Hay fever occurs when susceptible individuals inhale airborne pollen released from the anthers of wind-pollinated vascular plants. The dispersal of the pollen is haphazard, not being directed by the actions of insects or other pollination vectors. To ensure that some of this pollen arrives on the stigmas of flowers of the same species, flowers of wind-pollinated species must produce very large amounts of pollen. It is these wind-pollinated species, that release very large amounts of pollen into the air, that are the major causes of hay fever. Wind-pollinated flowers, not requiring insects or other vectors for effective pollination, tend to be very small and lack special attractants. This is in contrast to plants that have flowers that are mainly or exclusively pollinated by insects or other vectors. This type of plant typically produces fairly small amounts of sticky pollen, has large flowers or flower targets with colors and markings in wavelenghth clearly visible to the potential pollination vectors, and has attractants or rewards of special interest to the pollination vectors. Common ragweed (Ambrosia artemisiifolia L.), a common weed along the margins of roadsides and in other disturbed habitats, and the major cause of hay fever in many areas is a typical wind-pollinated plant. It has very inconspicuous flowers that produce very large amounts of airborne pollen. Unfortunately, since the flowering period of common ragweed usually coincides with that of Canada goldenrod (Solidago canadensis L.), a plant with heavy sticky pollen and very showy yellow flowers that often forms a solid stand in adjacent fields, Canada goldenrod is frequently wrongly considered to be the plant causing the hay fever symptoms. The following list of hay fever plants is adapted from the publication I. John Bassett, C. W. Crompton, and J. A. Parmelee, An atlas of airborne pollen grains and common fungus spores of Canada, Research Branch, Canada Department of Agriculture, Ottawa, Monograph No. 18, 1978, 321 pages. Dr. Parmelee also has an excellent section on fungus spores that cause hay fever in this publication.
alders - Alnus species ashes - Fraxinus species aspens - Populus species atriplexes - Atriplex species basswoods - Tilia species bayberry - Myrica pensylvanica Loisel. beeches - Fagus species birches - Betula species butternuts - Juglans species cattails - Typha species cedars - Thuja species chestnuts - Castanea species cockleburs - Xanthium species docks - Rumex species elms - Ulmus species grasses - family Gramineae (Poaceae) greasewood - Sarcobatus vermiculatus (Hook.) Torr. hemlocks - Tsuga species hickorys - Carya species junipers - Juniperis species kochia - Kochia scoparia (L.) Schrad. lamb's-quarters - Chenopodium album L. maples - Acer species mugwort - Artemisia vulgaris L. nettle, stinging - Urtica dioica L. nettle, wood - Laportea canadensis (L.) Gaudin oaks - Quercus species pines - Pinus species pigweed, redroot - Amaranthus retroflexus L. pigweed, Russian - Axiris amaranthoides L. pigweed, tumble - Amaranthus albus L. plantains - Plantago species poverty weed - Iva axillaris Pursh ragweed, common - Ambrosia artemiisifolia L. ragweed, false - Iva xanthifolia Nutt. ragweed, giant - Ambrosia trifida L. ragweed, perennial - Ambrosia psilostachya DC. sagebrushes - Artemisia species sages - Artemisia species sedges - Carex species sorrels - Rumex species sweet-fern - Comptonia peregrina (L.) Coult. sweet gale - Myrica gale L. sycamores - Plantanus species thistle, Russian - Salsola kali L. walnuts - Juglans species willows - Salix species wormwood, biennial - Artemisia biennis Willd.