Fertilization programs are utilized to maintain the vigorous and healthy growth of trees and to increase their resistance to damage from insects and diseases. When applied judiciously they can be a valuable part of a plant health care program.
The foundational tenet of tree fertilization is that it should be an as necessary procedure, not a standard, one-size-fits-all program.
Fall versus spring
When needed, there are two general time frames to fertilize trees: early to mid-spring and late fall when plants are dormant. In spring, applied nutrients are converted to essential plant compounds – sugars, carbohydrates and amino acids – that bolster stem, trunk and root growth. In late fall, some nutrients are used in root growth, with the remainder stored in other plant tissues ready to be used when the roots resume absorption and expansion in the spring.
Early fall applications aren’t recommended because the resulting encouragement of growth that often occurs may not have adequate time to harden off before the onset of winter. If not winter-ready, the stems have a greater potential to suffer winter injury because of their soft and supple nature. Depending on where you live, root function can continue into December.
The amount of fertilizer applied in fall versus spring should also be considered. A good rule of thumb for fall applications is to apply about a third of the amount normally used for spring applications – enough to encourage root and future shoot growth, but not more than will be absorbed. Judicious applications don’t facilitate ground or surface water pollution. Slow-release nutrient formulations decrease the odds of leaching and runoff, but always use caution when fertilizing trees.
When determining whether to fertilize and how much, lean on factors such as soil testing, appearance, growth rate and the surrounding plant material in the landscape.
Soil testing — The critical indicators provided by a soil test report are nitrogen, phosphorous, potassium, pH, secondary nutrients and the micronutrients. Cation exchange capacity, salt index, sodium absorption ratio and organic matter content should also be considered when determining need.
Before submitting a sample for soil testing, ask the testing lab how to best obtain a representative sample for the appropriate testing procedure and follow their instructions carefully. Once the report is back, consider the tree and its specific needs. For example, if the tree in question is a red maple or pin oak in northern Illinois, the soil test report is likely to indicate a high pH and a suggestion to lower it to make the iron in the soil more available. As such, it’s important to match up the specific needs of the tree species with the test results.
Species aside, nitrogen is generally the nutrient required in the greatest quantity. However, because it is a mobile nutrient, test reports tend to be transitory and reliable only for a short time. As a result, it’s common for soil tests to indicate a need for low to moderate amounts of nitrogen and that an adequate amount of phosphorous and potassium, which are more stable and required in lesser quantities, already exist. Again, good communication with the soil-testing lab is essential.
Appearance — The color of foliage, condition of the branch tips, size of leaves, thickness of crown, and comparison with other specimens of the same species are good indicators of the need for added nutrients. If foliage is pale green, the tree has undersized leaves, a thin canopy is present, or stems have dieback, fertilization may be beneficial.
Surrounding plant material – If the tree is in the middle of a lawn that receives periodic nutrient applications, the need for tree fertilization is reduced, as small amounts of the applied materials will move downward in the soil profile and be absorbed by the tree roots. If grass clippings are returned to the lawn during mowing operation, then small amounts of the nutrients present in the grass blades are recycled to the soil, where both grass and tree roots can grab them.
The situation is different for a tree growing in a mulch bed or in a parking lot. Neither of these landscape spaces offer the benefit of applied nutrients to turf; however, depending on the amount of leaf debris that’s recycled back to the tree roots, the need will change. In mulch beds, the decomposition of debris that falls on the mulch layer is helpful in maintaining the fertility level of the soil, thereby decreasing the need for added nutrients. Parking lots, on the other hand, offer little potential for turf fertilizer or tree debris to encourage healthy tree growth. Fertilization may be helpful in landscapes that offer minimal inputs. In addition, fertilizer injections may be best in these scenarios, as they are generally not conducive to water lance or surface applications.
Regardless of the application method, the rate should be based on the area occupied by the roots. Unless severely restricted because of planter boxes or tree pits, roots typically spread well beyond the branches on established trees and shrubs. Therefore, the target area for fertilization should be two to three times beyond the diameter of the branch spread.
Growth rate over time — A tree that grows at a rate typical for its species for several years followed by a decline in growth rate over successive years should be considered a candidate for fertilization. If a slowed growth rate is observed in conjunction with an indicated need in a soil test report for a tree in an area that is well aerated but not routinely fertilized (such as an ornamental bed), then a low to moderate application of nitrogen and other elements may be useful in improving the tree’s health.
On the other hand, if a tree has been receiving nutrients from lawn applications, has received adequate rainfall, and there are no visible pests, then added nutrients are generally not recommended. Also, if a tree has produced growth that is healthy and typical of its species over time, the need for fertilization is not justified.
Fertilizing trees under stress isn’t generally recommended. It’s better if the tree’s resources are used to defend it against insects and diseases and for root regrowth, rather than using up stored sugars to produce new leaves. Other trees and shrubs that should not be fertilized include newly planted specimens and those with root damage from recent trenching or construction. The root systems need time to re-establish before fertilizers are applied.
For mid-spring and late fall applications, there’s a fine line when it comes to fertilizing specimens under stress. Yes, applied nutrients can increase resistance to stressors; however, if the stressors are causing the tree to decline, it’s best to avoid fertilization until the tree recovers.
Also, a specimen may not need fertilizer if an application has been made and the tree still looks thin or produces inadequate growth. In other words, if your fertilizer application doesn’t produce the results you were looking for, investigate to determine the factors that are affecting the tree’s health.
There are some other treatments that can produce a similar effect on tree health as fertilization.
Aeration — This opens up compacted soils, increasing the movement of oxygen into the pore spaces and facilitating greater water infiltration. Many specimens struggle because the roots cannot expand laterally to support the needs of the tree.
Expanded mulch area — Mulch, placed in a manner that simulates Mother Nature, could be the ticket to improved tree health. Expanded mulch areas offer the additional benefits of moisture retention and weed suppression.
Compost application — Depending on the landscape situation, thin applications of compost will provide a dose of slow-release nutrients. The key is thin, not more than 1/3 inch deep. Compost applications are best following lawn aeration in landscapes where a tree is in the midst of a sward of turfgrass.
When to Fertilize
- Do apply in the late fall.
- Do consider the amount of fertilizer applied in fall versus spring. A good rule of thumb for fall applications is to apply about a third of the amount normally used for spring applications – enough to encourage root and future shoot growth.
- Do consider nitrogen, which is generally the nutrient required in the greatest quantity for fall fertilization.
- Do fertilize if a tree’s foliage is pale green, a tree has undersized leaves, a thin canopy is present, or stems have dieback.
- Do fertilize if a tree is growing in a mulch bed or in a parking lot. These landscape spaces don’t offer the benefit of nutrients applied to turf.
- Do fertilize if a tree that grows at a rate typical for its species for several years is followed by a decline in growth rate over successive years.
When Not to Fertilize
- Don’t apply in the early fall when fertilizer can encourage growth. Depending on where you live, root function can continue into December.
- Don’t apply if trees have root damage from construction or trenching.
- Don’t fertilize if a tree is in the middle of a lawn that receives nutrient applications, as small amounts of the applied materials will move downward in the soil profile and be absorbed by the tree roots.
- Don’t fertilize trees under stress.