Mulberry Pollen: The Tree Planting Cities Had to Ban

Mulberry pollen holds a distinction almost no other pollen in this series can claim: it was potent enough, in enough places, that cities legislated against the tree that makes it. Mulberry pollen pours off male mulberry trees each spring — trees that were planted by the millions, on purpose, precisely because they grew fast, shaded streets, and never dropped fruit. The catch is written into the botany: a mulberry that makes no fruit is a male, and a male makes pollen instead. Southwestern cities spent decades planting pollen factories, then spent the following decades trying to stop.

Key takeaways

• Mulberry pollen was potent enough that cities legislated against the tree: a University of Nevada, Las Vegas thesis documents Clark County's 1991 ban on planting or selling non-native fruitless mulberry — and European olive — within the county.
• The trap was botanical: a mulberry that makes no fruit is a male, and a male makes pollen instead, so the fast-growing "fruitless" street trees cities planted by the millions were all pollen producers.
• Mulberry season is earliest and strongest in the desert Southwest, running February or March through April or May in Phoenix, Tucson, and Las Vegas.
• The bans stopped new plantings, not the mature trees already in the ground, which is why mulberry pollen concentrations stayed high long after the ordinances passed.

The fruitless trap: how mulberry pollen got planted everywhere

Mulberries (Morus) come in several species. White mulberry (Morus alba), introduced from Asia, was promoted for shade and — in an earlier American chapter — as food for silkworms. Red mulberry (Morus rubra) is the eastern native. The USDA Forest Service's woody plant seed manual for Morus records the flowering pattern behind the season: white mulberry flowers in April–May in the Southwest and Oklahoma (May in the East), and the Southwest's littleleaf mulberry flowers April–May as well.

Male trees, planted by design

Mulberry trees are dioecious — male and female flowers grow on separate trees. Nurseries and cities loved male clones sold as "fruitless" mulberry: no messy fruit on sidewalks, fast growth, deep shade in hot climates. In desert cities building suburbs at speed, fruitless mulberry became a default street and yard tree. Every one of those trees was male. Every spring, every one of them flowered.

Small pollen in dry desert air

The pollen itself is small, light, and produced in large quantities — built for wind, and very effective at reaching noses. In dry desert air, with little rain to wash it out during the season, mulberry counts in southwestern cities could climb to levels that dominated the entire local tree-pollen picture.

When mulberry pollen season runs

Mulberry is a spring tree, earliest in the desert Southwest:

In Phoenix, Arizona, Tucson, Arizona, and Las Vegas, Nevada, mulberry is not one tree among many — for weeks it can be the tree story of spring, and it shares the season with the other famous southwestern planting covered in our olive pollen guide.

The bans: Las Vegas, Tucson, and the prohibited lists

The legislative record is specific, and it is the strongest evidence of how seriously these cities took mulberry.

What the ordinances banned

• Clark County, Nevada (Las Vegas), 1991. A University of Nevada, Las Vegas thesis on airborne tree pollen in the Las Vegas Valley documents that in 1991 the Clark County Board of County Commissioners banned non-native fruitless mulberry (Morus alba "Fruitless") — and European olive — from planting, selling, or offering to sell within the county, acting on evidence that airborne pollen levels in urban areas were high during spring. The same research notes the sobering follow-up: mulberry pollen concentrations in Las Vegas remained high long after the ban, because a ban stops new plantings, not the mature trees already in the ground.
• Tucson and Pima County, Arizona. Tucson is widely reported to have banned new mulberry plantings in 1984, and Pima County has long deterred the worst pollen offenders; a University of Arizona Cooperative Extension prohibited-plant list for the Tucson area still lists mulberry (Morus alba) among its prohibited trees, alongside fruiting olives. El Paso, Texas is also commonly reported among the cities that followed with mulberry restrictions in the early 1990s.

The pattern behind the bans is sometimes discussed under the heading of "botanical sexism" — the argument that preferring male, fruitless clones of dioecious species concentrated pollen production in cities. Whatever label is used, the municipal record stands on its own: these were public-health ordinances aimed at a specific, measurable pollen.

Living with the legacy trees

Bans grandfathered the existing canopy. Decades later, mature male mulberries still shade streets in El Paso, Texas and Albuquerque, New Mexico, and their pollen still arrives each spring. That is why mulberry remains a daily tracking problem rather than a history lesson — the source trees are long-lived, and a city-scale canopy turns over slowly.

Tracking mulberry pollen on Pollen Atlas

Mulberry rides the daily Tree Index on Pollen Atlas city pages (0 to 100 — Low 0–33, Moderate 34–66, High 67–100), with per-plant detail that shows when mulberry is the driver. In March and April, compare the desert cities on the national city rankings: on strong mulberry weeks, Phoenix, Tucson, and Las Vegas climb on tree pollen while much of the country is still in early-spring mode. The broader sequence is in the tree pollen season guide.

If mulberry weeks are your hardest of the year and you want your specific sensitivities tested, an allergist can help — search the Pollen Atlas Find an Allergist directory. This article is general information, not medical advice.

Frequently Asked Questions

Why did cities ban mulberry trees?

Cities banned mulberry trees because airborne mulberry pollen levels in urban areas were high during spring. Clark County, Nevada banned planting and selling non-native fruitless mulberry in 1991, and Tucson-area rules likewise prohibited mulberry plantings.

When does mulberry pollen peak?

Mulberry pollen peaks in spring, earliest in the desert Southwest. Phoenix, Tucson, and Las Vegas run February or March through April or May, while the Northeast and Midwest flower in April through May.

What is a fruitless mulberry?

A fruitless mulberry is a male clone sold for landscaping because it drops no messy fruit. Because mulberries are dioecious — male and female flowers grow on separate trees — every fruitless tree is a male, and males produce pollen instead of fruit.

Are mulberry trees still a problem after the bans?

Yes — the bans grandfathered the existing canopy, so mature male mulberries still shade streets across the Southwest. Their pollen still arrives each spring, and a city-scale canopy turns over only slowly.

Cities banned the planting; nobody banned the season. Know when it runs where you live, watch the Tree Index through it, and spring in mulberry country gets much more predictable.

In mulberry country, spring is a season to schedule: check today's Tree Index and 5-day forecast for your city, free, on PollenAtlas.com.

Sources cited in this post: University of Nevada, Las Vegas thesis, "A Comparison of Airborne Tree Pollen in the Las Vegas Valley from 2017–2021" (Clark County's 1991 prohibition on fruitless mulberry and European olive under its Air Quality Regulations; mulberry concentrations remaining high after the ban); USDA Forest Service, Woody Plant Seed Manual, Morus chapter (flowering April–May Southwest/Oklahoma, May East); University of Arizona Cooperative Extension prohibited plant list (mulberry prohibited, Tucson area). Tucson's 1984 ban date is as commonly reported in secondary accounts of the ordinance; the UNLV-documented 1991 Clark County ban is the primary-sourced date in this post.