If you're a ragweed allergy sufferer who's been feeling more miserable in recent years, you're probably not imagining things. New research shows the ragweed pollen season has grown longer, all thanks to climate change.

Plant and allergy experts with the U.S. Department of Agriculture just completed a study in which they analyzed ragweed pollen counts from Texas to Saskatoon over a 15-year period. They found that ragweed now has a longer flowering season that extends further north than it did years ago.

The study authors say the pollen season has been lengthening due to an increase in the number of frost-free days and a delay in the first autumn frost.

Five sites that they looked at, from Wisconsin north to Saskatoon, saw significantly longer ragweed pollen seasons of at least 13 days.

And the further north they went, the longer the pollen season had extended.

Saskatoon's season, for example, is now 27 days longer than it was 15 years ago. That's about a full month more of sneezing, wheezing and watery eyes.

The researchers report that the trend they observed is consistent with the Intergovernmental Panel on Climate Change's view that climate change will increase temperatures to a greater extent at higher latitudes.

Even in places where the ragweed season didn't get longer -- such as Texas and Oklahoma -- there was more pollen, causing more intense symptoms for allergy sufferers.

Lewis Ziska, the study's lead author and a plant physiologist with the Agricultural Research Service at the U.S. Department of Agriculture, said even he was surprised by how much the ragweed season had changed in such a relatively short period of time.

"I thought maybe 10 days, or a couple of weeks, but to see it up to almost four weeks was kind of interesting," he told Reuters.

The study appears in Proceedings of the National Academy of Sciences.

Ragweed allergies are one of the most widespread allergies in Canada. Not only does the allergy cause hay fever symptoms, it can also cause asthma flare-ups.

Ragweed is not just one weed, but actually a group of about 15 species of plants. Short or common ragweed is found all over the places, such as along roadsides and fallow fields. The weeds can produce enormous amounts of airborne pollen, with each weed generating up to 1 billion grains of pollen.

Ragweed begins blooming when the days start getting shorter: after the summer solstice on June 21, and keeps flowering right until the first frost.