When most people picture seasonal allergies, they picture spring – bright yellow pollen coating everything in sight. But for millions of people, the worst allergy season is that one that’s just about to start.

Fall allergy season is really about one plant’s season: ragweed. 

Ragweed pollen peaks in September

Ragweed plants start producing pollen in August, but it doesn’t peak until mid-September, and a lot of it has to do with the weather. 

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In September, the humidity drops, conditions dry out and breezy days become more common.

This is the ideal recipe for the ragweed plants to release pollen and for that pollen to stay in the air, traveling farther and lingering longer. 

(WATCH: Top 5 worst cities for allergies)

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It’s abundant, and hard to escape

Ragweed is almost impossible to avoid. It grows wild in nearly every state, plus Canada, and is especially abundant across the Midwest and East. It thrives in suburban and rural areas, along roadsides, in fields and in any patch of ground nobody’s tending to. 

A single ragweed plant can produce up to 1 billion grains of pollen. Each grain is a tiny, powdery particle built to travel – light enough to float on the smallest breeze. 

(WATCH: Wondering How Pollen Will Affect You Each Day? We’ve Got You!)

Some of the pollen fertilizes other ragweed plants, but the rest ends up in your nose, eyes and mouth, or settles in your hair, skin and clothing.

And they don’t stay local. Ragweed pollen has been documented traveling as far as 400 miles from where it was released. So even if there isn’t any in your own backyard, pollen a state away could be making you feel miserable. 

Common symptoms 

Almost 50 million people in the U.S. have symptoms from an allergy to ragweed pollen. It's a leading cause of seasonal allergic rhinitis (aka hay fever) as well as a major asthma trigger. For people with allergic asthma, that’s not a minor concern: ragweed exposure can set off symptoms that become serious. 

The most common complaints are a cough, runny nose and an itchy throat. Others include:

• Nasal congestion
• Thin, watery nasal discharge
• Post nasal drip
• Sneezing
• Watery itchy or irritated eyes
• Eye swelling 
• Weakness or fatigue
• Hives

Direct contact with the plant itself, or heavy exposure to airborne particles, can also cause a ragweed rash. 

(WATCH: What doctors want you to know about your allergies)

How to get through it 

You can’t avoid ragweed pollen, but you can plan around it. 

Watch the forecast for daily pollen counts, sudden temperature swings and air quality alerts all tell you when a day is going to be rough. Warm, dry and breezy conditions are a bad combination. A good soaking rain temporarily clears the air. 

Airborne pollen concentration runs highest in the morning, right after the dew dries and on through late morning, and it can stay elevated into the late afternoon. If you have a choice about when to run, walk the dog or work in the yard, choose the evening. 

Shower when you come inside. Pollen rides in on your hair, skin and clothes and then follows you to bed. A quick rinse and a change of clothes keeps you from sleeping in it.

Keep your windows closed on high pollen count days, and run air conditioning instead. I know it’s tempting to open up the house on the first cool morning of fall, but that's the same morning ragweed is at its worst. 

The good news is that ragweed season has a pretty hard stop once that first frost hits. But until then, keep your eye on the forecast. 

Miriam Guthrie is an associate meteorologist with weather.com who graduated from Georgia Tech with her undergrad and graduate degrees in Earth & Atmospheric Sciences.