About Ragweed

It may seem odd to focus an article on a site dedicated to the love of Nature on a scourge such as ragweed, but I see it in a similar vein to an article on sharks or tornadoes which can cause great suffering and yet still spawn much interest. Since seventy-five percent of people with hay fever are sensitive to ragweed, I feel it is important to give this plant a little more attention. (For my article on hay fever and its connection with weather, see Aeroallergens: Misery Blowin' in the Wind.) Indeed, early pollen allergy research gave it almost undivided attention in the 1950s and 1960s.

Ragweeds are green- or yellow-flowering, perennial herbs with lobed or divided leaves. They are hairy, coarse-looking plants with unattractive flowers and an unpleasant scent. There are 21 species of ragweed, but the two most often encountered in North America are common or short ragweed (Ambrosia artemisiifolia), common in most of the Northeast US, the Maritime Provinces, southern Quebec and southern Ontario; and giant ragweed (Ambrosia trifilda) found mostly in the Mississippi River Valley and Great Lakes region.

Ragweeds are pioneer plants that are well adapted to invading newly disturbed soils. While in natural habitats they are restricted by competition with other plants, but in areas where humans have cleared existing vegetation, ragweed quickly becomes widely and aggressively established. Thus, ragweed is very abundant along rural roadsides, fence lines, waste lands, new excavations, cultivated fields, gardens, and poorly kept lawns.

A. artemisiifolia, or common ragweed, grows to 30 to 150 cm (1 to 5 ft) in height. Its stems are green and hairy, holding leaves divided into narrow, irregularly lobed segments. The green- to yellow-coloured flowers of common ragweed are small, 2 to 5 mm (0.08 to 0.2 inch) across, and grown on 5 to 10 cm (2 to 4 inch) long terminal spikes or elongated clusters at the end of branches. Common ragweed is best distinguished by its finely divided leaves, which are opposite in the lower plant stem and alternate in the upper part of the plant. Leaves are yellow-green initially and become gray-green with age.

A. trifilda, or giant ragweed, has a much-branched and hairy stem with leaves opposite (2 per node) each other throughout the plant -- except towards the ends of the smaller branches. Its leaves are large and rounded with 3 to 5 lobes that are either smooth or coarsely toothed. The mature plant stands 0.9 to 1.5 metres (3-5 feet) tall and may reach 4m (13 ft) with terminal spikes bearing 50 to 100 tiny, pale green, nobby flower heads.

These spike heads look as though they are about to blossom at any moment into yellow flowers, but they never do. A mature ragweed flower shows only a hint of yellow, and thus, ragweed is often mistaken for the not-yet-blossoming goldenrod (which does not contribute to hay fever).

On both varieties, the individual florets are either male or female, but never both. All flowers within one head being either only male or only female. However, both male and female flower heads are usually present on the same plant. The male flowers that produce the offending pollen form in elongated clusters at branch ends, each head on a short stalk hanging downward like a tiny inverted umbrella so that pollens may drop into the wind.

Decreasing day length is the trigger for ragweed pollination. Therefore, the ragweeds begin pollinating in late August and continue to do so through September, and even into October, until the first killing frost halts pollen production.

Ragweed is a prodigious producer of small, light pollen: forty grains laid end to end would span about one millimeter. Just one ragweed plant can produce a billion airborne pollen grains during an average season, and the pollen are extremely potent allergens. As few as 20 ragweed pollen grains per cubic meter of air -- less than 1,000 grains in an average bedroom -- can trigger an allergic reaction.

In fact, ragweed pollens can produce such strong and unexpected reactions that each fall at the University of Michigan in the 1950s and 60s, many new foreign students, particularly those from Asia where ragweed is not common, flocked to student health services suffering from sneezing and congestion. This spate of illness soon became known as Ann Arbor Flu until the true cause was finally determined. I too was a sufferer from that Flu, but unfortunately I knew the cause of my misery and fervently prayed for Jack Frost to come to town.

The situation may be getting worse for those allergic to ragweed pollens according to research by Lewis H. Ziska, a plant physiologist with USDA's Agricultural Research Service. He has shown that pollen released by ragweed plants grown in indoor chambers at various levels of atmospheric CO2, increased substantially in number as the gas concentration increased. Under early 1900 CO2 concentrations of 280 parts per million (ppm), today's levels at 370 ppm and a predicted future level at 600 ppm, ragweed pollen production rose from 5.5 grams to 10 grams to 20 grams, respectively.

For a look into the role weather plays in hay fever, see Aeroallergens: Misery Blowin' in the Wind