’t as common, and typically only asked when homeowners notice something is not quite right with their tree.
Do trees need fertilizer? In short, yes, trees, like all plants, need an adequate amount of essential nutrients to maintain plant health and good growth. Whether or not we need to provide supplemental nutrients in the form of fertilizers will depend on the age and species of the tree, location, and soil composition.
The annual cycle of growth and decay in natural settings is efficient in recycling nutrients back into the soil. As fallen leaves and plant material decay, nutrients are released back into the soil. This can provide trees in forest settings with all the fertilizer they need.
Trees planted in urban and suburban areas grow in very different conditions from those in the forest. We alter the environment by raking and removing leaves, reducing natural sources of nutrients. We change soil composition by removing topsoil and adding fill material for construction projects which can reduce soil fertility and result in soil compaction.
When trees are planted in these settings, they likely need additional fertilizer. The age, health, and species of a tree are factors that need to be considered when deciding how much and when to fertilize. Trees that naturally have a faster growth rate, such as red maple, tulip polar, and sycamore, need less fertilizer than slower growing trees.
Newly planted trees and trees that have been severely root pruned should be allowed to establish and re-establish root systems, respectively, before fertilizers are applied. Fertilizer applications the year following planting through 5 to 10 years of a tree’s life will help establish a healthy root system and canopy to sustain good, long-term growth.
Mature trees do not need fertilizer applications annually unless there is a noticeable decrease in plant vigor such as pale foliage, smaller than normal leaves, chlorotic leaves, twig growth of less than 2 inches per year, or noticeable tip or branch dieback. These signs can indicate either a nutritional deficiency, disease presence, or insect attack. A soil test will determine if specific nutrients are deficient along with recommendations for amounts of N, P, and K that need to be applied.
A fertilizer application every three years is generally adequate to help maintain good growth of healthy trees. Of course, if the lawn is being fertilized annually, then the tree is already benefiting from those applications. General recommendations of how much fertilizer to apply are based pounds of N per square feet of the tree’s root zone. To determine the root zone area, you can use these formulas:
Columnar trees: (3 x length of farthest branch in feet)2 x 3.14
Broad shaped trees: (2 x length of farthest branch in feet)2 x 3.14
A container of fertilizer will have concentrations of nitrogen (N), phosphorus (P), and potassium (K) percentages found on the label in that order. These are represented with the abbreviations NPK. The numbers correspond to pounds of those nutrients (nitrogen, phosphoric oxide, and potash) per 100 lbs. of fertilizer product.
Apply fertilizer at a rate of 3 lbs. N per 1000 square feet. Do not apply more than 1 lb. of N per 1000 square feet if a lawn or ground cover is under the tree. Fertilizer can either be broadcast over the root zone or placed in 1-2-inch-wide holes 8-10 inches deep scattered in the root zone of the tree. This is preferred for areas with compacted soil or areas prone to excess runoff. The root zone extends from the root flare at the base of the tree to past the dripline of the tree. Fertilizer should be watered in after application.
Late February and early March are a great time to fertilize healthy trees and woody shrubs. However, wait until late spring, after leaf-out, to fertilize stressed or otherwise unhealthy-looking trees though. Taking care of our trees now can help provided us with the much needed shade come summer.
