The Aster plant family (Asteracea) has a genus called Ambrosia. The Greek word ambrosia translates to “food or drink of immortality.” Gardeners like pretty composite aster flowers and most people like ambrosia pudding, fruit salad and custard. What’s not to like?

There are about 40 species in the Ambrosia genus; most originally evolved in North America. Some are from the subtropical and tropical Americas, and several are invasive introduced weeds in Europe. Their common name is ragweed. Ragweeds produce billions of microscopic pollen grains that disperse in the wind for hundreds of miles. September is “hay fever” time. In North America, the pollen count is based primarily on ragweed and dogfennel pollen in the air.

Ragweeds are tall, coarse annual plants with rough hairy stems and lobed or divided leaves. Monoecious ragweeds have both tiny staminate (male) flowers that produce pollen and even tinier pistillate (female) flowers that produce seeds. Staminate flowers are on terminal spikes while the pistillate flower stems rise from the upper leaf axils where leaves join the stem. Flowers are greenish and inconspicuous. The USF Plant Atlas lists four ragweed species in Florida.

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A single plant of Common Ragweed, Ambrosia artemisiifolia, can produce a billion grains of pollen. Its thin, divided ragged leaves may appear alternate or opposite on a 3.5-foot stem. Common Ragweed ranges across North America in southern Canada, the U.S., northwestern Mexico and throughout Florida.

Giant Ragweed, Ambrosia trifida, can grow between 1 meter and 5 meters tall. It ranges across southern Canada from British Columbia to Quebec and south to California, Arkansas and scattered areas of Florida. Leaves have three to five lobes.

Cut down annual ragweed plants in late summer before they flower. That eliminates pollen and seeds for next year’s crop. In the latter half of August, I saw roadside mowing in Citrus, Levy and Marion counties. Homeowners and farmers could also cut down annual ragweed before flowering to reduce airborne pollen.

The lists 19 species of Eupatorium genus plants growing in Florida, including perennial, sticky-stemmed Dogfennel, Eupatorium capillifolium. Dogfennel is found in wet and drier disturbed sited like roadsides, flatwoods, pastures and abandoned gardens throughout Florida. Its tiny pollen spreads in the slightest air movement to cause allergic reactions in sensitive people like me.

In mid to late summer, before dogfennel flowers produce pollen or seed, dogfennel is easily removed and eradicated. Wear gloves, as dogfennel and ragweed sap can cause contact dermatitis in sensitive people. After a rain, irrigation or hand watering, pull tall dogfennel stems toward the center of the basal rosette. The roots will easily pull up from moist sandy soil. As there are no flowers, pollen or seeds, gardeners can toss the pulled stems onto turfgrass and mow the plants into fine mulch. The nutrients in the dead plant material are reusable by grasses, groundcovers and wildflowers. The carbon from the plants is recycled and stored in the soil.

Unfortunately, plants, animals and people can be falsely considered guilty by association. Attractive flowering goldenrods, Solodago species, and dotted horsemint, Monarda punctata, flower at the same time as allergy-causing ragweed and dogfennel. Large goldenrod pollen is heavy, so falls to the ground rather than becoming airborne. Goldenrod pollen is critically important for native bees and honeybees. There are 21 Solodago species and varieties of goldenrod in Florida, plus eight other genus species with goldenrod in their common names.

Ragweed and dogfennel pollen are prime causes of allergies in humans in North America. Contact with pollen causes the release of histamine in a human body. Histamine irritates blood vessels and mucus-secreting glands, causing sneezing, itchiness, mucus dripping and swelling. Knowing I will get allergic pollen reactions in late summer and early fall is an emotional stress that makes symptoms worse.

I find wearing a mask while outdoors works well at preventing breathing in pollen dust and fungi. Rain washes pollen from the air. Ragweed and dogfennel pollen starts to be released in the morning and peaks around solar mid-day. Cooler early mornings or late afternoons and after a rain are better and more comfortable times to garden during pollen allergy season.

Jane Weber is a professional gardener and consultant. Semi-retired, she grows thousands of native plants. Contact her at or phone 352-249-6899.

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