Ideally, growing trees should be fertilized throughout the year but a bit differently as trees age. A tree needs larger amounts of nitrogen (N) based fertilizer during the growing season. Nitrogen-based solutions should be applied during the early spring and summer months.
Several light applications a year are preferred as the tree gets older to a point where they need very little fertilizer. A soil test may be needed to determine the amounts of phosphorus (P), potassium (K). Read the label for proper ratios and application rates of N, P, and K for trees.
Important Age Considerations
Here is how you should fertilize a tree as it ages:
- Newly planted tree phase - these trees are still babies and should have only minimal applications of a quick release fertilizer and more of a type that releases slowly. High nitrogen release rates on newly planted trees will burn roots and leaves on contact. Note: Liquid and fully composted fertilizers have the fastest release rates while slow release forms tend to be granular and less water soluble.
- Rapidly growing young tree phase - encouraging the rapid growth of young saplings may be in your tree management plan. It is certainly desirable and appropriate to up the fertilization rates, especially with adequately spaced trees on sites low in organic matter. When using the recommended rate labeled on your fertilizer container, a twice a year feeding is perfect.
- Mature and stable tree phase - As trees mature their growth rate naturally slows down. The need for fertilization drops and your applications need to be reduced. You have now arrived at a low maintenance level for fertilizing established trees. The purpose of this low maintenance level is to maintain trees in a healthy condition without excessive vegetative growth.
Again, for young trees, the time to put out fertilizer is late March through early June. When a tree reaches the desired height you may want to decrease the fertilizer application to only once a year.
How to Fertilize a Tree
You do not need to remove mulch to fertilize! Scatter or drop pellet fertilizer under the tree's drip zone but avoid touching the tree trunk with the material. Do not over-fertilize.
An application of between .10 and .20 pounds of nitrogen per 100 sq. ft. will be adequate. Again, read the label. Keep solid or concentrated fertilizer off stems and leaves and adequately water the fertilizer into the soil as that prevents fertilizer burn injury to roots.
Stick with the higher ratio nitrogen fertilizers unless your tree is determined to be deficient in potassium or phosphorus (soil test). N-P-K rates of 18-5-9, 27-3-3, or 16-4-8 are good bets. Not all trees are alike and conifers rarely need high rates of fertilizer so you might want to skip applications or stop feeding after a year.
Some uncomposted Organic fertilizers come from plant and animal sources. These fertilizers have a slower release of nutrients as they need to be decomposed by soil microorganisms. They are easy on plant roots but take longer to become effective.
Organic fertilizers are harder to find than inorganic fertilizers and often more expensive but they are the least harmful and less exacting when applying. The best organic fertilizers are cottonseed meal, bone meal, manure and chicken litter. Read the label (if packaged) for application methods and amounts to use.
Inorganic fertilizers are inexpensive and are the most frequently used fertilizers for trees. Inorganic nitrogen based tree food sources are sodium nitrate, ammonium nitrate, and ammonium sulfate.
General purpose fertilizers are complete with N-P-K which is usually defined as the ratio of nitrogen, phosphorus, and potassium in the mixture. You can use these excellent fertilizers but don't overdo. Use high-ratio nitrogen products unless a soil test suggests a lack of other nutrients. Inorganic fertilizers can come in slow-release, liquid or water-soluble for foliar application.
Read the label for application rates.
Remember Organic Soil Amendments
The greatest value of most organic materials is in the change they bring to soil structure. Remember that chemical fertilizers have no positive physical effect on soil structure.
Peat moss, leaf mold, aged pine bark, or sawdust and stable manure can improve the soil while adding nutrients. These amendments increase the fertilizer and water-holding capacity of many soils. Mulching with these amendments aids in root development.