Street tree diversity: the 10-20-30 rule and how city and county documents word it
The 10-20-30 formula caps a single species, genus and family in an urban forest. This page gives the formula as three sources state it, then how a county, two cities and a state agency wrote it into their own documents.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named documents say. It does not set a diversity target for any place. Several of the documents say their own figures are goals for a whole inventory, not a limit for one street. The species lists cities publish are in the approved street tree lists family, and the tools that tally an inventory are on the tree inventories page.
The formula
Cecil Konijnendijk's 2021 article on the 3-30-300 rule credits Frank Santamour with the 10-20-30 rule. It states the rule as caps of 10 percent for any one species, 20 percent for any genus and 30 percent for any single family of a municipality's urban forest. It says the rule has been debated but has become widely known and adopted. Michael Galvin's 1999 paper in the Journal of Arboriculture, which the Maryland Department of Natural Resources hosts, cites Santamour 1990 for what it calls the 10-20-30 formula, stated as protection against pest outbreaks. Galvin's paper describes how Maryland took an inventory, tallied it by species, genus and family and used the formula to recommend future species composition. It notes that many jurisdictions use approved tree lists that include many species not generally available locally and rarely build diversity limits into them.
Galvin points to the losses in places that over-planted American elm, which Dutch elm disease hit, as an example of the risk of near monocultures. The 2023 Ambio paper by Hilbert and colleagues starts from the fact that urban forests continue to be dominated by a limited number of species despite high richness in urban areas, and offers a framework of how tree producers and consumers narrow the species that reach the street.
How documents word it
| Document | Wording |
|---|---|
| Westchester County, New York, street tree species selection page | Says it is recommended that a municipality aim for no more than 10 percent of one species, 20 percent of one genus and 30 percent of one family. Ties diversity to blights such as Dutch elm disease and to pests it lists, among them emerald ash borer and spotted lanternfly, and says municipalities will need regularly updated street tree inventories. |
| City of New Westminster, British Columbia, urban forest management strategy, implementation page | Lists as an objective better urban forest diversity through planting capped at 10 percent per species, 20 percent per genus and 30 percent per family. The strategy page also lists a goal of increasing canopy cover to 27 percent city-wide. |
| City of Saint Paul, Minnesota, street tree master plan text (2010) | Says the street tree inventory being developed will help track diversity, and that species diversity will be considered at city scale. Says a goal that caps any one species at 10 to 20 percent of the inventory supports diversity planning. Calls the 30-20-10 model a useful guide while noting urban site conditions and the species available in Minnesota may limit full implementation. |
| Maryland Department of Natural Resources, Galvin paper | Uses the 10-20-30 formula to assess an existing street tree population from an inventory and to recommend future species composition. |
The Saint Paul text also sets a maximum linear distance for one species, one to three blocks in the text, which is a layout limit and not a share of the inventory.
Sources
- Konijnendijk, The 3-30-300 Rule for Urban Forestry and Greener Cities, Biophilic Cities Journal, July 2021, copy hosted by the University of Toronto, read October 9, 2026
- Galvin, A methodology for assessing and managing biodiversity in street tree populations, Journal of Arboriculture, 1999, hosted by Maryland Department of Natural Resources, read October 9, 2026
- Westchester County, New York, street tree species selection for the municipal right-of-way, read October 9, 2026
- City of New Westminster, urban forest management strategy implementation, read October 9, 2026
- City of Saint Paul, street tree master plan text 2010, section 3 guiding principles and design standards, read October 9, 2026
- Hilbert and colleagues, Conceptualizing the human drivers of low tree diversity in planted urban landscapes, Ambio, September 2023, abstract read on Europe PMC October 9, 2026