Red alder in Washington and Oregon yards

How to identify it

OSU’s plant profile gives the quick checks. The bark is smooth and light gray, usually so covered in lichen it looks nearly white, and the inner bark is orange-red, so fresh wounds turn red. Leaves are alternate, 7–13 cm long, with coarse teeth and edges rolled under. Seed catkins are small, barrel-shaped cones 12–25 mm long.

The silvics manual puts its range along the Pacific coast from southeast Alaska to southern California, mostly within about 125 miles of the ocean and below about 2,500 feet.

How fast it grows and how big it gets

Alder is built for open ground. The USDA Forest Service silvics manual records seedlings growing 3.3 feet or more in their first year, and height growth over 9.8 feet a year between ages 2 and 5. On good sites trees reach 100–130 feet tall and 22–30 inches in diameter; OSU gives a more typical planted height of 40–50 feet.

The same manual says alder matures at about 60 to 70 years and that maximum age is usually about 100. An alder that started 60 years ago is at the age the manual calls mature, and the decay described below becomes the main concern.

Alder also thrives where other trees struggle. OSU explains that it forms root nodules with the soil bacterium Frankia, which turn it into a nitrogen fixer and let it tolerate infertile soil; the silvics manual reports fixation of up to 290 pounds of nitrogen per acre a year in pure stands. ODF’s 2024 forest health report lists it among Oregon’s most abundant hardwoods alongside bigleaf maple, Oregon white oak, Oregon ash and black cottonwood.

How an old alder fails

• Heart rot. The silvics manual names Phellinus igniarius, a white heart rot, as probably the major cause of cull in older trees. Look for conks, the hard fungal shelves that grow from decayed wood, on the trunk; the guide to mushrooms on trees covers what conks mean.
• Shade decline. Red alder is intolerant of shade; the manual says suppressed young trees grow very little and die.
• Fast decay after death. The manual notes that alder wood decays rapidly once cut. Do not leave a dead alder standing over a target for long; see dead tree removal.
• Not usually windthrow. The manual says windthrow is not common in alder, which forms extensive fibrous roots. Breakage of decayed stems and limbs is the bigger risk.

Two things alder is not vulnerable to: laminated root rot, to which ODF lists hardwoods such as maple and alder as immune (see laminated root rot), and Swiss needle cast, which is why OSU’s coastal guide suggests red alder among the replacements for Douglas fir in the near-coast zone. OSU does note that it is a favored host for tent caterpillars.

Why cities treat alder differently

Many Puget Sound cities leave alder out of their “significant tree” counts or set a higher size threshold for it. That can be the difference between needing a permit and not. In Renton, for example, a developed lot may remove up to two significant trees in 12 months and five in five years without a permit (RMC 4-4-130); a 7-inch alder does not use up one of those slots, while a 7-inch fir does.

Two cautions. These exclusions apply to the size counts, not to critical areas: Renton, for one, bars removal in critical areas unless an exemption applies (RMC 4-4-130.D.3), and an alder in a stream buffer, wetland or steep slope falls under those rules. Oregon rules differ: Portland regulates private trees 12 in DBH and larger regardless of species (PCC 11.40.020). City-by-city rules are in Washington tree removal permits and Oregon tree removal permits.

Removal: when and what it involves

A certified arborist with a Tree Risk Assessment Qualification can sound the trunk and write the report that supports a hazard permit; see tree risk assessment.

Modeled estimate, not quotes (method): a 50–80 ft alder models at $1,700–$3,400 in Washington and $1,800–$3,600 in Oregon with chipping and haul-away but no stump; 25–50 ft models at $850–$1,700 and $900–$1,800. See tree removal cost. Other native species are under trees, including bigleaf maple.