Tree and Shrub
Fertilizing Options
Trees and shrubs in your landscape require fertilizing for continued vigor and health. Consider these important factors as you inspect the plants in your yard:
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- Why are two fertilizer applications needed when planting trees and shrubs?
- What’s the best time to fertilize landscape trees and shrubs?
- How to recognize tree and shrub nutrition problems and stunted growth?
- When is a soil nutrient test necessary?
- How and where should fertilizers be applied to landscape trees and shrubs?
- What methods should be used to apply various solid and liquid fertilizers?
- What fertilizers are right for trees and shrubs?
- Why is watering important after applying fertilizer to trees and shrubs?
Fertilizing Trees and Shrubs
Help trees or shrubs thrive by giving them additional nutrients. The conditions in our yard rarely are a perfect match to every plant’s specific needs.
When to Fertilize
Trees and shrubs growing in soils to which they are naturally adapted—whether it be the rich loam of a forest or the sandy terrain of a beachfront—typically thrive on their own and rarely need supplemental fertilizers.
Fertilize trees and shrubs before planting and during the planting itself. Thereafter, fertilize only when you need to correct symptoms of a nutritional deficit or because of shortages noted in a soil test [See: Soil for Trees and Shrubs].
Before-planting fertilizing is important for trees and shrubs grown in nursery containers. Apply a mix of liquid organic fish fertilizer, kelp extract, and other micronutrients to their rootball with a probe applicator a week before planting to rouse them from dormancy.
Apply fertilizer to the soil beneath the rootball at time of planting, making sure at least 2 in. (5 cm) of soil covers it to prevent direct contact with the tree’s or shrub’s roots after the plant is set into the hole.
Recognizing Nutrition Problems
You can spot most deficiencies early by unusual coloration in your tree’s and shrubs’ leaves or needles.
Yellow leaves on a tree that normally bears green foliage is often a sign of either a nitrogen or iron deficiency. With a nitrogen shortage, the entire leaf turns yellow and may drop off. With an iron deficiency the veins of the leaf remain green even as the rest of the leaf turns yellow.
Leaves affected by specific nutrient shortages may be smaller and their canopies less dense compared to those of other trees of the same species. Sometimes new twig growth may be less vigorous than in prior years.
If you notice any of these symptoms, first rule out other causes [See: Tree and Shrub Pests and Diseases] before choosing to fertilize.
Soil Testing
For certainty, also conduct a soil test [See: Soil for Trees and Shrubs]. The test will indicate the nature and amount of shortage of the three key nutrients, nitrogen, phosphorous and potassium. Follow the soil test’s recommendations regarding the amount and type of fertilizer to apply.
Choosing Fertilizers
Shrubs, especially flowering species, may need to be fed more often than trees. Use a light application of all-purpose fertilizer in the spring and again in the summer.
Feed more frequently if there is a nutritional deficiency, as indicated by unusual leaf coloration and confirmed by a soil test.
Generally, flower and fruit trees occasionally need potassium and phosphorus for root health, flowering and fruit development. Most other species may require more nitrogen to promote foliage growth. Iron deficiency—a common cause of foliage yellowing—is usually cured by amending the soil with an additive containing chelated iron.
On loose sandy soils broadcast the needed fertilizer over an area from its trunk to twice the diameter of the tree’s or shrub’s foliage canopy.
In clayey or compacted soils, bore holes in the soil around the tree’s drip line—the area beneath the outer edge of the tree’s canopy—and fill the holes with a slow-release fertilizer, which will feed the roots at a prescribed rate over a period of time.
Always water after applying fertilizers and amendments to avoid burning your plants. Keep in mind a simple tip: when in doubt, less is better than more. More fertilizer can always be added later, but stopping the damage done by too-heavy applications is hard work.
Five Fertilizer Choices
Fertilizers are available in a confusing number of options: natural, organic, inorganic, synthetic, liquid, dry, and water-soluble forms.
Eliminate the confusion by referring to the composition analysis on the package. It’s a trio of numbers such as 10–10–10 that tells the relative percentage by weight for the nitrogen, phosphorus, and potassium the fertilizer contains.
Nitrogen fuels foliage and fruit development, while phosphorus and potassium help nutrient transport between roots, stems, and leaves. They also increase a tree or shrub’s general vigor.
For most tree and shrub applications, a complete, balanced fertilizer with equal measures of all three macronutrients is best, as in the 10-10-10 example. To apply various fertilizers, choose from these options.
Fertilizer Options
Apply liquid and water-soluble fertilizers to the soil around the plant’s drip line, an imaginary circle on the ground under the outermost foliage.
Apply dry, granular, and synthetic fertilizers by sprinkling them onto the soil around the plant, then working them into the soil with a hand fork or claw. Water after application.
Apply natural manure and inorganic mineral fertilizers by spreading them across the soil surface around the plant. Work them into the surface with a rake, fork, or claw. Water after application.
Apply organic compost to the soil in a layer, 2–3 in. (50–75 mm) thick, avoiding contact with shrub stems or tree trunks. Water after application.
Apply foliar fertilizers using a hose-end sprayer set to the package-recommended application rate. Choose a time early on a calm, warm day to allow foliage to dry before evening.