The 10-20-30 rule is a planting guideline used by urban foresters and city planners. It says a city’s trees should include no more than 10% of any one species, 20% of any one genus, and 30% of any one plant family. The goal is straightforward: keep tree populations diverse so one disease or pest cannot wipe out a large portion of the canopy.
Dr. Frank Santamour, a research geneticist at the US National Arboretum, is credited with publishing the rule in 1990. He introduced it as a practical way to manage urban tree populations at the city scale. It has been widely used by arborists, municipal foresters, and city planners ever since, including throughout Wisconsin.
The rule does not tell you which trees to plant. It tells you how much of any one type is too much. That distinction matters. A city can have excellent individual tree choices and still be dangerously overexposed if those choices repeat too often.
Why Does Tree Diversity Matter?
When a city or property plants too many of the same tree, it creates a single point of failure. One targeted disease or invasive insect can cause massive, rapid losses across an entire canopy. The trees may all look healthy for years. Then one threat arrives and spreads with nothing to slow it down.
Milwaukee knows this firsthand. The emerald ash borer arrived in Wisconsin in 2008 and has killed millions of ash trees across the state. Cities that had planted ash heavily, and both green ash and white ash were popular street tree choices for decades, took the hardest hits. The Wisconsin DNR now discourages planting any ash species due to the ongoing risk.
Dutch elm disease tells a similar story. American elms once lined streets across Milwaukee County. Their broad, arching canopies created cathedral-like corridors over roads throughout the region. When Dutch elm disease spread through the area, entire blocks lost their tree cover in a short period. Communities spent years and significant budget replanting from scratch.
Both examples follow the same pattern. A tree species proves itself useful and attractive. It gets planted widely. A single threat emerges and, because the trees are everywhere, the threat spreads everywhere too. The 10-20-30 rule exists to interrupt that cycle before it starts.
Diverse tree populations are also more resilient to climate shifts. Different species tolerate heat, drought, heavy snow, and strong winds in different ways. A mixed canopy absorbs those stresses better than a uniform one. That matters in Milwaukee and Waukesha Counties, where winters average 48.7 inches and 38.1 inches of snowfall respectively and where freeze-thaw cycles put real physical stress on trees every season.
How the Rule Breaks Down
The three tiers of the rule work together. Each one addresses a different level of biological classification.
Think of a city with 1,000 trees total.
- 10% Species: No more than 100 trees should be the exact same species. For example, no more than 100 sugar maples. Sugar maple and red maple are different species, so each has its own 10% limit.
- 20% Genus: No more than 200 trees should belong to the same genus. All maple varieties combined, including red, sugar, and silver maple, cannot exceed 200 trees. Even though they are different species, they share a genus and vulnerabilities.
- 30% Family: No more than 300 trees should belong to the same plant family. This is the widest tier. It groups together trees that share deeper biological relationships, including species that may be vulnerable to the same family-level threats.
Each tier builds on the one before it. Staying within the species limit does not automatically keep you within the genus limit. You have to track all three independently.
The wider the spread across species, genera, and families, the better the protection. A canopy with 20 species drawn from 15 genera and 10 families is far more resilient than one with 8 species drawn from 3 genera.
Does the Rule Apply to Private and Commercial Properties?
The 10-20-30 rule was written for city-wide or neighborhood-scale planning, not individual lots. A single residential yard with three or four trees is not the target audience. But the thinking behind the rule applies meaningfully to any property with a substantial tree inventory.
Commercial property managers in Milwaukee and Waukesha County often oversee sites with dozens of trees. Office parks, retail centers, manufacturing facilities, and large institutional campuses can hold significant tree populations across parking lots, green spaces, and property borders. At that scale, species concentration becomes a real exposure.
If a disease affecting northern red oaks spreads through your area, for example, and your property holds 15 red oaks out of 20 total trees, you stand to lose 75% of your canopy from a single problem. A more varied mix, including evergreens like eastern white pine or blue spruce alongside deciduous species like red maple, bur oak, or hackberry, distributes that risk.
Property managers with large grounds benefit from periodically reviewing their tree composition. This is especially true now, as new pest and disease threats continue to emerge across Wisconsin. Knowing what you have and how concentrated it is gives you a clearer picture of your exposure.
Does the Rule Have Limits?
Yes. The 10-20-30 rule is widely respected but not without valid criticism.
One concern is scale. A city might appear to meet the 10% species threshold when measured across its entire tree inventory, while individual neighborhoods are planted almost entirely with one tree type. Those neighborhoods carry the same concentration risk as a city that ignores the rule entirely. Recent research has suggested applying the rule at smaller scales, including at the census block level, to address this gap.
Another concern is that the rule may not be strict enough for today’s threats. The emerald ash borer does not stop at species lines. It attacks all ash species. A city with 10% white ash and 10% green ash still has 20% of its canopy at risk from a single insect. Some researchers and foresters have proposed tightening the targets to no more than 5% of any one genus to account for pests and diseases that operate at the genus level rather than the species level.
There is also a practical tension with design. Planting for strict diversity can conflict with aesthetic goals. Designers often prefer visual repetition and uniformity. A row of identical trees along a boulevard reads as intentional and orderly. A mix of six species in that same row looks inconsistent to some eyes. Urban forestry and landscape design do not always want the same things, and the rule does not resolve that tension on its own.
The rule is a useful compass, not a complete answer. It identifies where risk accumulates. What to do about it still requires judgment, local knowledge, and an understanding of which trees actually perform in a given climate.
What Trees Work Well for Diversity in Milwaukee?
Milwaukee and Waukesha Counties present specific conditions that not every tree handles well. Heavy snow loads, ice accumulation, freeze-thaw cycles, lake-effect snow from Lake Michigan, road salt exposure near parking lots and roadways, and urban heat in paved commercial areas all factor into how trees perform over time.
Species that hold up well in these conditions and support a more diverse canopy include the following.
On the evergreen side, eastern white pine is one of Wisconsin’s most familiar native trees and grows well across a range of sites. Blue spruce offers excellent snow load tolerance due to its stiff branches and works well as a focal specimen or windbreak. White spruce is cold-hardy to negative 50 degrees Fahrenheit and adapts to various soil types. Northern white cedar thrives in wet areas where other evergreens struggle and is widely used for screening and hedges.
On the deciduous side, bur oak is one of the most drought-tolerant native trees in the region and lives for centuries when healthy. Red maple grows at a moderate pace, handles occasional wet soils well, and delivers reliable fall color. Hackberry is one of the most adaptable urban trees available, tolerating poor soils, pollution, and road salt better than most species. American basswood provides dense shade and supports pollinators through its fragrant early summer flowers. River birch offers attractive peeling bark and holds up better against the bronze birch borer than other birch species. Northern red oak provides strong structure, acorn production for wildlife, and striking fall color.
Mixing species from different genera and families across a property gives the best long-term protection. It also supports wildlife through a broader range of food and habitat, improves the visual interest of the property across seasons, and reduces the chance that one problem spreads unchecked through everything at once.
What to Do if You Are Unsure About Your Tree Composition
If you manage a property in the Milwaukee area and have not taken stock of your tree inventory recently, that is a reasonable place to start. Knowing what species you have, how many of each, and what conditions they are in gives you the information you need to make good decisions going forward.
A professional tree assessment looks at species, structure, and health. It can identify trees that are already showing signs of disease or stress, flag species that carry higher risk given current pest pressures in the region, and give you a clearer picture of where your canopy is concentrated.
This kind of information is useful at budget planning time. It helps you prioritize which trees need attention now and which are likely to need replacement in the coming years. It also provides documentation that can support insurance and liability management.
Tree Services in Milwaukee, WI
Tree & Snow Removal Services has worked with property owners and commercial clients across Milwaukee and Waukesha Counties since April 2012. We handle tree trimming, tree removal, stump grinding, and emergency tree service throughout Milwaukee, Menomonee Falls, Brookfield, Germantown, Mequon, Wauwatosa, Oak Creek, and surrounding communities.
We offer free estimates with no deposit required. Payment is due only when the job is complete.
For a property-specific assessment in the Milwaukee area, contact Tree & Snow Removal Services at 414-215-4499. We will take a look at what you have and give you an honest picture of the condition and composition of your trees.