On-Site Tree Density Requirements

Where on-site tree density/tree planting requirements apply 

On-site tree density/tree planting requirements apply when any of the following are present:

• New development
• Exterior alterations to existing development with a project valuation is that is more than the non-conforming upgrade threshold, which is updated annually and can be found in Section 33.258.070(D)(2)(a) of the zoning code

Exemptions from on-site tree density/tree planting requirements 

You may be exempt from on-site tree density/tree planting requirements in the following circumstances: 

• A specific condition of land use review approval exempts the site from these density standards
• On portions of sites located within an IH zone
• The site is within the Portland International Airport Plan District or Cascade Station/Portland International Center Plan District and is subject to the Airport Landscaping Standards; see Title 33, Planning and Zoning
• Work conducted under Demolition, Site Development, Septic, Plumbing or Zoning permits

This project valuation is the non-conforming upgrade threshold, which is updated annually and can be found in Section 33.258.070(D)(2)(a) of the zoning code.

On-site tree density requirement

The required tree density is based on the size of the site and type of proposed or existing development, as listed in Table 50-1 (below). You may select either option for calculating the required tree area. However, only Option A may be used to apply standards to a Development Impact Area. 

Title 11.50 Table 50-1 Determining Required Tree Area

Meeting on-site tree density requirements by preserving trees, planting trees or paying a fee

You can meet your on-site tree density requirement through any combination of preserving trees, planting trees or paying a fee.

Preserving trees

Existing trees may be used to meet the on-site tree density requirements as follows:

• On sites that are up to 3,000 square feet, healthy non-nuisance species Street Trees can be credited towards density requirements.
• Trees 1.5 inches or larger but less than 6 inches in diameter count as one small canopy size tree.
• Trees that are 6 inches or larger in diameter count as one medium canopy size tree for each full increment of 6 diameter-inches.
• Trees straddling property lines count as one medium canopy size tree for each full increment of six diameter inches on the property where development is occurring. 

Read more in the related Protecting Trees During Construction and Tree Preservation Inspections webpage.

Planting trees

Applicants may plant any combination of small, medium, and/or large canopy trees to meet the on-site tree density requirement, where:

• Small Canopy Trees  =  300 square feet, 50 s.f. (3’ x 3’) min. planting area
• Medium Canopy Trees  =  500 square feet, 75 s.f. (5’ x 5’) min. planting area
• Large Canopy Trees  =  1,000 square feet, 150 s.f. (10’ x 10’) min. planting area

To learn which trees are in each canopy size category, see Determining Canopy Size below.

Pay a fee

Payment may be made in lieu of planting if there is not room on the site to plant the required trees. This fee supports the planting and establishment of one 1.5-inch caliper tree in the same watershed. See the fee schedule to calculate this cost.

Determining tree canopy size category

Tree canopy size category can be determined by the canopy factor, which is a number calculated from the dimensions of the tree at maturity. 

The Portland Tree and Landscaping Manual has calculated the tree canopy factor for many tree species. You can calculate the tree canopy factor for other tree species with the following formulas:

• Canopy factor = (mature height of tree) x (mature canopy spread) x (growth rate factor) x 0.01
• Growth rate factor = 3 for fast-growing trees, 2 for medium-growing trees, and 1 for slow-growing trees

Find the tree’s mature height, mature canopy spread, and growth rate from a reputable source such as published sources, nursery spec sheets, or internet sources.

Once you have calculated the tree's canopy factor, you can find the canopy size category from the following table: