Many shade trees require no additional fertilization. However, if a tree is showing signs of a nutrient deficiency or is growing in a soil low in organic matter (e.g., rocked areas or new construction) a fertilizer may be beneficial.
Nitrogen (N) is the most limiting nutrient to tree growth. Trees with nitrogen deficiencies have slow canopy growth rates and may show yellowing of their inner leaves (many other biotic and abiotic issues have similar symptoms). Nitrogen fertilizer encourages canopy growth. Because of this, it can also upset the ratio of roots to shoots for a tree, leading to stress.
When applying nitrogen, a slow release formulation is preferable. Alternately you can split applications of faster releasing products by applying half the recommended amount, waiting a few weeks then applying the other half.
It’s uncommon for trees to be deficient in phosphorous or potassium in Colorado as soils in the western United States typically contain adequate amounts of these nutrients.
Iron and other micronutrient deficiencies are common in some tree species. Refer to GardenNotes #223, Iron Chlorosis, for additional details.
Fertilizer Application Rates
The need for fertilizer varies with the tree’s growth phase. For additional details on the growth phases of a tree, refer to GardenNotes #101, Plant Health Care.
Establishment Phase
During the root establishment phase, the growth objective is root growth. Nitrogen fertilizer increases canopy growth with a corresponding decrease in root growth, which is undesirable in this phase.
As a rule-of-thumb for Hardiness Zones 4-5, the establishment phase for recently transplanted trees lasts one year for each inch of trunk caliper (measured at 6” above ground level). In other words, the establishment period for a one-inch caliper tree is typically one year, and three years for a three-inch caliper tree. The establishment phase may be longer on sites with poor soil tilth, limited irrigation, and with poor planting techniques. Refer to GardenNotes #633, The Science of Planting Trees, for additional details on tree planting.
During the root establishment phase, fertilizer applications should be kept to a minimum, as follows:
- If the soil organic matter content is moderate to high (3-5% organic matter), no additional fertilizer is warranted.
- If the soil organic matter content is low (1% or less), a slow release nitrogen fertilizer may be beneficial if applied a few months after planting. Application should not exceed 0.1 pounds of N per 100 square feet (based on the area of the planting hole). Do not apply fertilizer on a site with growth limiting factors such as limited irrigation.
- If the tree is planted in turfgrass which is being fertilized, additional fertilization will likely not be beneficial.
Growth Phase
Significant branch growth indicates a shift from the root establishment phase into the growth phase. In this phase of the tree’s life, fertilization can encourage faster canopy growth. Though again this may not be desirable due to slowed root growth. Application rate is based on several factors:
- Growth limiting factors such as limited irrigation, severe soil compaction, or limited root spread potential – Do not force growth.
- Heavy fertilization promotes canopy growth that the roots cannot support in summer heat and wind.
- If the soil organic matter content is moderate to high (3-5% organic matter), no additional fertilizer is warranted.
Table 1 illustrates rate adjustments based on these factors.
Mature Phase
As trees reach a mature size and their growth slows, the need for nitrogen decreases. In this phase, the application rates is 0.1 to 0.4 pounds N per 100 square feet per year based on soil factors such as organic matter content, the species of tree, landscape maintenance practices, plant health and other landscape specific factors. Over fertilization may push more canopy growth than the tree’s root system can support. If a tree is not showing signs of deficiency (lack of growth, dieback, yellowing of older leaves) it is likely that fertilization is not necessary.
Application Rate Based on Growth Phase and Soil Organic Content
The fertilizer application rate should be adjusted according to soil organic matter content as indicated in the table below.
Table 1: Annual tree fertilizer rates per 100 ft2 based on growth phase and soil organic content. N application rate1,2 (lbs. N per 100 square feet)
| Low Soil Organic Content (0-1%) | Medium Soil Organic Content (2-3%) | High Soil Organic Content (4-5%) | |
|---|---|---|---|
| Establishment phase | 0 to 0.1 lbs. | 0 | 0 |
| Growth phase (routine) | 0.1 to 0.2 lbs. | 0.05 to 0.1 lbs. | 0 |
| Mature phase (Only when there is a deficiency) | 0.2 to 0.4 lbs. | 0.1 to 0.2 lbs. | 0 |
Time of Year
The best time of year to fertilize trees is early spring (4-6 weeks before bud break) or late fall after leaves drop (and soil temperatures are above 40o). Avoid late summer and early fall fertilizations as they may interfere with winter hardiness.
Area to Fertilize
Fertilizer should be applied to the rooting zone of the tree; ideally where fine feeder roots are more likely to be present. For many trees this means fertilizing the entire yard or bed the tree is in. For mature trees in larger spaces than the extent of their rooting zones, a safe estimate is to fertilize a circular area twice the diameter of the tree’s canopy.
Fertilizer Application Methods
Broadcast application of fertilizers can be effective for many formulations of fertilizer. Some fertilizers are applied by drilling 4-6 inch deep holes at two-foot intervals in a grid in the tree’s rooting area, then applying the fertilizer in the holes.
Fertilizing Stressed Trees
When plants appear stressed, a common reaction is to fertilize. However, this can aggravate stress. Before fertilizing a stressed tree, evaluate whether an increase in canopy growth with the corresponding decrease in root growth is desirable.
Nitrogen fertilization shifts the tree’s balance of growth, favoring the canopy. If the stress is root related (i.e., soil compaction, restricted root spread, construction damage, extensive storm damage), this shift will aggravate it. Do not apply high levels of fertilizer to trees with root problems.
Additionally, trees invest energy reserves to take up nutrients. Thus, the short-term effects of a heavy fertilization will be an immediate reduction in a tree’s carbohydrate levels, aggravating stress. If the tree shows severe stress, do not apply high levels of fertilizers. Work to alleviate stress factors to the extent possible.
This publication, reference GardenNotes #651, is developed as part of the Colorado State University Extension Master Gardener Program.