Feed Trees & Shrubs Right
Learn how to fertilize your landscape, diagnose nutrient deficiencies, and improve the heavy clay soil common in the Triangle.
Lawn Fertilizer vs. Plant Fertilizer
Never use a high-nitrogen lawn fertilizer (like 32-0-4) on your trees and shrubs. Lawn fertilizers are designed to push fast, green blade growth. If you use them on a tree, you'll get a flush of weak, sappy growth that is highly susceptible to aphids, disease, and winter freeze damage.
Instead, use a balanced, slow-release fertilizer. Look for numbers like 10-10-10 or 14-14-14 for general ornamentals. For acid-loving plants (azaleas, camellias, gardenias), choose a fertilizer specifically formulated for them, such as Holly-Tone.
Acid-Loving Plants
Many popular southern landscape plants — including Azaleas, Camellias, Gardenias, Rhododendrons, and Blueberries — require acidic soil (pH 4.5 to 5.5) to thrive. Our native NC clay is generally acidic, but new construction often leaves alkaline soil behind due to mortar and concrete dust.
If you see yellowing leaves with green veins on these plants, they are likely suffering from iron chlorosis because the high pH is locking up the iron in the soil. You must lower the pH. Use a fertilizer designed for acid-loving plants, and consider adding elemental sulfur if a soil test confirms high alkalinity.
Diagnosing Iron Chlorosis
Iron chlorosis is one of the most common nutrient issues in the Triangle. It's rarely caused by an actual lack of iron in the soil; instead, high pH (alkalinity) or poor drainage/compacted clay prevents the roots from absorbing the iron that is already there.
Symptoms:
- Leaves turn pale green or yellow.
- The veins of the leaf remain distinctly dark green (interveinal chlorosis).
- Newest leaves show the worst symptoms first.
- In severe cases, leaf edges may scorch and turn brown.
Treatment:
- Short term: Apply chelated iron (liquid or granular) to the soil or as a foliar spray. The plant can absorb this form of iron even in alkaline soil.
- Long term: Conduct a soil test to determine the pH. If it's too high (above 7.0), incorporate elemental sulfur to lower the pH over time.
- Improve drainage: Compacted, waterlogged clay also limits iron uptake. Aerate and incorporate organic matter.
Seasonal Feeding Calendar
Timing is everything. Fertilizing at the wrong time wastes money and can harm your plants.
| Timing | Action | Details |
|---|---|---|
| Early Spring (Late Feb–Mar) | Primary Feeding | Apply a balanced, slow-release fertilizer just before new growth starts. This provides nutrients for the spring flush. |
| Late Spring (May) | Acid-Lovers | Feed azaleas and rhododendrons immediately after they finish blooming. |
| Summer (Jun–Aug) | Hold Off | Avoid applying high-nitrogen fertilizers during the hottest months, as this stresses the plant and requires more water. |
| Early Fall (Sep) | Secondary Feeding (Optional) | A light application of balanced fertilizer or compost can help roots store energy for winter. |
| Late Fall/Winter (Nov–Jan) | Stop Feeding | Do not apply nitrogen. It can push tender new growth that will be killed by hard freezes. |
Fertilizing New Plantings
The biggest mistake homeowners make is dumping strong granular fertilizer into the planting hole of a new tree or shrub. This burns the delicate new roots and forces canopy growth when the plant needs to be focusing energy below ground.
Wait one full year before applying standard NPK granular fertilizers. At planting time, you can use a root stimulator containing mycorrhizae or apply a layer of compost over the root zone.
Mulch & Organic Matter
In our dense clay, adding organic matter is often more important than adding synthetic fertilizer.
- Compost: Top-dress your beds with 1-2 inches of high-quality compost every spring. As it breaks down, it improves soil structure, drainage, and nutrient availability.
- Mulch: Maintain a 2-3 inch layer of shredded hardwood mulch or pine straw year-round. Keep mulch 2 inches away from the trunk to prevent rot.
- Do not 'volcano mulch' (piling mulch high against the trunk). This kills trees slowly by encouraging stem-girdling roots and bark rot.
Soil Testing for Ornamentals
Before making major amendments, get a soil test. The NCDA&CS provides soil testing for NC residents (free from April-November, small fee in peak winter season). It's the only way to accurately determine your soil pH and base nutrient levels.
Visit the NCDA&CS Agronomic Services page to learn how to submit a sample.
Fun Fact: The Wood Wide Web
Trees in a forest communicate and share nutrients through a vast underground network of mycorrhizal fungi. These fungi attach to tree roots, extending their reach to absorb water and minerals (like phosphorus). In exchange, the tree provides the fungi with sugars from photosynthesis. You can add mycorrhizal inoculants when planting new trees to help establish this beneficial relationship in your yard!
Plant Nutrition FAQ
Should I use lawn fertilizer on my trees and shrubs?
No. Lawn fertilizers are often extremely high in fast-release nitrogen (e.g., 32-0-4) to push blade growth. This can cause trees to produce weak, sappy growth that attracts aphids and is susceptible to winter damage. Use a balanced, slow-release ornamental fertilizer (like 10-10-10 or 14-14-14) or an organic option like Holly-Tone.
When is the best time to fertilize established trees and shrubs in NC?
Early spring (late February to March), just before new growth begins. A second, lighter application can be made in early fall (September). Avoid fertilizing in late fall or winter, as it can push tender growth that gets killed by frost.
My oak tree's leaves are turning yellow with green veins. What's wrong?
That is classic iron chlorosis. In the Triangle, it's often caused by high soil pH (alkalinity) or compacted clay, which locks up iron so the plant can't absorb it. You can apply chelated iron for a quick fix, but long-term you need to acidify the soil with elemental sulfur and improve drainage.
Do newly planted trees need fertilizer right away?
Wait at least a year before applying strong granular fertilizers. Newly planted trees need to focus on establishing a root system, not pushing canopy growth. You can use a mild organic root stimulator or compost tea when planting, but skip the NPK until year two.
How much compost should I add to my ornamental beds?
A 1- to 2-inch layer of high-quality compost spread over the root zone every spring is excellent. Cover it with 2-3 inches of hardwood mulch to protect it and retain moisture. Over time, the compost breaks down and improves our heavy clay soil naturally.