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Most of us do not look forward to that first hard frost in late September or early October.

Farmers and gardeners know it as the end of the growing season. Lovers of home-grown fresh vegetables know this as well. They must now turn to foods shipped in from the South or West. And all of us know frost as a sign of coming winter.

But there is at least one group that can hardly wait. Those are the hay fever sufferers.

Sniffling and sneezing, their eyes puffy and red, their heads aching, sleep-deprived, they have been terribly uncomfortable ever since mid-August. No wonder they are ready for cold weather to end the pollen season.

Hay fever -- more formally, to the medical profession, allergic rhinitis -- is, of course, only one of many allergies. But it is the most common, affecting 10 to 20 percent of us. The name is misleading, as hay has nothing to do with hay fever.

Until 1873, when English researcher Charles Blackley identified the illness with plant pollen, it also was thought to be caused by hot weather. And even today, many people blame it on goldenrod. That is not an unreasonable guess, as goldenrods and asters are widespread though hay fever season. But the pollen of those plants is heavy and is not easily airborne.

That pollen is indeed allergenic, but it is not so readily available.

The real culprit is ragweed. It isn't the only villain. At other times of the year, even elms, maples, willows, poplars, dock and timothy create problems for a few especially allergic people, but lighter, and therefore carried by wind, ragweed pollen is far and away the worst troublemaker for hay fever sufferers.

There are many species of ragweed, but the one most often encountered locally is common ragweed. The Latin name for this plant is Ambrosia artemisiifolia, that genus name surely misleading because it could hardly be called food for the gods.

One author, perhaps a hay fever sufferer himself, says "food of the devil" would be a better term. When cows eat it, the ragweed spoils their milk. For this reason, it is called bitterweed by dairymen.

Common ragweed could easily be mistaken for not-yet-blossoming goldenrod, as it stands 3 to 5 feet tall with terminal spikes, each bearing 50 to 100 tiny, pale green, nobby flower heads. These bracts look as though they would any day now blossom into nice yellow goldenrod -- but they never do.

The mature flowers show only a tint of yellow. Ragweed leaves are deeply cut, quite fernlike. The species name comes from the fact that they are like the leaves of the artemisias, or wormwoods. This is a plant of disturbed areas and is very common along our rural roadsides.

Just one ragweed plant can produce a billion pollen grains. The name pollen also derives from Latin, this time from a word meaning "fine flour" or "dust." They are so small that we cannot see them.

To get an idea of their size, consider the smallest space on a metric ruler. That's one millimeter. Forty or more ragweed pollen grains laid end to end would be that long.

Weather conditions influence pollen concentrations, with warm, dry, breezy days optimal for the plant, worst for the allergic.

Pollen avoidance is a suggested response for hay fever sufferers, but that is not at all easy, as some individual grains among the hundred million tons produced in North America each year have been known to travel 400 miles.

So think of those poor souls before you complain about that first frost.

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