Do trees need fertilizer? A Northwest guide to fertilizing trees

Do trees need fertilizer?

Usually not, once they are established. A tree’s roots run far beyond the trunk, and OSU’s fact sheet on fertilizing shade and ornamental trees notes that trees absorb nutrients whenever the area above their roots is fertilized. ISA’s plant health care brochure makes the same point from the other side: the roots of a single mature tree may extend well into your lawn or flower beds, and every lawn treatment can affect the tree.

Young trees are the exception. OSU says trees with a trunk under 6 inches across can benefit greatly from regular fertilizing, and its new-tree guide suggests feeding established young trees every two or three years, in fall after leaf drop or early spring. Even then, both ISA and WSU Extension horticulturist Linda Chalker-Scott say to test the soil before adding any fertilizer.

USDA’s PLANTS database backs this up for Northwest trees. It rates the fertility each species needs for normal growth as low, medium or high, and none of these common Washington and Oregon trees is rated high.

Source: USDA NRCS PLANTS Database, species characteristics (fertility requirement; soil pH minimum and maximum; nitrogen fixation rated high for red alder), checked 2026-10-02.

Yellow leaves or thin crown: deficiency or something else?

Pale or yellowing leaves look like hunger, but OSU warns that reduced shoot growth, leaf discoloration and insect or disease problems are sometimes not due to nutrient deficiencies at all. Its list of other causes: wet, cold or poorly aerated soil; long stretches of dry soil; salt from overfertilizing, excess calcium or salty irrigation water; insect and disease damage to roots or crown; and trunk damage.

Sources: OSU FS 103; PNW Plant Disease Management Handbook (plant nutrition diagnosis; key to nutrient deficiencies); WSU Extension (phosphate fertilizer; needle loss; laminated root rot); ISA; Oregon Department of Forestry drought fact sheet; OSU arborvitae hedge guidance.

Don’t trust your eyes alone. The PNW Plant Disease Management Handbook says visual diagnosis of nutrient deficiencies is often inaccurate, and that the only accurate way to diagnose a nutritional problem is a soil test and a leaf tissue analysis done together. It adds that root rot, cankers and trunk wounds can produce deficiency symptoms by cutting the tree’s plumbing, which no fertilizer fixes.

In the Northwest, check summer water first. OSU’s new-tree guide puts a young tree’s need at about 1 inch of water a week from June through September, roughly 17 inches over the season. NOAA’s normals show how far short the rain falls.

Even rainy Olympia gets under a third of that in an average summer, and Yakima gets about a fifteenth. When leaves yellow or brown in August, check the water before you buy fertilizer. Oregon’s Department of Forestry is blunt for drought-stressed conifers: do not fertilize during drought, because more growth means the tree needs more water.

How to test your soil in Washington and Oregon

1. Sample where the roots areOSU’s garden soil-testing guide suggests digging about 6 to 8 inches deep, removing roots, grass and mulch from each sample, and mixing soil from several spots into one sample.
2. Pick an accredited labOSU’s lab-selection guide recommends a lab enrolled in the North American Proficiency Testing program or the Agricultural Laboratory Testing Association that uses methods calibrated for Western soils.
3. Ask for the basicsA pH test plus a nutrient analysis covering phosphorus, potassium, calcium and organic matter. If you are chasing a symptom, add a leaf tissue test, as the PNW handbook advises.
4. Read it, then decideStart with the lab’s own interpretation, and ask Extension Master Gardener volunteers, as OSU suggests, if the numbers don’t make sense.

West and east of the Cascades are different soils. OSU’s lab guide matches tests to western Oregon’s acidic soils and eastern Oregon’s alkaline soils, so tell the lab where the sample came from. If the pH is the problem, ISA notes that nutrients may be present but out of the roots’ reach, and that sulfur, lime and some mulches can shift soil chemistry; piling on more fertilizer does not fix a pH problem.

Fertilizing trees: when, how much and where (if the test says so)

Nitrogen is the nutrient trees usually need; OSU says phosphorus and potassium are used at lower rates and most soils already contain enough. OSU suggests a fertilizer with roughly a 3-1-2 ratio of nitrogen, phosphorus and potassium, such as 10-4-6 or 20-5-10, but if your test shows plenty of phosphorus, choose one without it.

When to fertilize trees

OSU’s window is late summer or fall, or early spring as growth begins. In late summer or fall, use an insoluble, slow-release product: a soluble, high-nitrogen fertilizer can push a late flush of growth that an early freeze then damages.

How much

OSU’s rate is 1/4 pound of actual nitrogen per inch of trunk diameter, measured 4.5 feet above the ground. To get pounds of product, divide the pounds of nitrogen by the bag’s first number and multiply by 100; OSU’s own example is a 4-inch tree and 20-5-10, which works out to 5 pounds of fertilizer, about 10 cups.

Source: OSU Extension FS 103 (1/4 lb actual nitrogen per inch of trunk diameter; five holes per inch of diameter). Product weights = nitrogen ÷ the bag’s nitrogen fraction, computed by Westcoast-Tree.org.

Where to put it

Spread it over the roots, not at the trunk. OSU says most of the roots that take up nutrients are in the top 12 inches of soil, from the trunk out to about 1½ times the canopy diameter. Broadcast over the lawn or mulch above that zone, or dig 6- to 12-inch holes around the drip line, five per inch of trunk diameter, and split the fertilizer among them. Tree fertilizer stakes, the spikes sold in garden stores, go in at the same spacing. Water afterward to carry the nitrogen down to the roots.

Skip the trunk injections. WSU’s Chalker-Scott reviewed the research and found that injected trees greened up at first but looked no different from untreated trees after a few seasons, while the injection holes let in decay and pests, especially in trees already in poor shape; she also found soil injection no more effective than spreading fertilizer on the surface.

Fertilizing evergreen trees and arborvitae

Northwest conifers ask for little: the PLANTS ratings above run from medium for Douglas fir, western redcedar, western hemlock and grand fir down to low for ponderosa and shore pine. The test-first rule applies to them like any tree, with three evergreen traps to rule out before you feed one.

• Normal needle drop. WSU says conifers periodically shed older needles, and western redcedar often drops them dramatically in early fall after a dry summer; it is a natural process that needs no treatment.
• Drought. The Oregon Department of Forestry lists missing or shorter branches, thinning crowns that may start at the top, and top kill as drought symptoms in conifers, and says not to fertilize during drought.
• Root disease. WSU lists yellowing and thinning crowns and a distress crop of cones as signs of laminated root rot in Douglas fir, with no chemical treatment recommended; see laminated root rot.

Arborvitae hedges add one more reason for restraint. WSU’s fact sheet on spider mites in arborvitae says high nitrogen in the foliage encourages mite reproduction and that drought-stressed plants are more susceptible, and it points toward lower-nitrogen or slow-release products where mites are a problem. OSU’s arborvitae advice centers on good drainage and a deep soak every two to three weeks in dry spells, not on feeding. If your hedge is already brown, start with why arborvitae turns brown; for an ornamental favorite, see Japanese maple care.

Too much fertilizer: salt, pests and water quality

Overdoing it costs more than money. OSU lists salt buildup from overfertilizing among the causes of poor growth and discolored leaves, and WSU’s Chalker-Scott notes that phosphorus is rarely deficient in landscaped urban soils, that excess phosphorus competes with the iron and manganese roots take up, causing yellow leaves, and that high phosphorus harms the mycorrhizal fungi that help roots.

What the roots don’t use can leave the yard. Chalker-Scott writes that excess phosphate eventually reaches waterways and feeds algal blooms, followed by bacterial activity that lowers oxygen and kills fish. Washington’s Department of Ecology describes the same chain in Puget Sound: too much nitrogen from human sources drives algae growth, and its decay uses up the oxygen marine animals need. Washington also limits phosphorus in lawn fertilizer, under the rule shown above.

Watch out: A lawn and the tree in it share the same soil. ISA’s plant health care brochure warns that every treatment applied to a lawn can affect the trees growing in it, so read the label before spreading anything over a tree’s roots.

Why fertilizer won’t save a stressed or dying tree

Fertilizer feeds growth, and growth costs a struggling tree water it may not have; that is why the Oregon Department of Forestry says not to fertilize drought-stressed conifers. If roots are rotting, or cankers or a trunk wound have cut the tree’s plumbing, the nutrients cannot reach the leaves, as the PNW handbook points out.

ISA lists discolored leaves, dead branches and early leaf drop as symptoms that may call for an arborist, and notes that a problem left undetected too long can leave removal as the only option. If the tree is losing ground despite water, mulch and time, the guide to how to tell if a tree is dead covers the field checks, and an arborist can diagnose what you cannot see; a big tree near the house may also warrant a tree risk assessment.

Mulch: the better default

Mulch is the low-risk alternative. ISA’s mature tree care brochure says organic mulch evens out soil temperature and moisture, cuts competition from grass and weeds, keeps mowers off the trunk, and adds nutrients and soil life as it breaks down.

For a free source, the guide to free wood chips in Washington and Oregon covers how to get a load of arborist chips and how deep to spread it.