For thousands of residents, the misery begins in late February or early March, when fruitless mulberry trees across the valley sprout a yellow-green coat of inch-long tassels known as catkins.

Each catkin is loaded with pollen ready to be scattered by the wind.

Mulberry levels in excess of 1,500 grains of pollen per cubic meter of air are considered very high by Clark County's Department of Air Quality and Environmental Management. It's not unusual for readings to top 10,000 grains per cubic meter in older neighborhoods with lots of trees.

In 2005, a level of almost 70,000, the valley's highest in 20 years, was recorded on a mulberry-lined stretch of Alta Drive between Decatur and Valley View boulevards.

Nothing like that has been seen so far this allergy season.

"Instead of a quick burst of pollen, we're seeing a gradual release," said Monte Symmonds, the county's senior air quality monitoring technician. "It's a fairly average year."

Don't tell that to former Democratic legislator Larry Spitler.

He has lived in Las Vegas for 30 years. Spitler said he didn't have any allergy problems his first two decades here, even when he lived in an older southeast valley neighborhood thick with mulberry trees.

Now he lives near the southern edge of Henderson, with nary a mulberry in sight, and he endures a mild bout of allergies every spring.

"It's usually a one- or two-day event," he said.

This year, though, it was like a truckload of the "smallest gravel imaginable" had been dumped beneath his eyelids. The only thing that brought him relief was lying down with a cool washcloth draped over his eyes.

"I spent Easter in bed with my eyes swollen, red and itching like mad," Spitler said. "Had I hid Easter eggs ... I wouldn't have been able to find them."

According to Las Vegas allergist Dr. Joram Seggev, allergies afflict 20 percent to 30 percent of the population, while another 20 percent to 30 percent of valley residents suffer reactions to the dry, dusty air itself.

Some "may have a beautiful combination of both," Seggev deadpanned.

"Mulberry pollen is not a very potent allergen. It's a problem because it reaches such astronomical numbers," he said.

When they bloom, mulberry trees cause a wide range of reactions, from runny noses and itchy eyes to severe seasonal asthma attacks. Those too miserable to sleep can even slide into depression, Seggev said.

Allergic reactions to pollen tend to grow progressively worse each year, and long-term exposure can lead to the development of asthma.

Every year at this time, Seggev's waiting room fills up with what he called his "mulberry and olive patients."

"It's already worse than last year," Seggev said.

Starting in the 1950s, mulberry trees were planted by the thousands to provide shade from the relentless desert sun.

In a lot of ways, it was the perfect choice, said Bob Morris, area horticulturalist for the University of Nevada Cooperative Extension.

The tree grows quickly and stands up well to hot weather and overzealous pruners. Its broad leaves provide a lot of shade and cool the air around them through evaporation.

"You stick them in the ground and it's a sure thing," Morris said. "You butcher them, and they come right back."

But valley landscapers made a crucial mistake decades ago, when they decided to stop planting the female variety because its dark fruit would stain the ground and make a mess.

As a result, Morris said, nearly all the mulberry trees you see are males, which don't bear fruit but do churn out pollen in great irritating clouds.

In 1991, Clark County commissioners banned the planting and sale of fruitless mulberry trees.

Commissioners also banned all fruited varieties of the European olive tree, which is widely considered the valley's second worst allergen-producing plant.

Other Southwestern cities have enacted similar bans, among them Phoenix and Tucson in Arizona, Albuquerque and Las Cruces in New Mexico, and El Paso in Texas.

Horticulturist and Review-Journal columnist Linn Mills did not support the ban when it was approved, but he said it has succeeded in localizing the pollen problem somewhat.

"I think the impact that the ban has had is higher concentrations in older parts of town," he said.

Mills feels a little sorry for the poor, put-upon mulberry. After all, it has served its role as a shade tree quite well.

"Here's a plant that does almost everything we want, and because it's got a problem three weeks a year, we ban it," he said.

As long as the ban holds, the seasonal problem of mulberry pollen should eventually solve itself, though probably not quickly enough for some allergy sufferers.

Mills said mulberry trees typically live 40 to 50 years. Some local trees are older than that, but all of them should die off naturally within a few decades.

Olive trees, on the other hand, can linger for hundreds of years.

Clark County monitors pollen levels at eight locations in the Las Vegas Valley and Boulder City.

Samples are collected in 18-second intervals every 10 minutes using an automated device that spins at 2,400 revolutions per minute and gathers airborne particles on clear plastic rods coated with silicone grease.

The pollen itself is too small to see with the naked eye.

Symmonds and his staff use microscopes to count individual grains, each no more than 20 microns in diameter.

A line of mulberry pollen 35 grains wide would pass comfortably through the eye of a sewing needle.

Many of the trees around town have begun to shed their catkins, a sure sign that pollen season is almost over.

"We're on the downswing," Symmonds said.

The bad news for your nose: After mulberry season ends, olive season begins.

Relief, if you can call it that, won't come until summer, when the temperature rises well past 105 degrees and all the flowering finally stops.

Review-Journal writer Lynnette Curtis contributed to this report.

Contact reporter Henry Brean at hbrean@reviewjournal.com or (702) 383-0350.