Established cedars rarely need fertilizer; a soil test decides whether nitrogen, phosphorus, or potassium is missing, and it is the only reliable guide to which NPK ratio your tree needs.
Yellowing foliage on a cedar looks like a nutrient problem, and often it isn’t — drought, soggy roots, mites, and transplanted shock all produce the same symptom. Feeding a stressed tree wastes money and can make the decline worse. What follows is how NPK numbers work, what rate verified extension guidance supports, and how to apply fertilizer so the roots actually get it.
What Do The Three NPK Numbers Actually Mean?
The three digits on a fertilizer bag are the percentage of nitrogen, phosphorus, and potassium by weight, always in that order. Nitrogen drives green foliage and leafy growth; phosphorus supports root development; potassium handles general plant function and stress tolerance. A bag marked 30-0-10 is 30% nitrogen and 10% potassium, with no phosphorus, and the 70% left over is carrier material.
Fertilizer labels express phosphorus as phosphoric acid and potassium as soluble potash, so the digits are not bare elemental percentages. That distinction matters when you calculate how much product delivers a target amount of nitrogen.
Which Cedar Are You Feeding, And Does It Even Need It?
“Cedar” covers different plants — true cedars in the genus Cedrus and white cedar, which is arborvitae (Thuja occidentalis) — and needs vary by species, soil, age, and moisture. A healthy, established landscape cedar usually gets nothing. Fertilize when a soil test, poor foliage color, or weak growth says something is short.
Run through the non-nutritional causes first: drought, poor drainage, pests, disease, transplant stress, or root injury. If any of those are present, fix the underlying problem before you feed. Yellow foliage is not proof of a nitrogen deficiency, and treating it as one is the most common way cedar fertilizing goes wrong.
What NPK Ratio Should Cedar Fertilizer Use?
Let the soil test set the formula. When it calls for phosphorus, general landscape-tree guidance favors a ratio near 3:1:1 or 3:1:2. When it doesn’t — and many soils already hold enough — a phosphorus-free option such as 3:0:1 or 3:0:2 works, and 30-0-10 is a common example.
| Nutrient | What It Supports | When To Skip It |
|---|---|---|
| Nitrogen (N) | Green foliage and vegetative growth | Tree is healthy and growing at a normal rate |
| Phosphorus (P) | Root development | Soil test shows adequate phosphorus |
| Potassium (K) | Overall function and stress resistance | Soil test shows adequate potassium |
| Steady low-P ratio (3:0:1) | Foliar greening without phosphorus buildup | You have not tested and soil P runs high |
| Balanced ratio (3:1:1) | General feeding where soil is truly low in P | Soil is already adequate in phosphorus |
Do not default to a high-phosphorus “bloom” product just because it sounds stronger. Extra phosphorus does not push shoot growth, and it can accumulate. If you want the specific formulas sold for this use, this roundup of fertilizer options for cedar trees lists products with labeled NPK percentages you can compare against a soil test.
How Much Fertilizer, And When Should It Go Down?
Rates are stated in pounds of actual nitrogen, not pounds of product. ANSI A300 guidance allows 2–4 lb of total nitrogen per 1,000 sq ft per year for slowly available sources, capped at 6 lb per 1,000 sq ft. A typical mature tree takes 1–2 lb per 1,000 sq ft per year, and 2–4 lb when you want young trees growing fast.
Convert that to bag weight using the nitrogen percentage and the product label. For a 30-0-10 product, roughly 3.3 lb of product carries 1 lb of nitrogen; the label’s directions take precedence over any general figure. Early spring, as active growth begins, is the standard window for landscape cedars in the North. Skip late-season feeding that pushes tender growth before dormancy — one cedar product specifically stops applications after the end of July. Clemson Cooperative Extension’s guide to fertilizing trees and shrubs walks through the same calculation.
Spread granules evenly across the root zone, weighted toward the drip line rather than the trunk, because fine absorbing roots live in the outer root zone. Keep granules off the foliage and off the trunk. Apply to moist soil or irrigate afterward so nutrients reach the roots and the salts don’t burn them. On dry soil, water first.
Sources
- Clemson Cooperative Extension. “Fertilizing Trees & Shrubs.” Supports application rates, timing, and root-zone placement guidance.
- North Carolina Cooperative Extension. “A Gardener’s Guide to Fertilizing Trees and Shrubs.” Supports NPK ratio selection and soil-test-first guidance.
- Nuway. “Cedar Fertilizer 18-3-10.” Supports the late-July cut-off for cedar applications.